Limiting factors in the operation of photosystems I and II in cyanobacteria

Abstract

Cyanobacteria are important targets for biotechnological applications due to their ability to grow in a wide variety of environments, rapid growth rates, and tractable genetic systems. They and their bioproducts can be used as bioplastics, biofertilizers, and in carbon capture and produce important secondary metabolites that can be used as pharmaceuticals. However, the photosynthetic process in cyanobacteria can be limited by a wide variety of environmental factors such as light intensity and wavelength, exposure to UV light, nutrient limitation, temperature, and salinity. Carefully considering these limitations, modifying the environment, and/or selecting cyanobacterial species will allow cyanobacteria to be used in biotechnological applications.

Cyanobacteria are attractive targets for many biotechnological applications including antibiotics, fertilizers, biofuels, and bioplastics. In this review, we discuss the photosynthetic process in cyanobacteria and summarize the major abiotic factors that limit or stress this activity. We highlight how these factors impact the potential use of cyanobacteria in biotechnology. Created with BioRender.com.

PHOTOSYNTHESIS IN CYANOBACTERIA

Cyanobacteria are attractive targets for biotechnological applications because they are globally distributed and survive in a wide variety of environments including the nutrient‐limited open ocean, hot springs, deserts, and ice‐covered lakes in polar regions. Cyanobacteria also typically have higher photosynthetic efficiency, biomass production, and growth rates than plants (Melis, 2009). Furthermore, many cyanobacteria have tractable genetic systems that allow for genetic engineering (Behler et al., 2018) and advances in genetic engineering in these systems promise to improve their ability to be used in biotechnology (Hitchcock et al., 2019). Cyanobacteria are promising targets to produce many secondary metabolites (reviewed in Abed et al., 2009; Kumar et al., 2019), including those with antibacterial, antifungal, or anti‐phytopathogenic properties (e.g. Ghareeb et al., 2022; Jaki et al., 2000; Kajiyama et al., 1998). Cyanobacteria are also a good source of biofuels and bioplastics in the form of linear polyesters of polyhydroxyalkanoates (PHAs) (Agarwal et al., 2022; Steinbüchel et al., 1997). Cyanobacteria have other desirable applications such as fertilizers (Vaishampayan et al., 2001; Peng & Bruns, 2019), soil stability (Singh et al., 2011), colourants (Pandey et al., 2013), and carbon capture (Jansson & Northern, 2010).

Photosynthesis in cyanobacteria requires effective light harvesting and the transfer of high‐energy electrons through two photosystems (PSII and PSI) for carbon fixation. This activity is most often limited by light, CO₂, or RuBisCO (D‐ribulose‐1,5‐bisphosphate carboxylase/oxygenase) activity (Ducat & Silver, 2012). While carbon fixed via the Calvin–Benson–Bassham (Calvin) cycle is channelled to products of interest (e.g. Ducat et al., 2012), photon capture and the efficient conversion of light to chemical energy remain challenging for biotechnological applications. Indeed, light‐to‐biomass conversion efficiency in cyanobacteria is <10% (Blankenship et al., 2011). As a result, efforts to use cyanobacteria for biotechnological applications have focused on expanding the range of light captured, decreasing the detrimental impacts of photodamage, optimizing electron transport, and increasing carbon fixation (Durall & Lindbald, 2015; Luan & Lu, 2018). This review provides a brief overview of oxygenic photosynthesis in cyanobacteria. It summarizes the major abiotic factors that limit this activity and thus impacts the potential use of these bacteria in biotechnology (summarized in Table 1).

TABLE 1.

Examples of inhibitors of cyanobacterial photosynthesis and the response.

Inhibitor	Mechanism for inhibition	Cyanobacterial response
Light
Low or far‐red light	Non‐low‐energy photons are limited, reducing the potential growth rate	Produce photoacclimated photosystems that can capture long‐wavelength photons
High light	High‐energy photons and the generation of free radicals damage PSII	Decrease light captured by reducing the production of PSII. Increase energy‐consuming processes
Blue light	High PSI activity, low PSII activity	Alter the phycobilisome core or generate orange carotenoid proteins to decrease the absorbance of blue light
UV light	UV light breaks phycobilisomes, bleaches pigments, and damages PSII. Damages DNA leading to cell death	Generate compounds that intercept UV radiation before it interacts with the photosystems or other sensitive portions of the cell
Nutrient limitation
Nitrogen limitation	Photodamage, oxidative stress	Decrease electron flow and degrade PBS
Iron	Oxidative stress and lack of Fe prevent the formation of functional PS components	Reorganization of PSI, reduction of chlorophyll and PBS, replacement of iron‐containing proteins, IsiA antenna rings to protect from oxidative damage
Manganese	Lack of Mn prevents the formation of functional PSII	Generation of PSI monomers
Temperature
High temperature	PSII electron transport inhibited, Mn released from oxygen‐evolving complex, conformational changes in D1 and D2 proteins	PsbU and PsbV protect PSII from heat inactivation and reverse heat‐induced changes to the acceptor side of PSII
Low temperature	Decreased membrane fluidity, osmotic stress, and cell lysis	Alteration of membrane lipids, generation of EPS, disruption of ice formation
Salinity	Inhibition of electron transport on the acceptor (and potentially donor) side of PSII, ROS accumulation	Decreased PSI electron transport

