Acclimation to Very Low CO₂: Contribution of Limiting CO₂ Inducible Proteins, LCIB and LCIA, to Inorganic Carbon Uptake in Chlamydomonas reinhardtii^,

Two complementary inorganic carbon uptake systems show distinct functions in the CO₂-concentrating mechanism and both are essential for limiting CO₂ acclimation.

Abstract

The limiting-CO₂ inducible CO₂-concentrating mechanism (CCM) of microalgae represents an effective strategy to capture CO₂ when its availability is limited. At least two limiting-CO₂ acclimation states, termed low CO₂ and very low CO₂, have been demonstrated in the model microalga Chlamydomonas reinhardtii, and many questions still remain unanswered regarding both the regulation of these acclimation states and the molecular mechanism underlying operation of the CCM in these two states. This study examines the role of two proteins, Limiting CO₂ Inducible A (LCIA; also named NAR1.2) and LCIB, in the CCM of C. reinhardtii. The identification of an LCIA-LCIB double mutant based on its inability to survive in very low CO₂ suggests that both LCIA and LCIB are critical for survival in very low CO₂. The contrasting effects of individual mutations in LCIB and LCIA compared with the effects of LCIB-LCIA double mutations on growth and inorganic carbon-dependent photosynthetic O₂ evolution reveal distinct roles of LCIA and LCIB in the CCM. Although both LCIA and LCIB are essential for very low CO₂ acclimation, LCIB appears to function in a CO₂ uptake system, whereas LCIA appears to be associated with a HCO₃⁻ transport system. The contrasting and complementary roles of LCIA and LCIB in acclimation to low CO₂ and very low CO₂ suggest a possible mechanism of differential regulation of the CCM based on the inhibition of HCO₃⁻ transporters by moderate to high levels of CO₂.

The CO₂ concentration in Earth’s atmosphere has declined significantly since the origin of photosynthesis, and the current atmospheric CO₂ level is a major limiting factor for optimal photosynthesis in many plant species. Rubisco, the central enzyme catalyzing CO₂ assimilation, has a low affinity for CO₂ and a slow catalytic turnover rate for the carboxylation reaction, possibly as an evolutionary relic, and also catalyzes the competing oxygenation reaction between O₂ and ribulose-1,5-bisphosphate that releases fixed CO₂ through photorespiration. As a result, several adaptive strategies have evolved to improve photosynthetic efficiency by raising the CO₂ concentration at the site of Rubisco to increase the carboxylation rates and to suppress the wasteful photorespiration pathway. Among them, the cyanobacterial/microalgal CO₂-concentrating mechanism (CCM) appears to be one of the most effective strategies for CO₂ enrichment. These CCMs deploy diverse, active inorganic carbon (Ci) uptake systems, allowing cells to accumulate intracellular Ci up to 1,000-fold from low CO₂ environments (Badger and Price, 2003; Moroney and Ynalvez, 2007; Price et al., 2008; Spalding, 2008). Because microalgae and cyanobacteria contribute a great portion of global CO₂ sequestration through photosynthesis (Behrenfeld et al., 2001), and have shown enormous potential as an alternative future energy source (Sheehan et al., 1998; Wijffels and Barbosa, 2010), enhanced knowledge of the microalgal CCM will provide not only insights into the CCM’s influences on natural environments, but also guidance for the potential application of bioengineering approaches to enhance biomass productivity in economically important algae strains, or to improve photosynthetic carbon fixation in crop species that lack a CCM.

Although microalgal and cyanobacterial CCMs exhibit a very high level of diversity with regard to components and mechanisms (Badger and Spalding, 2000), these CCMs share some major molecular characteristics, including Rubisco sequestration in a specialized microcompartment, catalyzed interconversion of Ci species by carbonic anhydrase (CA), and energy-dependent, active Ci uptake systems. The eukaryotic CCM has been extensively studied in the model organism Chlamydomonas reinhardtii during the last 3 to 4 decades. Compared with the prokaryotic, cyanobacterial CCM, the eukaryotic, microalgal CCM appears complex because of the involvement of additional subcellular compartments and regulatory systems (Wang et al., 2011). Some areas of uncertainty regarding key features of the microalgal CCM remain to be addressed. First, Ci, including the charged species, bicarbonate (HCO₃⁻), must cross both the plasma membrane and the chloroplast envelope to reach Rubisco, yet the roles, if any, of proposed and confirmed Ci transporters, including the plasma membrane proteins HLA3 and Limiting CO₂ Inducible1 (LCI1) and the chloroplast envelope proteins LCIA (NAR1.2), CCP1, and CCP2 (Im and Grossman, 2002; Miura et al., 2004; Pollock et al., 2004; Mariscal et al., 2006; Duanmu et al., 2009a; Ohnishi et al., 2010), in acclimation and Ci uptake under different limiting CO₂ conditions are not yet clearly defined. Second, previous physiological studies have demonstrated that active uptake of both CO₂ and HCO₃⁻ occurs in C. reinhardtii (Moroney and Tolbert, 1985; Sültemeyer et al., 1989), but the molecular components and the underlying mechanism responsible for active CO₂ uptake are still largely unknown. Third, the tight regulation of CCM functional components is still not well understood (Spalding et al., 2002). Physiological studies in wild-type C. reinhardtii cells have suggested that within the range of so-called limiting or low CO₂ (the meaning of low CO₂ is not universally defined in many previous publications, often ranging from air level of CO₂ to nearly zero CO₂), at least two distinct acclimation states, low CO₂ (approximately 0.03%–0.5%) and very low CO₂ (<0.02%), can be defined (Vance and Spalding, 2005). However, recent transcriptome studies (Brueggeman et al., 2012; Fang et al., 2012) failed to identify any changes that occur at the transcription abundance level between low CO₂ and very low CO₂ conditions, so it is still unclear how the CCM is differentially regulated between these two limiting CO₂ acclimation states.

