Light spectra influence biomass and phenolics while sustaining high fucoxanthin in the tropical indigenous diatom Thalassiosira weissflogii

Abstract

Microalgae are promising sustainable sources of bioactive compounds for food and pharmaceutical applications. In this study, the tropical indigenous diatom Thalassiosira weissflogii TRG10-P105 (TW P105) was investigated under various light spectra to evaluate biomass and metabolite productivity for industrial cultivation. Cultures were grown under white, red, blue, combined red–blue, and white supplemented with UV-A light, and assessed for growth, pigment composition, fucoxanthin (Fx), and total phenolic content (TPC). Broad-spectrum white light supported the highest growth (30.17 ± 2.06% d⁻¹), biomass productivity, chlorophylls, carotenoids, and TPC (8.63 ± 0.07 mg GAE g⁻¹ dw). Conversely, growth in the white-UV group was initially suppressed but resumed from day 5 onwards as UV-A was introduced, resulting in a specific growth rate (SGR) of 10.94 ± 3.96% d⁻¹, biomass accumulation of 0.017 ± 0.00 g L⁻¹, and productivity of 0.002 ± 0.000 g L⁻¹ d⁻¹. The reduced pigment content and TPC observed in the white–UV treatment suggest that the short UV-A exposure period (days 5–7) limited the induction of photoprotective and antioxidant responses. However, Fx levels remained consistently high across spectra (17.27–18.49 mg g⁻¹ dw), indicating that TW P105 maintains near-maximal Fx accumulation even under low-light conditions or suppressive white-UV conditions (15.06 ± 0.69 mg g⁻¹ dw). These values exceeded most previously reported values by 1.9–13.2 fold. Collectively, these findings show that spectral quality can be used to fine-tune phenolic and antioxidant production, while white light maximizes biomass, carotenoid, and TPC yields. This study provides a technical foundation for spectral control in large-scale algal production and supports the industrial potential of TW P105 as a scalable, cost-effective source of fucoxanthin-rich biomass for nutraceutical, functional food, and pharmaceutical applications.

Introduction

Driven by rapid anthropogenic development and global population growth, nutrient-rich microalgae have emerged as a key alternative to meet the rising demand for sustainable food and pharmaceutical feedstocks (Li et al., 2021). At the same time, the depletion of fossil fuels and related environmental degradation have intensified the search for carbon-neutral bioresources (Ambaye et al., 2021). Microalgae demonstrate high photosynthetic efficiency, rapid growth, and a remarkable ability to capture carbon dioxide (CO₂) while recycling nutrients from wastewater and flue gas (De Bhowmick, Sarmah & Sen, 2019). Their resilience to extreme conditions, such as high salinity, temperature, or pH, has made them increasingly valuable not only for producing food, feed, and bioenergy, but also for supporting wastewater bioremediation (Richmond & Hu, 2013). However, despite their potential, the commercial viability of microalgae remains limited due to high cultivation costs (Nwoba et al., 2019). Therefore, optimizing cultivation parameters to enhance both metabolic productivity and processing efficiency is a critical research priority.

Microalgal carotenoids have attracted increasing interest due to their strong antioxidant properties and potential applications in nutraceutical and cosmetic products. As light is the primary energy source for photoautotrophic microalgal growth, it plays a crucial role in determining both biomass and carotenoid productivity. Although natural sunlight is cost-effective, its diurnal and seasonal variability limits consistent carotenoid production (Blanken et al., 2013). In contrast, artificial lighting provides precise control over photosynthetic photon flux density, illumination duration, and spectral quality, which can enhance algal productivity (Ramanna, Rawat & Bux, 2017). While many studies focus on irradiance, growing evidence indicates that spectral quality is equally important for regulating photosynthesis and secondary metabolism in microalgae (Lima et al., 2018). In particular, ultraviolet (UV) radiation, especially in the UV-A range, induces oxidative stress by generating reactive oxygen species (ROS), which can damage cellular components (Isaia et al., 2024). To counter this stress, microalgae activate antioxidant defense systems, including the upregulation and accumulation of photoprotective carotenoids (Salguero et al., 2005; Forján et al., 2011). Among the various carotenoids produced by microalgae, fucoxanthin (Fx) is notable. It is a marine xanthophyll mainly found in diatoms and has received significant attention for its wide range of biological activities, including antioxidant, anti-obesity, anti-inflammatory, anti-cancer, antidiabetic, and antihypertensive effects (Zaragozá et al., 2008; Beppu et al., 2009; Maeda et al., 2009; Heo et al., 2010; Woo et al., 2010; Tanaka, Shnimizu & Moriwaki, 2012). Fx is also a major light-harvesting carotenoid in diatoms, playing a central role in photosynthesis and photoprotection. Despite its importance, Fx production in diatoms with high antioxidant capacity remains underexplored, particularly regarding the effects of specific light spectra on pigment synthesis and stress responses. For example, Schellenberger Costa et al. (2012) showed that blue light supports photoprotection and acclimation of Phaeodactylum tricornutum under monochromatic light regimes (blue, red, and white). While Fx content was generally higher under blue and white light than under red light, these differences were not statistically emphasized, and no combinatory or dynamic light treatments were evaluated. Yang & Wei (2020) reported that red–blue combined LED lighting enhanced both Fx accumulation and biomass productivity, suggesting a synergistic effect, although their study did not examine white or UV-A light or assess photoprotective mechanisms. These gaps highlight the need to investigate a broader range of light spectra to optimize both pigment synthesis and stress-related responses in diatoms.