PSII and PSI form supercomplexes with light‐harvesting proteins. They have a core complex and an associated antennae system. The PSI supercomplex also includes soluble electron donors and acceptors. Both photosystems bind chlorophylls to sense light, use chlorophylls and other pigments to transfer light to a photosynthetic reaction centre, and excite chlorophylls to initiate proton translocation. During oxygenic photosynthesis, electrons from the photo‐oxidation of water in PSII are transferred through two plastoquinone to cytochrome b ₆ f and then to soluble electron carriers such as plastocyanin or cytochrome c ₅₅₃ and onto PSI (Shevela et al., 2023). These electrons are eventually transferred to ferredoxin and NADP⁺ to form NADPH. NADPH and ATP are produced from a proton gradient formed coincident with the electron transfer reactions and fuel downstream processes, including carbon fixation and other assimilatory processes. This process is summarized in Figure 1.

FIGURE 1.

Components of photosynthesis and simplified electron transport in oxygenic photosynthesis. Multiple arrows from ferredoxin (Fd) indicate potential electron transfer to many reactions and processes. Calvin–Benson–Bassham (CBB) cycle; cyclic electron transfer (CET); ferredoxin (Fd); Fd‐NAD(P)H‐ oxidoreductase (FNR); oxygen‐evolving complex (OEC); NADH dehydrogenase‐like complex 1 (NDH‐1): PBS, phycobilisome (PBS); plastoquinone (PQ); plastocyanin (PC).

Light harvesting at phycobilisomes

Cyanobacteria capture light energy using three types of pigments: phycobiliproteins, chlorophylls, and carotenoids (Maresca et al., 2008). Photons are captured at specialized structures called phycobilisomes. These structures contain bundles of light‐harvesting pigments called phycobiliproteins connected to a core by linker proteins and anchored to the thylakoid membrane. Four types of phycobiliproteins capture light energy and transfer it to the photosystems: allophycocyanin (AP, λ_max ∼650 nm), phycocyanin (PC, λ_max ∼620 nm), phycoerythrin (PE, λ_max ∼560 nm), and phycoerythrocyanin (λ_max ∼570 nm) (Bryant & Canniffe, 2018; Glazer, 1989; Sidler, 1994). The spectral properties of phycobiliproteins depend on the number and different types of bilins (linear tetrapyrrole molecules) they bind to (Soulier & Bryant, 2023). Cyanobacteria can tailor the composition of the phycobilisomes to best suit their light environment in a process called photoacclimation (reviewed in Bogorad, 1975; Bryant & Cohen‐Bazire, 1981; Grossman, 1990; Tandeau de Marsac, 1983; Tandeau de Marsac et al., 1988; Sanfilippo et al., 2019).

Cyanobacteria also use chlorophyll to absorb and transfer light energy and for photoprotection (Frank & Cogdell, 1996; Frank & Brudvig, 2004; Fraser et al., 2001). Cyanobacteria synthesize chlorophyll Chls a, b, d, and f and divinyl‐Chls a and b (Maresca et al., 2008). Like photosynthetic eukaryotes, cyanobacteria primarily use Chl a, which absorbs most strongly in the blue and red regions of visible light, while Chl d and Chl f enhance far‐red light (700–800 nm) absorbance (Gan et al., 2014; Gan & Bryant, 2015). These light‐harvesting mechanisms allow cyanobacteria to respond to and capture light between ~300 and ~ 800 nm (Ho et al., 2017).

PSII

PSII is a large homo‐dimeric multi‐subunit pigment–protein–cofactor complex located in the thylakoid membrane of cyanobacteria. The multi‐subunit complex of PSII includes the oxygen‐evolving complex (OEC), Mn₄CaO₅ clusters, where water is oxidized to produce electrons, protons, and O₂ and reducing equivalents. A functional PSII is comprised of 17 intrinsic, transmembrane protein subunits, a heterodimer of the D1 and D2 proteins, two chlorophyll‐binding antennas (CP47 and CP43) (CBAs), 13 low‐molecular‐weight protein subunits (PsbE, F, H, I, J, K, L, M, T, X, Y, Z, and 30), and three membrane‐extrinsic low‐molecular‐weight subunits (PsbO, U, and V), as well as numerous cofactors (Knoppová et al., 2022). D1 and D2 proteins form the core of PSII in cyanobacteria.

Oxygen‐evolving complex

During photosynthesis, electrons and protons are extracted from water at the OEC, a Mn₄CaO₅ cluster bound by the D1 protein in PSII (Kok et al., 1970; Lubitz et al., 2019). Four light‐activation steps are needed to oxidize water in a single S‐state cycle (Siegbahn & Crabtree, 1999; Vrettos & Brudvig, 2002). The electrons from water are used to reduce plastoquinone (PQ) to plastoquinol (PQH₂) (Vinyard & Brudvig, 2017), and the protons generate an electrochemical gradient (Iverson, 2006). The photosynthetic electron transport chain, via PQ, funnels electrons to the cytochrome b ₆ f complex and onto PSI.