The first line of clear genetic evidence showing multiple, limiting-CO₂ acclimation states came from identification of mutants with an air dier growth phenotype (Wang and Spalding, 2006); these mutants die in air level of CO₂ (0.03%–0.05%) but can survive in either high CO₂ (5%) or very low CO₂ (<0.02%), and their ability to accumulate Ci is severely compromised in low CO₂ but fairly normal in very low CO₂. The mutation causing this air dier phenotype has been unequivocally linked to the LCIB gene (Wang and Spalding, 2006), which encodes a novel chloroplast protein that forms a heteromultimeric complex with its close homolog LCIC (Yamano et al., 2010; Wang and Spalding, 2014). It is unclear how LCIB is involved in Ci accumulation, largely because of its unknown biochemical characteristics and the lack of homologs of known function in other organisms. Genetic analysis of LCIB/CAH3 double mutants indicated that LCIB functions downstream of CAH3, a thylakoid lumen CA (Duanmu et al., 2009b). Because CAH3 catalyzes dehydration of accumulated HCO₃⁻ to provide CO₂ for Rubisco inside the pyrenoid (Spalding et al., 1983b; Moroney et al., 2011), it has been hypothesized that LCIB captures CO₂ leaked from the pyrenoid, possibly by unidirectionally hydrating CO₂ back to HCO₃⁻ (Duanmu et al., 2009b). If this hypothesis is correct, it also implies that LCIB may also function in active Ci accumulation by unidirectionally hydrating externally diffused CO₂ into the stromal HCO₃⁻ pool (Wang and Spalding, 2014). LCIB and the LCIB/LCIC complex change location within the chloroplast when cells are transferred from low CO₂ (or high CO₂) to very low CO₂ conditions (Yamano et al., 2010; Wang and Spalding, 2014; Yamano et al., 2014), suggesting that a regulatory mechanism exists to regulate the functions of LCIB, possibly associated with low CO₂ and very low CO₂ acclimation states.

The growth phenotype and physiological characteristics of LCIB mutants clearly demonstrate that LCIB is indispensable for Ci acquisition in low CO₂, but is dispensable in very low CO₂ when other Ci uptake systems are still functional. It is also clear, however, that even though it is dispensable, LCIB contributes to viability in very low CO₂ (Duanmu et al., 2009a), although neither its level of contribution to nor its precise role in Ci uptake under these conditions has been established. Two putative Ci transporters have been demonstrated to contribute LCIB-independent Ci uptake in C. reinhardtii_. Knockdown of HLA3, a gene encoding an ATP-binding cassette transporter protein, causes growth defects in LCIB mutants in very low CO₂ and decreased Ci accumulation especially at a high pH (Duanmu et al., 2009a). HLA3 is predicted to be localized on the plasma membrane, and therefore is very likely involved in HCO₃⁻ transport at this location. LCIA, another putative Ci transporter that might be involved in Ci uptake in very low CO₂, belongs to a formate-nitrite transporter (FNT) family (Mariscal et al., 2006), and is predicted to be targeted to the chloroplast envelope (Miura et al., 2004). Although it shows significant identity to the C. reinhardtii nitrate assimilation-related (NAR) gene family and has also been named NAR1.2, unlike other NAR genes, LCIA expression is regulated by CO₂ but not by nitrogen source (Miura et al., 2004; Mariscal et al., 2006), and introduction of an LCIA gene into Xenopus laevis oocytes reportedly increased HCO₃⁻ uptake (Mariscal et al., 2006). Although no LCIA single knockout or knockdown mutants have been reported, simultaneous co-knockdown of LCIA with HLA3 resulted in decreased Ci accumulation and decreased growth in very low CO₂, especially when combined with LCIB mutations (Duanmu et al., 2009a). Although it appears that their contribution to Ci uptake may be important for the growth of C. reinhardtii under very low CO₂, HLA3- or LCIA-associated Ci uptake in the absence of LCIB cannot support growth of LCIB mutants in air-level CO₂, as evidenced by the air dier phenotype. This suggests that the primary role for these putative Ci transporters may be in very low CO₂ conditions, but it is not clear whether, or how, HLA3 and LCIA are differentially regulated between very low CO₂ and low CO₂.

This study was undertaken primarily to dissect the molecular mechanisms underlying the functional acclimation to very low CO₂ in C. reinhardtii. We identified an LCIA-LCIB double mutant that is unable to survive in very low CO₂. The effects of the single LCIB mutation, the LCIA mutation, and the combined LCIA and LCIB mutations on growth and Ci-dependent photosynthetic O₂ evolution reveal possible functions of LCIB and LCIA in the CCM, and demonstrate that both LCIA and LCIB contribute to Ci acquisition and acclimation under very low CO₂ conditions but that they play distinct and complementary roles. Analyses of these data also revealed a possible mechanism to explain the curious air dier phenotype of LCIB mutants and the regulation between low CO₂ and very low CO₂ acclimation states.

RESULTS

Impact of LCIB Mutation on Ci-Dependent O₂ Evolution

LCIB mutations cause severe inhibition of Ci uptake and photosynthesis in low CO₂, but appear to have much less impact on photosynthesis in very low CO₂ (Spalding et al., 1983a; Wang and Spalding, 2006; Duanmu et al., 2009a; Yamano et al., 2010; Duanmu and Spalding, 2011). To better understand this dichotomy, we more closely examined the impact of the LCIB mutation on very low CO₂ acclimation and Ci species dependence of photosynthesis in the LCIB mutant by comparing photosynthetic O₂ evolution in very low CO₂ acclimated cells of the wild type and the LCIB mutant air dier1 (ad1; Wang and Spalding, 2006) at two pHs. As shown in Figure 1A, at an acidic pH (pH 6.0), where the relative abundance of CO₂ is higher, O₂ evolution in ad1 was severely inhibited in 10 to 100 µm Ci but regained wild type rates at 200 µm Ci. However, at pH 7.3, where HCO₃⁻ is much more abundant than CO₂, O₂ evolution rates in ad1 were very similar to those of wild-type cells at 0 to 50 µm Ci but were inhibited relative to the wild type in 50 to 200 µm Ci. Furthermore, photosynthetic rates in ad1 actually decreased as Ci concentration was increased between 50 µm and 200 µm. The different responses of ad1 at these two pHs suggest that the function of LCIB, which is absent in ad1, is Ci species dependent. When the photosynthetic O₂ evolution rates are plotted as a function of the calculated CO₂ or HCO₃⁻ concentrations (Fig. 1, B and C), the apparent inhibition of O₂ evolution in ad1 relative to the wild type occurs in the same CO₂ concentration range, but in different total Ci or HCO₃⁻ concentration ranges, at both pHs, and this CO₂ range corresponds to CO₂ concentrations associated with the low CO₂ acclimation state (Vance and Spalding, 2005), even though the algae were acclimated to very low CO₂. These results suggest that the LCIB mutant is compromised strictly in CO₂ uptake and specifically in the low CO₂ range. Furthermore, the different responses of ad1 at two pHs also suggest that another element, possibly a HCO₃⁻ transport pathway, is responsible for the apparently normal photosynthetic O₂ evolution at lower Ci concentrations (<50 µm at pH 7.3; <10 µm at pH 6.0), equivalent to < 7 µm CO₂ at either pH, which corresponds to the range of CO₂ concentrations associated with the very low CO₂ acclimation state (Vance and Spalding, 2005). The rates of photosynthetic O₂ evolution in ad1 are very similar at either pH when plotted as a function of the HCO₃⁻ concentration, but not the total Ci or CO₂ concentration (Fig. 1, D–F), which supports the suggestion that a HCO₃⁻ transport system is responsible for the growth and Ci uptake of the LCIB mutant in very low CO₂.

Figure 1.

Photosynthetic activity in the wild type and the LCIB mutant ad1. A, Ci-dependent O₂ evolution at pH 7.3 and 6.0. Strains cw10 (squares) and ad1 (triangles) were grown in high CO₂ and then switched to very low CO₂ for 24 h before the measurement. Two hundred units per milliliter of bovine CA was present to ensure the equilibrium of CO₂ and HCO₃⁻. B and C, The photosynthetic activities were plotted against CO₂ (B) and HCO₃⁻ (C) concentrations calculated from total Ci at pH 6.0 (black squares for cw10 or black triangles for ad1) and pH 7.3 (white squares for cw10 or white triangles for ad1). D to F, Comparison of the photosynthetic activities of cw10 and ad1 at pH 7.3 and 6.0 plotted against total Ci (D) or CO₂ (E) and HCO₃⁻ (F) concentrations calculated from total Ci. The experiments showed similar results with two independent cultures, although results from only one experiment are shown. Each data point represents an average of three technical replicates, and the coefficient of variation for each data point, the ratio of the sd (n = 3) to the average, is less than 10% for all data. The O₂ evolution rates in cw10 and ad1 at 4,000 µm Ci are as follows: in pH 7.3, cw10 (101 ± 2) and ad1 (97 ± 5); and in pH 6.0, cw10 (93 ± 3) and ad1 (91 ± 6).

Generation of Additional Mutations in an LCIB Mutant Background

Although the LCIB-associated Ci uptake is missing in LCIB mutants, other Ci uptake systems appear still functional, as evidenced by the nearly normal photosynthetic rates of ad1 in the lowest Ci concentrations (<50 µm at pH 7.3; <10 µm at pH 6.0; Fig. 1A) and by its apparently normal growth in very low CO₂ (Fig. 2). To facilitate identification of Ci uptake systems responsible in LCIB mutants for active Ci uptake and growth in very low CO₂, ad1 was mutagenized by random insertion of an aminoglycoside 3-phosphotransferase (Aph8) gene-bearing plasmid (Sizova et al., 2001), selected for paromomycin resistance (para^R) and screened for inability to grow in very low CO₂. Five mutants with the target phenotype were identified from about 2,000 screened para^R transformants. Whereas ad1 shows a typical air dier growth phenotype and thus can still grow in very low CO₂, these new mutants were unable to grow in either very low CO₂ or low CO₂, but grew as well as the wild type in high CO₂ (Fig. 2). The genomic DNA flanking the Aph8 insert from these new double mutants was isolated and sequenced, which confirmed that all were independent mutants. One mutant in which the insert was found to have disrupted the LCIA gene was named lab1 (LCIA-LCIB double mutant), and was further characterized.

Figure 2.

Identification of lab1, an LCIA-LCIB double mutant. A, Growth of the wild type (cw10) and mutant strains on minimal medium agar plates in different concentrations of CO₂: high CO₂ (5%), low CO₂ (0.04%), and very low CO₂ (0.01%). The mutant genes are shown in parentheses below the strain names. lab1-LCIA is a cell line generated from LCIA-complemented lab1. Strain cia5 is a classic mutant lacking a limiting CO₂ acclimation response. WT, Wild type.

LCIA Mutation in lab1 Responsible for the Lethal Growth Phenotype in Very Low CO₂

The plasmid bearing Aph8 in lab1 is inserted in the sixth exon of LCIA, causing a 243 base deletion, including 133 nucleotides in the coding sequence and 110 nucleotides in the 3′ untranslated region (UTR; Fig. 3A). The insertion in LCIA was confirmed by PCR with a pair of primers that amplify the genomic DNA in this region (Fig. 3B). In wild-type cells, expression of the LCIA protein is induced by limiting CO₂. However, even though the insertion in lab1 is located near the 3′ end of LCIA and causes a deletion of only 44 C-terminal amino acids, western immunoblots were unable to detect any LCIA protein in lab1 (Fig. 3C), either because LCIA is not properly transcribed or not properly translated or because its transcripts or polypeptides are quickly degraded.

Figure 3.

Analysis of lab1, an LCIA-LCIB double mutant. A, Insertion of Aph8 in lab1. The region of deletion in LCIA caused by the Aph8 insertion is shown with a black box above the LCIA gene. B, Amplification of genomic DNA fragments in LCIA by PCR from cw10 and lab1. The locations of primers in LCIA are shown in A. The primers A1 and AS1 were used to amplify a region without insertion; A3 and AS3 were used to amplify a region disrupted by the Aph8 insert in lab1, and the larger size of PCR product in lab1 confirmed the insertion. C, Western-blot analysis of expression of LCIA and LCI1 in cells acclimated to high CO₂ or very low CO₂. LCI1 is a protein encoded by LCI1, a limiting CO₂-induced gene, and was used as the control. No difference in LCI1 induction was observed between the wild type and lab1. D, Recovery of induced expression of LCIA in LCIA-complemented lab1. Wild-type cw10 and mutants ad1, lab1, and LCIA-complemented lab1 were grown in high CO₂, and then shifted into very low CO₂ for 12 h. LCIA protein was detected by western immunoblots with a specific LCIA antibody.