PSI

PSI is a membrane chlorophyll–protein complex. PSI catalyses the light‐dependent oxidation of plastocyanin and the reduction of ferredoxin (or flavodoxin when iron is limited, Zhao et al., 1998). PSI generates ATP and reducing power (NADPH), which is required to convert light energy into chemical energy (Shevela et al., 2023). In PSI, the antenna absorbs light energy, and the excitation energy is transferred P700, a dimer of distinct Chl a molecules, leading to charge separation. Excited electrons are then transported through a series of acceptors and redox centres including a Chl a monomer, phylloquinone, and several [4Fe‐4S] clusters. The reaction centre of PSI uses light to catalyse the transfer of electrons from the plastocyanin or cytochrome c6 on the lumenal side to ferredoxin on the stromal side across the thylakoid membrane.

The monomer of PSI contains 11 protein subunits (PsaA, PsaB, PsaC, PsaD, PsaE, Psaf, PsaI, PsaJ, PsaK, PsaL, and PsaM) and ≈100 antenna chlorophyll molecules (Nagao et al., 2023). The PsaA and PsaB polypeptides of PSI contain several cofactors, including Chl P700, phylloquinone, and a [4Fe‐4S] cluster. These cofactors are essential for light absorption and energy and electron transfer from the PSI reaction centre to ferredoxin (Nagao et al., 2023). The organization of the PSI complex contributes to balancing energy distribution and electron flow between PSI and PSII (Karapetyan et al., 1999; Katayama, 2022).

Carbon fixation

Electrons from ferredoxin via Fd‐NAD(P)H‐ oxidoreductase (FNR) reduce NADP⁺ to NADPH, which can be used in biosynthesis, including carbon fixation. Cyanobacteria use RuBisCO for carbon fixation. RuBisCO cannot efficiently discriminate between O₂ and CO₂ and acts as a carboxylase and an oxygenase. The oxygenase side reaction of RuBisCO (photorespiration) produces the toxic compound 2‐phosphoglycolate, which must be recycled (Hagemann & Bauwe, 2016), ultimately releasing previously fixed CO₂, NH₃, and energy. Many aquatic environments are low in dissolved inorganic carbon. To overcome poor catalytic efficiency and low concentrations of inorganic carbon, cyanobacteria employ carbon concentrating mechanisms (CCMs) to increase the cellular CO₂ concentration and maximize the rate of carboxylation while competitively inhibiting the oxygenase side reaction (Price, 2011). CCMs enable relatively rapid CO₂ fixation by elevating intracellular inorganic carbon as bicarbonate (HCO₃ ⁻), then concentrating it as CO₂ (following conversion from HCO₃ ⁻ to CO₂ via carbonic anhydrase) around the RuBisCO enzyme in specialized protein micro‐compartments called carboxysomes (Rae et al., 2013).

Ferredoxin also donates electrons from photosynthesis to NADH dehydrogenase‐like complex 1 (NDH‐1) which functions in cyclic electron transport (CET) around PSI by donating electrons back to the PQ pool. CET, along with linear photosynthetic electron transport, helps generate a proton gradient across the thylakoid membrane for ATP synthesis and can increase ATP synthesis when cyanobacteria are exposed to environmental stress (Gao et al., 2016; Nixon & Mullineaux, 2001). CET also reduces the production of reactive oxygen species (ROS) and thus plays an important role in protecting components of photosynthesis from oxidative stress. Electrons from photosynthesis are also funnelled to multiple metabolic processes via ferredoxin (indicated by the multiple arrows from Fd in Figure 1).

MAJOR FACTORS AFFECTING PHOTOSYSTEMS

Light intensity and wavelength

Photosynthesis relies on the efficient use of light. Cyanobacteria have diverse mechanisms that facilitate responding to changing light conditions, particularly light between 300 and 800 nm. These mechanisms include complementary chromatic acclimation (CCA) where the composition of phycobiliproteins in the phycobiolisome changes based on wavelength of light (reviewed in Ho et al., 2017), altering the stoichiometry of PSII: PSI (Fujita et al., 1985; Myers et al., 1980), changing the cellular abundance of photosystem components and chlorophyll (Raps et al., 1983), state transitions that alter the association of PBS with PSII and PSI (Liu et al., 2013), circadian rhythm (reviewed in Piechura et al., 2017), phototaxis (Bhaya, 2004), morphology (reviewed in Montgomery, 2015), and quenching excess light using an orange carotenoid protein (OCP) (Kirilovsky & Kerfeld, 2013) or flavodiiron proteins (Allahverdiyeva et al., 2013; Bersanini et al., 2017). Below, we discuss several examples of cyanobacterial responses to light conditions that can inhibit photosynthesis.

Modified photosynthetic complexes in response to low light or far‐red light

In response to far‐red light, some cyanobacteria can remodel PSII, PSI, and PBS to harvest longer wavelength light using Chl d, Chl f, and an altered form of allophycocyanin in a process known as far‐red light photoacclimation or FaRLiP (Gan et al., 2014; Gan & Bryant, 2015). The genomes of these cyanobacteria contain a cluster of genes that encode alternative PS and PBS components, expressed when cells are exposed to far‐red light. Isolates with the FaRLiP gene cluster can grow more efficiently at longer wavelengths than visible light wavelengths, suggesting these cyanobacteria are specifically adapted to far‐red light conditions. Evidence from Synechococcus spp. 7335 indicates that far‐red light acclimated cells retain complexes to harvest visible light, suggesting this organism can rapidly reverse acclimation from far‐red to visible light (Ho et al., 2020). Some cyanobacteria have a subset of the FaRLip genes and make a modified photosynthetic complex to grow under low‐light conditions (LoLiP, low‐light photoacclimation) (Nowack et al., 2015; Olsen et al., 2015). Efficiently harvesting far‐red or low light is likely most advantageous to cyanobacteria growing in mats, blooms, or shady locations in soils or soil crusts (Gan et al., 2014).

High light

High light can negatively impact cyanobacterial growth and photosynthetic potential. If the amount of light energy absorbed exceeds energy consumption, cyanobacteria tend to decrease transcription of PSI, PSII, and PBS – to harvest less light. At the same time, they increase processes that consume energy, like carbon fixation. However, the response to high‐light intensity is not the same in all cyanobacteria. For example, under high light, Synechocystis sp. PCC 6714 and PCC 6803 and Synechococcus sp. PCC 7942 decrease the ratio of PSI to PSII (Hihara et al., 1998; Murakami & Fujita, 1991; Samson et al., 1994; Sonoike et al., 2001), while the thermophilic isolate Synechococcus OS‐B′ increases the ratio of PSI to PSII (Kilian et al., 2007). A lower PSI:PSII ratio should decrease light‐harvesting capacity and minimize reactive oxygen intermediate formation (Hihara et al., 1998; Sonoike et al., 2001). In the hot spring strain OS‐B′, high light results in lower levels of phycobilisomes and chlorophyll (Kilian et al., 2007).

Photoinhibition is the balance between photodamage and the rate of PSII repair. Excess light energy impacts photosynthesis by inhibiting PSII repair, which environmental conditions, including low temperature and salinity, can further impair. PSII is particularly susceptible to damage due to highly oxidative redox conditions and the formation of ROS generated from oxygen reacting with the triplet state chlorophyll (Krieger‐Liszkay et al., 2008; MacPherson et al., 1993; Vass et al., 1992). The D1 and D2 proteins form a heterodimer in PSII and provide most of the ligands for redox‐active cofactors. D1, encoded by psbA, provides most of the ligands for the OEC. D1 is susceptible to photodamage and turns over more rapidly than other PSII components, particularly under high light, with new D1 proteins synthesized every 30–60 min (Mattoo et al., 1984; Ohad et al., 1984).

Many cyanobacterial genomes contain multiple copies of psbA and encode different forms of the D1 protein (Mulo et al., 2012). All genomes of photosynthetic cyanobacteria sequenced to date encode at least one copy of the most common form of D1 protein (D1:1 according to the classification in Cardona et al., 2015), which is also found in plants and algae. Containing multiple identical copies of D1:1 can be advantageous. For example, the negative impacts of photoinhibition can be diminished through increased transcription of identical psbA genes encoding D1:1, leading to more rapid replacement of D1 (El Bissati & Kirilovsky, 2001). Alternatively, some cyanobacteria have a D1:2 variant, which is less susceptible to photoinhibition (Vinyard et al., 2014). The D1:2 variant has a key glutamine to glutamate amino acid substitution. In Synechococcus sp. strain PCC 7942, the D1:2 variant was abundant at light intensities above 390 μE m⁻² s⁻¹, indicating this form is preferred under high‐light intensity (Schaefer & Golden, 1989), while in thermophile OS‐B′ genes encoding the D1 isoforms are regulated by high light (Kilian et al., 2007).