A genetic cross between lab1 and wild-type strain CC620 was performed to confirm whether the lethal growth phenotype in lab1 is caused by the mutation in LCIA. Because lab1 has both a bacterial phleomycin resistance (Ble) gene, (confers zeocin resistance [zeo^R]) insert (causing the LCIB deletion) and an Aph8 insert (causing the LCIA truncation), progeny carrying both LCIB and LCIA mutations were selected for both zeo^R and para^R. More than 50 random progeny were tested for growth in different CO₂ concentrations, and all progeny carrying both mutations (confirmed by PCR) showed the same growth phenotype as lab1 (lethal in both low CO₂ and very low CO₂), indicating that the inability of lab1 to grow in very low CO₂ is genetically linked to the insertion in LCIA. We further cloned a genomic DNA fragment containing the wild-type LCIA gene, including 1.1 kb of the upstream sequence (presumably including the LCIA promoter region) and the entire 3′ UTR, introduced this gene into the lab1 mutant, and found that this LCIA genomic clone complemented growth of lab1 in very low CO₂. All complemented lines restored the air dier growth phenotype of the host strain ad1 (Fig. 2), and the presence of a wild-type LCIA gene and the expression of LCIA protein induced by limiting CO₂ in these lines were confirmed by PCR and western immunoblots (Fig. 3D). These results demonstrate that the lethal growth phenotype in very low CO₂ in the LCIA-LCIB double mutant is caused by the LCIA mutation.

Defective Photosynthesis in lab1 Double Mutants

The inability of lab1 to grow in very low CO₂ suggests that photosynthesis driven by active Ci uptake systems may be defective. Compared with that of the wild type, Ci-dependent O₂ evolution in lab1 was severely inhibited in Ci concentrations ≤ 200 µm at pH 7.3 and markedly decreased in Ci concentrations ≤ 100 µm at pH 6.0 (Fig. 4A). Photosynthetic O₂ evolution in lab1 was similar at either pH when plotted as a function of the calculated CO₂ concentration (Fig. 4B), suggesting that photosynthetic CO₂ assimilation remaining in the absence of both LCIB and LCIA is largely supported through Ci diffusion (Spalding and Ogren, 1983). Because photosynthesis in LCIB single mutants appears to be dependent on HCO₃⁻ uptake, as evidenced by the apparent dependence of photosynthetic O₂ evolution in ad1 on the concentration of HCO₃⁻ (Fig. 1C), the highly reduced and apparently CO₂-dependent photosynthesis in LCIA-LCIB double mutants implies that LCIA is required for much of the active HCO₃⁻ uptake in ad1.

Figure 4.

Ci-dependent photosynthetic oxygen evolution in cw10 and lab1. A, Ci-dependent O₂ evolution at pH 7.3 and 6.0. Strains cw10 (squares) and lab1 (diamonds) were grown in high CO₂ and then switched to very low CO₂ for 24 h before the measurement. Two hundred units per milliliter of bovine CA was present to ensure the equilibrium of CO₂ and HCO₃⁻. B, The photosynthetic activities of lab1 measured at pH 7.3 (white diamonds) and pH 6.0 (black diamonds) were plotted against CO₂ or HCO₃⁻ concentration calculated from total Ci. The experiments showed similar results with two independent cultures, although results from only one experiment are shown. Each data point represents an average of three technical replicates, and the coefficient of variation for each data point, the ratio of the sd (n = 3) to the average, is less than 10% for all data. The O₂ evolution rates of lab1 at 4,000 µm Ci are as follows: (78 ± 3) in pH 7.3 and (79 ± 6) in pH 6.0. The rates of cw10 are the same in Figure 1.

Decreased HCO₃⁻-Dependent Photosynthesis in LCIA Single Mutants

The impacts of an LCIA single mutation on growth and photosynthesis were evaluated in progeny bearing only the LCIA mutation. These were identified as para^R but zeocin-sensitive progeny from the genetic cross of lab1 with CC620 described above. PCR and western-blot analysis confirmed that these para^R but zeocin-sensitive progeny carry the LCIA mutation but retain the wild-type LCIB gene (Fig. 5, B and C). In contrast with lab1 or ad1, the LCIA single-gene mutants did not show any obvious growth defect in spot tests at either low CO₂ or very low CO₂ (Fig. 5A), indicating that loss of LCIA alone has no gross impact on growth and photosynthesis under these conditions.

Figure 5.

Generation and analysis of LCIA single mutants. A, Growth of the wild type, lab1 double mutant, and LCIA single mutants. Strains lab77 (an LCIA-LCIB double mutant progeny), lcia63, and lcia90 (LCIA single-mutant progeny) were derived from a genetic cross between lab1 and wild-type strain CC620. B, PCR with specific primers amplifying the disrupted regions in LCIA or LCIB. C, Western immunoblots with specific LCIA or LCIB antibody. Wild-type cells and mutants lcia-63 and lcia90 were grown in high CO₂, and then shifted into very low CO₂ for 12 h prior to analysis. WT, Wild type.

Ci-dependent photosynthetic O₂ evolution in several LCIA mutant progeny was similar to that of wild-type cells at pH 7.3, as illustrated by the comparison between one LCIA mutant, lcia90, and its wild-type parent CC620 (Fig. 6). Although the Ci-dependent photosynthetic activities of lcia90 and the wild type are very similar at pH 7.3, photosynthesis was significantly inhibited in lcia90 relative to the wild type at pH 9.0. Because almost all Ci at pH 9 is in the form of HCO₃⁻, decreased photosynthesis under these conditions is consistent with the mutation in LCIA causing a defect in active HCO₃⁻ uptake.

Figure 6.

Ci-dependent photosynthesis in the LCIA mutant. Ci-dependent photosynthetic O₂ evolution measured at pH 7.3 and 9.0. Strains CC620 (black squares) and lcia90 (white triangles) were grown in high CO₂ and then switched to very low CO₂ for 24 h before the measurement. The experiments showed similar results with two independent cultures, although results from only one experiment are shown. Each data point represents an average of three technical replicates, and the coefficient of variation for each data point, the ratio of the sd (n = 3) to the average, is less than 10% for all data. The O₂ evolution rates of CC620 and lcia90 at 4,000 µm Ci are as follows: in pH 7.3, CC620 (95 ± 2) and lcia90 (132 ± 4); and in pH 9.0, CC620 (87 ± 4) and lcia90 (112 ± 2).

DISCUSSION

We have demonstrated that two C. reinhardtii proteins, LCIA and LCIB, both contribute to CCM-mediated Ci assimilation in very low CO₂, exemplified by growth and photosynthesis of mutants defective in LCIB and LCIA. Whereas LCIB or LCIA single-gene mutants show no obvious growth defects in very low CO₂, combination of these mutations in the LCIA-LCIB double mutant results in a lethal phenotype under very low CO₂ conditions. This demonstrates that LCIB- and LCIA-associated Ci uptake systems act either complementarily or synergistically in Ci uptake. The effects of LCIA or LCIB single mutations and LCIA-LCIB double mutations on Ci-dependent photosynthesis at different pHs reveal that LCIA and LCIB play distinct roles in different active Ci uptake processes. Whereas LCIB appears involved in active CO₂ uptake, LCIA is more likely involved in a HCO₃⁻ transport pathway.

LCIB-Based CO₂ Uptake System

The essentiality of LCIB in CCM-mediated Ci accumulation was demonstrated by a number of earlier studies with LCIB mutants, and LCIB was previously proposed to function in direct Ci transport, Ci transport regulation, recapture of CO₂ escaping from the pyrenoid and/or the thylakoid lumen, or physically preventing CO₂ escape from pyrenoids (Spalding et al., 1983a; Miura et al., 2004; Wang and Spalding, 2006, 2014; Duanmu et al., 2009b; Yamano et al., 2010). Here we demonstrate that the absence of LCIB causes photosynthetic defects under low Ci conditions that correlate with CO₂ concentrations. At a slightly acidic pH (pH 6.0), where CO₂ comprises more than one-half of the total Ci, Ci-dependent photosynthetic O₂ evolution in the LCIB mutant ad1 was significantly inhibited at 10 to 100 µm Ci (approximately 7–70 µm CO₂), but at pH 7.3, where HCO₃⁻ dominates, photosynthetic O₂ evolution was not affected below 50 µm Ci (approximately 6 µm CO₂), but showed progressively greater inhibition at higher Ci concentrations. Yamano et al. (2010) reported an inhibition of photosynthetic O₂ evolution in the LCIB mutants above 200 µm Ci at pH 7.8 (approximately 7 µm CO₂), but no obvious inhibition was observed below 100 µm Ci. It is clear that the lack of LCIB in ad1 results in an apparent inhibition of O₂ evolution at similar CO₂ concentrations regardless of the pH, and that the lower limit of those CO₂ concentrations in all cases (6–7 µm or approximately 0.02%; Fig. 1C) is very similar to the gas-phase CO₂ concentration of 0.02% that we previously defined as the upper limit of the very low CO₂ acclimation state based on physiological characteristics in a wild-type strain and on the air dier phenotype (Vance and Spalding, 2005; Wang and Spalding, 2014). Below this CO₂ concentration, LCIB mutants grow, and above this concentration, up to at least air CO₂ level (0.03%–0.05% CO₂), LCIB mutants cannot survive.

When CO₂ significantly exceeds air CO₂ level, CO₂ diffusion alone may be enough to allow the mutants to survive. The CO₂ concentration at which LCIB mutants regained wild-type O₂ evolution rates at pH 6.0 (Fig. 1A; 200 µm Ci, corresponding to 140 µm or 0.42% CO₂,) is also close to the Ci concentration at pH 7.8 where cells were reported by Yamano et al. (2010) to regain wild-type photosynthetic rates (4500 µm Ci at pH 7.8, corresponding to 170 µm or 0.5% CO₂). This CO₂ concentration is also close to the upper limit of the low CO₂ acclimation state according to the previous physiological study with wild-type cells by Vance and Spalding (2005). On the basis of these data, LCIB likely plays a major role in the CCM via active CO₂ uptake, especially in the range of low CO₂ (0.02%–0.5%), although the underlying biochemical mechanism is not clear.

We recently proposed that LCIB may function similarly to the cyanobacterial ChpX/Y proteins in converting CO₂ to HCO₃⁻ (Duanmu et al., 2009b; Wang and Spalding, 2014). ChpX/Y proteins in cyanobacteria are proposed to convert CO₂ to HCO₃⁻ unidirectionally, in contrast with the bidirectional activity of CAs, via their linkage to NADH dehydrogenase complexes (Price et al., 2002). If LCIB does function in this manner, it should also accelerate HCO₃⁻ accumulation by hydrating externally diffused CO₂ in the chloroplast stroma, which has an alkaline pH in light. Although this proposed active role of LCIB in CO₂ uptake into the accumulated HCO₃⁻ pool is consistent with and can explain the observed physiological characteristics of LCIB mutants, LCIB/CAH3 double mutants (Duanmu et al., 2009b), and LCIB/LCIA double mutants, it is still speculative and needs to be rigorously tested at the biochemical level.

Function of LCIA in Ci Uptake

In contrast with the photosynthetic effect of LCIB mutations, the impact of an LCIA mutation, either alone or in combination with an LCIB mutation, on photosynthetic O₂ evolution is more significant at higher pH, indicating that LCIA is likely involved in HCO₃⁻ uptake. LCIA has a predicted chloroplast envelope location (Miura et al., 2004), which we confirmed by immunolocalization (see Fig. 8A). Although active Ci uptake across the chloroplast envelope has been demonstrated (Sültemeyer et al., 1988; Moroney and Mason, 1991; Amoroso et al., 1998) and LCIA was previously identified as a possible chloroplast envelope Ci transporter (Miura et al., 2004; Mariscal et al., 2006; Duanmu et al., 2009a), no clear, direct evidence links LCIA to active, chloroplast envelope Ci uptake. In addition to the lack of direct evidence for Ci transport by LCIA, other potential Ci transporters, such as CCP1 and CCP2, are also present on the chloroplast envelope (Ramazanov et al., 1993). In addition, chloroplast Ci uptake also might be explained by the activity of LCIB.

Figure 8.