In contrast to PSII, which undergoes rapid turnover and repair when photoinhibited (Yao, Brune, Vavilin, & Vermaas, 2012; Yao, Brune, & Vermaas, 2012), similar repair mechanisms do not exist for PSI. Instead, high light can irreversibly damage PSI, and PSI must be resynthesized. PSI damage will block complete electron transfer (Sonoike, 2010), leading to the suggestion that PSII photoinhibition is ultimately to protect PSI (Barbato et al., 2020; Tikkanen et al., 2014). Other adaptations, including long‐wavelength chlorophylls and trimerization, might be important for harvesting light, particularly under fluctuating conditions, in PSI (Chitnis & Chitnis, 1993; Schlodder et al., 2005; Sener et al., 2004). The PSI in Cyanobacterium aponinum 0216, which lives in the very high‐light environment of the Sonoran Desert, is in the trimer form and has small structural variations that change spectroscopic properties of PSI compared to other cyanobacterial PSI (Dobson et al., 2021). These structural variants along with other adaptations including shifting the distribution of excitation energy from PSII to PSI indicate PSI plays an important role in high‐light conditions in this taxon. These characteristics, along with subunit composition variation in PSI (monomeric, trimeric, and tetrameric oligomers), could be useful for engineering PS for specific light environments (Dobson et al., 2021). Downstream of PSI, high light, CO₂, or RuBisCO activity can impact photosynthesis (Ducat & Silver, 2012). For example, under high‐light conditions (mid‐day), cells receive more light than can be used for photosynthesis. The limiting step is the turnover rate of NADPH – the primary way NADPH is consumed is by the CBB cycle during carbon fixation.

Blue light

Cyanobacteria photosynthesize at lower rates in the presence of blue light (≤450 nm) compared to longer wavelengths of light (Luimstra et al., 2018). Oxygen evolution under blue light can be as low as 30% of the maximum when grown under other light conditions (El Bissati et al., 2000). Low rates of oxygenic photosynthesis under blue light may be caused because it is most strongly absorbed by Chl a which is primarily present in PSI but it does not excite PBS in PSII. An imbalance of PSI and PSII was observed in Synechocystis sp. PCC 6803 when grown under blue light which is consistent with blue photons being absorbed by PSI and a photon shortage at PSII (Luimstra et al., 2018). Blue light inhibition decreases the photosynthetic rate and, thus, the rate of oxygen evolution and carbon fixation in cyanobacteria. Therefore, blue light inhibition may limit the use of cyanobacteria for carbon capture biotechnologies in environments where blue light predominates. In contrast, some taxa increase their growth rate by up to 80% when grown under blue LED light (Duarte & Costa, 2018), but this effect is species‐dependent (Mahari et al., 2024) suggesting the degree of blue light inhibition species‐specific.

Blue light induces reversible fluorescence quenching in cyanobacteria which is mediated either by the core of the phycobilisome or an OCP (Rakhimberdieva et al., 2004; Scott et al., 2006). The OCP binds to the phycobilisome core, preventing light energy from being transferred into the photosystems and converting it into heat (Kirilovsky & Kerfeld, 2012). This response also occurs under saturating blue‐green or white light.

UV radiation

The Earth's surface receives small amounts of ultraviolet radiation (UV; 280–400 nm) in the form of UV‐B (280–315 nm; <1% of total irradiance) and UV‐A (315–400 nm; <7%) (Kirk, 1994). Ultraviolet radiation can inhibit cyanobacterial growth. For example, over 90% of Cylindrospermopsis raciborskii cells died when exposed to UV‐A light, and UV‐B light killed almost 50% of cells (Noyma et al., 2015). In general, UV radiation, particularly UV‐B, causes damage to DNA and proteins and ROS production, leading to cell death (Vincent & Neale, 2000). In addition, UV radiation negatively impacts key photosynthesis components and their repair; UV radiation breaks down phycobilisomes, bleaches pigments, and damages the D1 protein in PSII (reviewed in Häder & Rastogi, 2022). Many cyanobacteria use UV‐absorbing pigments like mycosporine‐like amino acids (MAAs) and scytonemin to intercept UV radiation before it reaches biomolecules (Sinha et al., 2007). The photon energy absorbed by MAAs is dissipated as heat and does not generate ROS (Conde et al., 2007). Small picoplankton cyanobacteria use repair mechanisms, fast replication, and vertical migration to overcome the negative impacts of UV radiation.

While high levels of UV‐A radiation can be detrimental, low levels of UV‐A, particularly under low or fluctuating light conditions, can enhance photosynthesis or facilitate the repair of UV‐B DNA damage (Xue et al., 2005). UV‐A also increases biomass production in some cyanobacteria commonly used as food supplements, including Spirulina (Wu et al., 2005) and Nostoc (Chen et al., 2024), compared to cells grown without UV‐A. In Nostoc, UV‐A increased PSI abundance, which could increase photosynthetic efficiency by accelerating electron transfer from PSII. The UV‐A acclimated Nostoc cells also generated more NADPH and ATP, enhanced RuBisCO activity, and accumulated more carbohydrates than non‐acclimated cells (Chen et al., 2024). Therefore, when optimizing biomass generation, it is essential to consider the response to UV‐A exposure.

Nutrient limitation

As oxygenic photoautotrophs, cyanobacteria require light, inorganic carbon, and water as an electron donor. Nitrogen, phosphorus, and sulphur are crucial for optimal growth and photosynthetic activity. As phototrophs, cyanobacteria also have specific needs and responses to trace elements including iron, cobalt, copper, manganese, magnesium, molybdenum, and zinc. The photosynthetic apparatus contains many trace element cofactors including Fe‐S clusters, cytochromes, nonheme Fe, Cu in plastocyanin, Mg in chlorophylls, and Mn in the oxygen‐evolving complex. A major or trace nutrient deficiency may lead to lower growth rates, impaired photosynthetic capacity, and decreased productivity. When nutrients are available in sufficient supply, reducing potential from the photosynthetic electron transport chain is funnelled to anabolic reactions. When nutrients are limited, the rate of reoxidation of final electron acceptors slows, and so does electron transfer activity (Grossman et al., 1993; Schwarz & Grossman, 1998).