Models for acclimation to low and very low CO₂ in C. reinhardtii. A and B, Immunolocalization of LCIA and LCIB and working models illustrating possible roles played by the LCIA-associated HCO₃⁻ uptake system and the LCIB-associated CO₂ uptake system in very low CO₂ grown cells (A) and low CO₂ grown cells (B). No obvious changes of LCIA localization were observed between low and very low CO₂ conditions. Dashed lines represent the hypothesized function of the LCIB-LCIC complex in conversion of CO₂ to HCO₃⁻, which is then delivered to CAH3 within the lumen of the pyrenoid-associated thylakoid tubules for dehydration to CO₂. A detailed explanation is provided in “Discussion.” ChE, Chloroplast envelope; PM, plasma membrane.

LCIA belongs to an FNT family (Mariscal et al., 2006). Multiple recent studies have shown that the bacterial FNT proteins form a pentameric complex with a structure similar to aquaporin and behave more like channels than active transporters (Wang et al., 2009; Lü et al., 2012a, 2013). Nonselective permeability of FNT proteins to several structurally similar small, anionic molecules also has been demonstrated in artificial lipid membranes (Lü et al., 2012b). It seems improbable that LCIA would function as a HCO₃⁻ channel in the chloroplast envelope, because the membrane potential of the chloroplast envelope is inside negative (Demmig and Gimmler, 1983; Wu et al., 1991; Fuks and Homblé, 1999), meaning that HCO₃⁻ (or any anion) would have to move against an electrical gradient to enter the chloroplast. However, if LCIA does function as a channel to facilitate passive entry of HCO₃⁻ into the chloroplast, it would have to depend on the activity of plasma membrane Ci transporters (e.g. HLA3 and LCI1; Duanmu et al., 2009a; Ohnishi et al., 2010) to generate a sufficiently high HCO₃⁻ concentration in the relatively small volume between the plasma membrane and chloroplast envelope to establish a HCO₃⁻ concentration gradient sufficient to overcome the electrical gradient and push HCO₃⁻ across the inner envelope. An LCIA channel would also have to be highly regulated to prevent reversed flow when the HCO₃⁻ gradient was not favorable for uptake. Ci uptake studies with isolated chloroplasts from LCIA mutants and LCIA overexpression lines will help to clarify whether LCIA is directly responsible for HCO₃⁻ uptake into chloroplasts and, if so, whether it performs as an active Ci transporter or as an anion channel. Future biochemical, biophysical, and structural characterizations of LCIA will also be required to clarify the molecular mechanism underlying LCIA-associated Ci uptake.

CO₂ Inhibition of LCIA-Mediated Ci Uptake

As shown in Figure 1B, the photosynthetic O₂ evolution in an LCIB mutant progressively decreased as the Ci concentration increased beyond 50 µm at pH 7.3. The O₂ evolution rate of ad1 at 200 µm Ci (approximately 22 µm CO₂) was significantly reduced compared with that at 50 µm at this pH. Yamano et al. (2010) reported similar responses at a higher pH (pH 7.8) in LCIB mutants, in which the mutants showed O₂ evolution rates similar to those of the wild type at 0 to 100 µm Ci, but reduced rates at higher Ci concentrations. The most significant inhibition occurred at 500 to 700 µm Ci (approximately 19–27 µm CO₂). In these higher pHs (pH 7.3 or pH 7.8), the dominant Ci species is HCO₃⁻, and the reduced activity at higher Ci concentrations cannot be simply explained by a defect in CO₂ uptake caused by an LCIB mutation, because the Ci-dependent photosynthetic rates in these mutants should then at least remain constant, if not increase, with increased Ci concentrations. Because it seems clear that an LCIA-associated system contributes significant Ci uptake in cells lacking LCIB, this observed Ci inhibition of photosynthetic O₂ evolution must represent inhibition of LCIA-associated Ci uptake activity.

We reasoned that if LCIA-mediated Ci uptake activity is inhibited by Ci, this should be evident in the calculated difference in photosynthetic activity between ad1and lab1, which should represent the contribution of LCIA to the photosynthetic performance of ad1. When we calculate this LCIA contribution from O₂ evolution data obtained at pH 6.0 and pH 7.3 and plot it as a function of calculated CO₂ and HCO₃⁻ concentrations (Fig. 7, A and B), it becomes evident that Ci inhibition of LCIA-associated Ci uptake activity responds to CO₂ rather than to either HCO₃⁻ or total Ci concentrations. Furthermore, the apparent minimum inhibitory CO₂ concentration occurs just above that defined as very low CO₂ (approximately 7 µm CO₂ at room temperature), and becomes more significant at a CO₂ concentration range equivalent to air level (approximately 12 µm CO₂ at room temperature) and above, implying that air level or low CO₂ inhibits the LCIA-associated Ci uptake system.

Figure 7.

Contributions of LCIA and LCIB to O₂ evolution and CO₂ inhibition of LCIA-dependent photosynthesis. A and B, Contributions to O₂ evolution activities of LCIA and LCIB. Contribution of LCIA to O₂ evolution activities in ad1 as a function of CO₂ (A) or HCO₃⁻ (B) concentration at pH 6.0 (white squares) and pH 7.3 (black diamonds). The LCIA contribution was calculated by subtracting photosynthetic O₂ evolution activity in lab1 from that in ad1. C, Contributions of LCIA and LCIB to photosynthesis as a function of the CO₂ concentration. The contribution of LCIA (black circles) was calculated as above, and the contribution of LCIB (black triangles) was calculated by subtracting photosynthetic O₂ evolution activity in ad1 from that in wild-type (cw10) cells. In all cases, CO₂ and HCO₃⁻ concentrations are calculated from total Ci concentration and pH, assuming that the addition of CA maintains CO₂-HCO₃⁻ equilibrium.

The Ci response of the LCIB mutant at lower pH (pH 6.0) is different from that at higher pH, in that the photosynthetic O₂ evolution rates of ad1 are inhibited relative to the wild type at pH 6.0 but plateau rather than decreasing with increasing CO₂ concentration, as observed at pH 7.3 (Fig. 1). In this CO₂ concentration range, the rate of O₂ evolution is much higher at pH 7.3 than that at pH 6.0 (Fig. 1B), possibly because the HCO₃⁻ concentration is higher at pH 7.3 and can support a much higher rate of HCO₃⁻ transport. Therefore, the inhibition of this higher HCO₃⁻ transport rate at pH 7.3 has a greater impact on the overall rate of O₂ evolution, resulting in a quantitative decrease in the O₂ evolution rate. The observation that the rate of O₂ evolution at about 20 µm CO₂ is the same at pH 6.0 and pH 7.3 supports this interpretation.