Without sufficient nutrients, cyanobacteria typically slow the rate of metabolic processes, as evidenced by the downregulation of many cellular processes (Karsikov et al., 2012; von Wobeser et al., 2011). Nutrient limitation, particularly of nitrogen and phosphorus, can lead to changes in the photosynthetic apparatus, the degradation of nitrogen‐rich phycoerythrin and phycocyanin from PBS, and ultimately decreased rates of photosynthesis (Allen & Smith, 1969; Grossman et al., 1993; Schwarz & Grossman, 1998 and Görl et al., 1998; Kiyota et al., 2014).

In cyanobacteria that cannot fix nitrogen, nitrogen limitation causes a decrease in anabolic reactions – those that consume ATP and reducing equivalents and an over‐reduction of photosynthetic electron carrier pools. This decrease requires cyanobacteria to decrease the electron flow through the photosynthetic components to avoid photodamage due to excess energy and reaction oxygen species (Salomon et al., 2013; Schwarz & Forchhammer, 2005). One typical response to nitrogen limitation is to degrade PBSs (Allen & Smith, 1969; Collier & Grossman, 1992; Grossman et al., 1993). The degradation of PBS decreases photon capture and, therefore, regulates the distribution of excitation energy between PSI and PSII (Calzadilla & Kirilovsky, 2020). Prochlorococcus marinus, an abundant low‐light adapted marine cyanobacteria, grows under constant nitrogen limitation. In this taxon, nitrogen starvation caused an increase in non‐functional PSII (including a lack of D1 protein synthesis and PSII repair) and a modified or altered acceptor side of PSII, resulting in decreased electron transfer efficiencies (Steglich et al., 2001). Similar responses have been observed under sulphur and phosphorus‐limiting conditions (Kharwar & Mishra, 2020). Phosphorus deficiency can also be accompanied by lowered ATP synthesis and NADP⁺/NADPH regeneration, leading to PSII damage.

Cyanobacteria require large amounts of iron for photosynthetic components, including electron transport, photosystems, removal of reaction nitrogen species, and synthesis of pigments. The demand for iron per cell is 10‐fold higher than for non‐phototrophs (Keren et al., 2004; Terry & Low, 1982). The linear electron transport chain in the photosynthetic apparatus requires ~20 iron atoms (Jia et al., 2021), and PSI has three [Fe₄S₄] clusters (Jordan et al., 2001; Keren et al., 2004). Although cyanobacteria can scavenge for iron in low‐iron environments using ferric iron chelators (e.g. siderophores) (reviewed in Qiu et al., 2022), they are still often iron limited. When iron is limited, cellular PBS content and pigment content decrease (Grossman et al., 1993; Ivanonv et al., 2000; Sandström et al., 2002; Schrader et al., 2011), and the number of thylakoid membranes decreases (Sherman & Sherman, 1983). These changes are typically accompanied by a downregulation in gene expression of all iron‐containing photosynthetic components, particularly PSI, resulting in an increase in the PSII:PSI ratio and an overall decrease in light‐dependent electron transfer and photosynthetic activity (Ivanonv et al., 2000; Sandmann, 1985).

Because iron is critical to photosynthesis and cyanobacteria often thrive in low‐iron environments (e.g. the open ocean), these organisms have several specific mechanisms for dealing with low‐iron concentrations. For example, when iron is limiting, cyanobacteria can compensate by replacing iron‐dependent proteins with analogues that do not require iron. For instance, Cyt c ₅₅₃ is replaced by plastocyanin, which is copper‐dependent, and iron stress‐induced (Isi) protein IsiB, an iron‐free flavodoxin, can serve as an electron carrier in place of ferredoxin (Hutber et al., 1977; Sandmann & Malkin, 1983; Straus, 1994). Several enzymes that detoxify ROS require iron as a cofactor (e.g. catalase, peroxidase, or some superoxide dismutases). Thus, iron limitation leads to a diminished ability to combat ROS (Gonzalez et al., 2018) and exacerbates oxidative stress. PSII is particularly susceptible to oxidative stress (Aro et al., 1993; Bhaya et al., 2000). Iron deficiency‐induced protein A (IdiA) is upregulated and associated with the acceptor side of PSII to protect against oxidative damage, particularly under mild iron limitation.

Iron limitation and/or oxidative stress also leads to the expression of IsiA. IsiA binds chlorophyll and can shield the photosynthetic apparatus from oxidative damage (reviewed by Chen et al., 2018). IsiA forms an antenna ring around PSI trimers when iron is limiting forming a PSI‐IsiA supercomplex (Bibby et al., 2001; Boekema et al., 2001). However, the specific role of IsiA is not yet fully understood. IsiA likely plays a role in storing Chl a, serving as an accessory antenna for PSI and dissipating light energy (Burnap et al., 1993; Park et al., 1999; Riethman & Sherman, 1988; Sandström et al., 2002). These functions are critical during iron limitation when combating the negative impacts of oxidative damage is essential.