The inhibition of LCIA activity by air level of CO₂ could explain the puzzling air dier growth phenotype of LCIB mutants, because HCO₃⁻ uptake associated with LCIA (and other Ci transporters) should otherwise allow LCIB mutants to survive in low CO₂ as it does in very low CO₂. This CO₂ inhibition appears to occur very quickly, because it can be seen almost immediately after Ci concentrations exceed the inhibitory CO₂ concentration in photosynthetic O₂ evolution measurements. If CO₂ is responsible for the apparent inhibition of LCIA activity, this inhibition could be caused directly by CO₂ itself or through an unidentified CO₂ sensor protein, and this inhibition might extend to other target proteins, such as other Ci transporters. If this inhibition occurs and impacts survivability of LCIB mutants in air CO₂ level (i.e. is responsible for the air dier phenotype), it must take place at the post-translational or allosteric level, because it occurs so rapidly, and because no significant changes beyond those detected in low CO₂ conditions have been detected for LCIA or other potential Ci transporter genes at the transcriptional level under very low CO₂ conditions, (Brueggeman et al., 2012; Fang et al., 2012).

We can use logic similar to that we used to calculate the contribution of LCIA to ad1, and plot the apparent contribution of LCIB to photosynthetic O₂ evolution of the wild-type strain cw10 (i.e. subtract the O₂ evolution rate of ad1 from that of cw10). If we then compare that calculated LCIB contribution with the apparent contribution of LCIA to ad1 (Fig. 7C), it becomes evident that the apparent LCIA contribution declines at CO₂ concentrations where the apparent LCIB contribution becomes substantial. This relationship would explain why LCIA and LCIB both contribute to growth in very low CO₂ but only LCIB appears essential for growth in low CO₂ (e.g. air level of CO₂).

Distinct Roles Played by LCIA and LCIB in Low CO₂ and Very Low CO₂ Acclimation States

Because of slow diffusion of CO₂ in water, Ci concentration can often change dramatically on a daily basis in an aquatic environment, especially in the natural soil environment of C. reinhardtii when cell population increases and photosynthesis activities are high. The presence of multiple limiting CO₂ acclimation states in microalgae may reflect an evolutionary adaptation to this aspect of their natural habitats. The apparent inhibition of LCIA activity by CO₂ concentrations at and above air level, together with the subcellular localizations of and apparent complementary roles for LCIA and LCIB (Fig. 8), suggest a possible explanation for how and why C. reinhardtii acclimates to different limiting CO₂ conditions. Figure 8 illustrates a proposed working model that could explain the functions of LCIA and LCIB in the C. reinhardtii CCM in these two acclimation states.

In this hypothetical model, at very low CO₂ (Fig. 8A), both LCIB and LCIA contribute to Ci accumulation. An LCIA-associated HCO₃⁻ transport pathway is largely responsible for HCO₃⁻ uptake into the chloroplast, possibly in concert with other HCO₃⁻ transporter(s) on the plasma membrane and/or the chloroplast envelope. LCIB, located around the pyrenoid, traps CO₂, either escaping from the pyrenoid or entering from outside the cell, into the stromal HCO₃⁻ pool. This stromal HCO₃⁻ pool then provides substrate to the pyrenoid thylakoid tubules for CAH3-catalyzed dehydration to CO₂ for Rubisco, as described in previous CCM models (Spalding et al., 1983b; Mitra et al., 2005; Spalding, 2008; Moroney et al., 2011).

In low CO₂ (Fig. 8B), the majority of LCIB protein complex in chloroplasts are located throughout the stromal space. In this condition, the hypothetical model predicts that LCIB-associated CO₂ uptake into the stromal HCO₃⁻ pool plays a more dominant role, and Ci uptake associated with LCIA and other HCO₃⁻ transporters is inhibited by CO₂ in the low CO₂ concentration range. Again, the stromal HCO₃⁻ accumulated through LCIB activity provides substrate to the pyrenoid thylakoid tubules for CAH3-catalyzed dehydration to CO₂ for Rubisco as described in previous CCM models.

Assuming that the LCIB-mediated CO₂ uptake system provides an adequate supply of substrate CO₂ for Rubisco in the low CO₂ range and that simple CO₂ diffusion can do so at higher CO₂ concentrations, such CO₂ inhibition of HCO₃⁻ transport at air concentrations and above may reflect a versatile regulatory mechanism present in eukaryotic algae for acclimating quickly to changes in CO₂ availability that frequently occur in their natural environments. By switching rapidly from energy-intensive HCO₃⁻ transport systems to a CO₂ uptake system (possibly energetically less costly) when CO₂ becomes more abundant, algae cells can divert more solar energy for photosynthetic carbon fixation and other metabolic reactions to enable faster growth at a lower energy cost.

The well-documented regulation by transcript abundance should prevent even the synthesis of the HCO₃⁻ transporters under high CO₂ conditions, in which the transporters clearly are not needed. However, in lower CO₂ concentration ranges, such as 5 to 50 µm (0.015%–0.15%) CO₂, it may be advantageous for the cells to have the capacity to respond quickly to either activate or inactivate HCO₃⁻ transport in response to CO₂ availability. Fluctuations in CO₂ concentration near the surface of the natural soil environment of C. reinhardtii may occur over time frames too short for transcriptional regulation to act effectively, but the post-translational or allosteric regulation proposed here would allow HCO₃⁻ transporters to shift rapidly between active and inactive states in response to CO₂ concentration fluctuations. Further research will be required to determine whether this rapid regulation actually occurs via CO₂ inhibition of the CCM HCO₃⁻ transporter pathway.

MATERIALS AND METHODS

Cultures and Growth Conditions

The wild-type strain cw10 (CC849) and CC620 and the mutant strains of Chlamydomonas reinhardtii were maintained and grown as previously described (Wang and Spalding, 2006). For inducing very low CO₂ acclimation, liquid cell cultures were first grown in minimal medium (pH 7.3) to late log phase in high CO₂ gas (5% CO₂ [v/v] in air), and then switched to very low CO₂ by bubbling very low CO₂ gas though the liquid phase for 20 to 24 h. The very low CO₂ (0.008%–0.01%) gas was obtained by passing normal air through a saturated sodium hydroxide solution and remixing with normal air to achieve the desired final concentration.