Manganese is a critical component of the oxygen‐evolving complex. It is also a cofactor for other enzymes that catalyse reactions involving oxygen, including Mn superoxide dismutase, Mn peroxidase, and Mn‐catalase. Manganese is not typically limiting in most natural environments, but when limiting, it causes decreased oxygen evolution. Widespread degradation of pigments is not observed when Mn is limiting (Cheniae & Martin, 1967, 1969; Salomon & Keren, 2011) as with other nutrients (as discussed above). Decreased oxygen evolution is due to a lack of functional PSII – in Synechocystis sp. strain PCC 6803, Mn limitation causes the loss of PSI core proteins and the dissociation of PSI trimers to monomers (Salomon & Keren, 2011). These PSI monomers might facilitate direct energy transfer from PSII (McConnell et al., 2002) and help combat photooxidative damage to PSII under Mn limitation (Salomon & Keren, 2011).

Temperature

Cyanobacteria tend to adapt to local environmental conditions, including temperature and pH. Cyanobacteria maintain a constant internal pH of around 7.1–7.5 while thriving in environments where the pH ranges from ~5 to 10 (Ritchie, 1991). Cyanobacteria can acclimate to higher temperatures (Inoue et al., 2001), and two proteins, PsbU and PsbV, protect PSII from heat inactivation (Nishiyama et al., 1997, 1999). Temperatures that exceed optimal conditions negatively impact photosynthesis by inhibiting electron transport, presumably due to the alteration of PSII, and inhibit the oxygen‐evolving activity of PSII. Heat stress causes the release of Mn from the oxygen‐evolving complex, conformational changes in the D1 and D2 proteins, and reversible changes to the acceptor side of PSII that negatively impact electron transfer rates. At low temperatures, the repair cycle of D1 proteins is suppressed (Gombos et al., 1994; Kanervo et al., 1993). Thus, at both high and low temperatures, high light can cause damage to the PSII (Inoue et al., 2001). Under temperature fluctuations, cyanobacteria can use state transitions and alter the abundance of photosynthetic proteins to both keep PSII reaction centres more oxidized and minimize PSII photodamage. For example, in Synechococcus spp. near or at optimal growth temperatures, PBS associates with PSII and shuttles electrons through the electron transport chain, whereas when temperatures are lower, the PBS will associate with PSI to favour the oxidation of PSII and linear electron transport (Mackey et al., 2013).

Cold temperatures can also negatively influence photosynthesis. In near‐freezing temperatures, membrane fluidity decreases, and membranes become thin and more rigid. Decreased fluidity negatively influences electron transport in photosynthesis, but cyanobacteria can desaturate the lipids in their membranes, keeping them more fluid at low temperatures (reviewed in Los et al., 2013). Ice formation outside the cell can cause osmotic stress and cellular dehydration by increasing the concentration of solutes in the liquid water surrounding it because solutes are excluded from the ice (Vincent, 2007). Cyanobacteria can produce osmoregulatory proteins to mediate this stress (Papageorgiou & Murata, 1995). In freezing conditions, ice formation in the cell can break apart the membrane. Some polar cyanobacteria can produce macromolecular substances that disturb ice formation, protecting them from damage during freeze–thaw cycles (Raymond & Fritsen, 2004). Cyanobacteria can also produce exopolymeric substances (EPS), which protect against desiccation and freeze–thaw damage (Tamaru et al., 2005), potentially by protecting the membrane integrity (Knowles & Castenholz, 2008). Most cold‐adapted cyanobacteria species are not psychrophiles but are merely psychrotolerant (Tang et al., 1996). Many cold tolerance mechanisms also protect against desiccation or other stressors (Chrismas et al., 2016). Therefore, cold‐protection mechanisms may be broadly distributed rather than restricted to cold‐adapted species.

Salinity

High salinity inhibits photosynthetic activity in cyanobacteria (Allakhverdiev & Murata, 2008; Vonshak et al., 1988). Salt stress inhibits electron transport on the acceptor side of PSII but may also negatively impact the donor side of PSII (Lu & Vonshak, 2002). At the same time, salt can increase PSI electron transport activity (Zhang et al., 2010), presumably to protect PSII from excess excitation energy or to aid in maintaining osmotic balance (Allakhverdiev & Murata, 2008; Vonshak et al., 1995). The negative impacts of salt stress are more substantial at higher light intensities. High salt, particularly Na⁺ and Cl⁻ ions, can also lead to ROS accumulation, damaging photosystem proteins and components and negatively affecting photosynthesis (Allakhverdiev & Murata, 2008).