Isolation of Mutants, Identification of Mutation, and Genetic Analysis

Spot growth testing, mutant screening, and transformation were performed as previously described (Wang and Spalding, 2006). Strain ad1 was transformed with linearized pSI103d plasmid (Sizova et al., 2001). Transformed cells were kept in high CO₂ and selected on minimal medium plates supplemented with 10 μg mL⁻¹ paromomycin. Paromomycin-resistant transformants were transferred to duplicate plates for screening by growth spot tests in high CO₂ and very low CO₂.

The genomic DNA fragments flanking the insertion in mutants were identified by the restriction enzyme site-directed amplification PCR (González-Ballester et al., 2005). The primary PCR was carried out in a volume of 25 µL containing 2.5 µL of 10× PCR buffer (Invitrogen), 0.25 µL of Taq polymerase (Invitrogen), 1 µL of 25 mm MgCl₂, 0.4 µL of 10 mm deoxynucleotides, 0.75 µL of dimethyl sulfoxide, 5 pmol of primer RB1, 15 pmol of degenerate primers (González-Ballester et al., 2005), and 50 ng of genomic DNA as the DNA template. The secondary PCR was carried out in a volume of 25 µL with the same reagents as the primary RCR except 5 pmol of Q₀ primer was used to replace the degenerate primers and 1 µL of 25:1 diluted PCR product from the primary reaction was used as the DNA template. The parameters for the PCR cycle were set up according to Dent et al. (2005).

The genetic crosses between lab1 and CC620 were performed according to Harris (2009). The progeny bearing both Ble and Aph8 were selected on agar plates made from Tris-acetate-phosphate medium supplemented with 10 µg mL⁻¹ paromomycin and 10 µg mL⁻¹ zeocin. The progeny bearing only Aph8 was selected on agar medium supplemented with 10 µg mL⁻¹ paromomycin, and then their zeo^R was tested on plates supplemented with 10 µg mL⁻¹ of zeocin. PCR amplification with primers specific for LCIB or LCIA was used to confirm the defects in LCIA or LCIB in these progeny. To complement lab1, a genomic DNA fragment including the LCIA coding sequence, putative promoter region, and 3′ UTR was cloned into pBluescript vector, and used to transform lab1. The successful transformants were directly selected in very low CO₂ on minimal medium plates and untransformed lab1 was used as the control. The genomic DNA from the transformants was isolated and PCR amplified with specific primers in the region of LCIA where the insertion is located.

Measurements of Photosynthetic O₂ Evolution

Photosynthetic O₂ evolution was measured at 25°C with a Clark-type oxygen electrode controlled by an Oxy-Lab unit (Hansatech). Cells from liquid cultures were collected by centrifugation and suspended in 4 mL of N₂-saturated buffers to a final chlorophyll concentration of 20 μg mL⁻¹. The buffers used were MOPS-Tris (25 mm, pH 7.3), MES-KOH (25 mm, pH 6.0), or AMPSO (25 mm, pH 9.0). For each cell preparation, internal and external Ci were depleted under illumination (500 μmol photons m⁻² s⁻¹) as judged by cessation of O₂ evolution before each measurement. The Ci-dependent O₂ evolution was initiated by addition of various concentrations of NaHCO_3. For most measurements, the subsequent addition of Ci was delayed until the previous O₂ evolution rate approached zero. When O₂ evolution rates were extremely slow, such as in high pH or in lab1 mutants, the subsequent Ci was added to a desired concentration after previous O₂ evolution proceeded at least 1 min or longer; and the Ci concentration at each next addition was calculated based on the amount of Ci consumed from the previous Ci addition as reflected by the amount of O₂ evolved. Because O₂, when reaching high levels, appears to have negative influence on photosynthesis, the O₂ level in cell suspension was kept low by flushing with N₂ gas when it exceeded 200 nmol mL⁻¹. Two hundred units per milliliter of bovine CA (C2624; Sigma) was present in the measurements at pH 6.0 and 7.3 to ensure the equilibrium of CO₂ and HCO₃⁻.

Western-Blot Analysis and Immunolocalization

For protein analyses, harvested cells were directly dissolved in a 1× SDS-PAGE buffer in the presence of 2-mercaptoethanol at 80°C for 5 min. The resulting lysates were passed through a 26-gauge needle to reduce viscosity and then separated on 12% (v/v) SDS-polyacrylamide gels. Immunoblotting with specific antibodies was performed to detect the specific proteins by chemiluminescence (SuperSignal Wester Pico; Thermo Scientific).

To generate LCIA-specific antibodies, five LCIA complementary DNA fragments encoding different predicted soluble portions in LCIA protein were PCR amplified from a complementary DNA library (Wang and Spalding, 2006) and combined by recombination PCR. The resulting mini-LCIA sequence was cloned into pET-28a vector, and transformed into BL21 (DE3) Escherichia coli cells for overexpression. The expressed mini-LCIA was purified from E. coli and used to raise LCIA polyclonal antiserum. An LCIA monoclonal antibody against a synthetic peptide ENAINVGAYK was also generated (Abmart). LCIB antiserum against purified LCIB was previously described (Duanmu et al., 2009b). LCI1 antiserum was a gift from James V. Moroney (Louisiana State University).

For immunolocalization, cell suspension was placed on precharged microscope slides (ProbeOn Plus; FisherBiotech) for 1 to 3 min, and then quickly fixed in −20°C methanol for 10 min. Immunofluorescence staining was performed according to Cole et al. (1998). Antiserum against LCIA or LCIB was used at a dilution of 1:1,000 as primary antibody, and fluorescein isothiocyanate-conjugated goat anti-rabbit IgG or goat anti-mouse IgG (Jackson ImmunoResearch Laboratories) was used at a dilution of 1:150 as the secondary antibody for immunofluorescence. After final washing with phosphate-buffered saline, the slides were mounted using ProLong Gold reagents (Invitrogen), and digital images of stained cells were acquired with a Leica SP5 X MP confocal microscope.

Glossary

CCM

CO₂-concentrating mechanism

Ci

inorganic carbon

CA

carbonic anhydrase

para^R

paromomycin resistance

UTR

untranslated region

zeo^R

zeocin resistance