Case studies

Researchers and industry scientists must carefully consider and control the environmental conditions under which cyanobacteria are grown to effectively use them in biotechnological applications (Figure 2; Table 2). Because there is variation in how species respond to stressors (Tables 1 and 2), it is also essential to consider how individual species respond to a particular environmental stress. Below, we highlight an example application in which the environment should be engineered to cyanobacterial species used (biofuels and carbon capture) and one in which the environment cannot be easily controlled, and thus the proper strain should be selected (biofertilizers in agroecosystems).

FIGURE 2.

Decision and action flow chat for use of cyanobacteria in biotechnological applications.

TABLE 2.

Examples of biotechnological uses of cyanobacteria, the environmental stressors or sources of limitation, and the ways in which the environment or the physiology can be controlled for desirable products or functions.

Biotechnological use	Stressors or limitations	Environmental controls	Physiological controls
Carbon Capture	Carbon concentrating mechanism, buffering capacity of the environment, competition with carbon fixation	Concentration of Ca and CO2, buffering (reviewed in Jansson & Northern, 2010)	Species selection (e.g. halophilic cyanobacteria, mutants with high HCO3 − uptake)
Biofuels	Self‐shading	Design of bioreactor	Consortium of low‐ and high‐light species (Litchman, 2003)
Biofertilization	High light, low light, desiccation	Water content	Species selection (e.g. Anabaena or Nostoc spp., Peng & Bruns, 2019)
Bioremediation	Salinity, metals, light environment	pH	Species selection for salinity, pH, and metal‐tolerant taxa (e.g. Nostoc, Anabaena, Synechococcus, Synechocystis, Oscillatoria, Phormidium; reviewed in Chakdar et al., 2022)
Biodegradable plastics	Low production of desirable products, light, CO2	Light, CO2	Metabolic engineering and high‐density cultivation (Tan et al., 2022)
Pharmaceuticals	Species‐specific	Species‐specific	Species selection for specific secondary metabolites (Vijayakumar & Menakha, 2015)

Biofuels and carbon capture

Cyanobacteria grown for biofuels or carbon capture in bioreactors are subjected to a fluctuating light environment and self‐shading (Andersson et al., 2019), which can create low‐ or no‐light microenvironments that are not ideal for biomass production. Systems can be designed to be turbulent or flowing to allow better access to light for all individuals or can be engineered to have the highest possible surface area (e.g. flat or narrow glass bioreactors) to provide increased access to light. In non‐flowing or non‐turbulent systems, using a consortium of high‐ and low‐light‐adapted species may be helpful. Although the low‐light species are likely to grow slower and generate less biomass, they may increase the overall yield by increasing growth in the lower‐light areas of the bioreactor.

Biofertilizers in agroecosystems

Nitrogen is a limiting nutrient for many agroecosystems. Nitrogen‐fixing cyanobacteria can be used as biofertilizers to provide nitrogen to agroecosystems, reducing the need for fertilizers and the loss of nitrogen (Peng & Bruns, 2019; Vaishampayan et al., 2001) while also increasing soil stability (Singh et al., 2011). Cyanobacteria used as biofertilizers are likely to be exposed to high‐light conditions when on the surface of the soil and low‐light environments when shaded by crops. They are also expected to experience fluctuating water levels. Controlling the local environment in a crop field is not feasible but selecting strains of cyanobacteria that are both high‐light and desiccation resistant should result in nitrogen fixation, even under less favourable photosynthetic conditions. A combination of controlled environmental conditions, species selection, and potentially genetic engineering or targeted evolution can increase the odds of successfully using cyanobacteria as biofuels.

CONCLUSION AND PERSPECTIVES

Cyanobacteria are promising candidates for biotechnology in numerous applications, including carbon capture, biofuels, biofertilizers, bioremediation, and biodegradable plastics. However, they are also sensitive to multiple environmental stressors and even in cultivated ecosystems; these stressors can limit their growth, oxygen evolution, and photosynthetic rate, limiting their potential in biotechnological applications, particularly at commercial‐scale production. As illustrated by the examples above, cyanobacteria can be successfully deployed in biotechnological applications if the physiological capabilities are well suited to the environment or that the environment is engineered to remove stressors that hinder cyanobacterial growth. Genetic engineering might be able to target specific aspects of cyanobacterial growth (such as increasing growth rates) or physiology (e.g. high‐light tolerance) to a make a more desirable chassis for biotechnological applications, but these efforts will always have to carefully balance environmental characteristics to maximize the benefits of photoautotrophic growth in cyanobacteria.

AUTHOR CONTRIBUTIONS

Christen L. Grettenberger: Conceptualization; writing – original draft; writing – review and editing. Reda Abou‐Shanab: Writing – review and editing. Trinity L. Hamilton: Conceptualization; funding acquisition; visualization; project administration; supervision; resources.

CONFLICT OF INTEREST STATEMENT

The authors declare no conflict of interests.

Grettenberger, C.L. , Abou‐Shanab, R. & Hamilton, T.L. (2024) Limiting factors in the operation of photosystems I and II in cyanobacteria. Microbial Biotechnology, 17, e14519. Available from: 10.1111/1751-7915.14519