In addition to carotenoids, phenolic compounds, often quantified as total phenolic content (TPC), play a crucial role in the oxidative stress defense of microalgae and are increasingly valued for their antioxidant properties in industrial applications. These secondary metabolites are typically synthesized in response to environmental stressors, scavenging reactive oxygen species (ROS) and protecting cellular components. Light influences their biosynthesis not only as an energy source but also as a regulatory signal, modulating phenolic metabolism through photoreceptor-mediated pathways. For example, in Scenedesmus falcatus, moderate light intensity produced the highest TPC (546 mg GAE g⁻¹ at 100 µmol photons m⁻² s⁻¹), while higher intensity caused a decline (231 mg GAE g⁻¹ at 1,000 µmol photons m⁻² s⁻¹) (Songserm, Nishiyama & Sanevas, 2024). Furthermore, blue light and red-blue light combinations have been shown to strongly enhance phenolic and flavonoid production in the green microalga Coelastrella sp. compared to white light (Georgieva et al., 2024). The effects of UV-A and UV-B exposure appear to be species- and strain-specific. In Tetraselmis sp. and Chlorella sorokiniana, UV-A and UV-B reduced phenolics and antioxidant capacity, likely due to antioxidant depletion under oxidative stress (Huarancca Reyes et al., 2023; Isaia et al., 2024).

In contrast, Chlamydomonas nivalis showed a 5–12% increase in phenolics, including stilbenoids, under UV light (Duval, Shetty & Thomas, 1999). These responses likely depend on both UV dose and species-specific stress tolerance. Despite these findings, the specific effects of light spectral quality and intensity on TPC accumulation in microalgae, particularly diatoms, remain poorly understood. This is partly because phenolics have only recently been recognized as important antioxidant and bioactive compounds in microalgae, while earlier studies primarily focused on carotenoids and other metabolites. Further evaluation of how light spectral quality and intensity influence phenolic and carotenoid production is therefore essential for optimizing both pathways.

Thalassiosira weissflogii TRG10-P105 (TW P105) is a newly isolated indigenous diatom recognized for its exceptionally high antioxidant capacity and compound productivity. It produces a notably high total carotenoid content, reaching up to 20.2 mg g⁻¹, including 11.5 mg g⁻¹ dw of fucoxanthin (Fx) (Katayama et al., 2022). TW P105 also demonstrates substantial phenolic content (12.46 ± 1.55 mg GAE g⁻¹ dw), further enhancing its antioxidant potential (Rahman et al., 2020). These characteristics distinguish TW P105 from many other diatoms and highlight its potential as a resource for bioactive compound production. Its tropical origin further increases its relevance. Microalgae that tolerate high irradiance and elevated temperatures are well suited for stable, large-scale cultivation in warm regions with abundant sunlight. While brown seaweeds are the primary commercial source of Fx (Guo et al., 2016), their use is limited by low productivity, poor Fx yield, and concerns about heavy metal contamination (Mori et al., 2004; Kanazawa et al., 2008). Microalgae such as TW P105 may therefore provide a sustainable and more efficient alternative for commercial Fx production. Nevertheless, despite increasing interest in optimizing light conditions to enhance microalgal metabolite production, the photobiological responses of diatoms like TW P105 to varying light spectra remain largely unexplored. Specifically, it is unclear how different light spectra may influence its growth performance and metabolic output, particularly carotenoid and phenolic content. Microalgal responses to light quality vary widely among species and are shaped by species-specific metabolic pathways and physiological traits (Li et al., 2023b). Given the strong antioxidant profile of TW P105 and its tropical origin, understanding how light spectra regulate its physiological and biochemical responses is crucial for evaluating its commercial potential. This study investigates how variations in light quality (spectral composition) influence the physiological and biochemical responses of TW P105. Specifically, it evaluates the effects of various light spectra on growth performance, pigment accumulation (including total carotenoids and fucoxanthin), and phenolic compound production. Overall, these findings aim to support the development of optimized cultivation strategies and improve our broader understanding of light-regulated metabolism in diatoms for sustainable bioresource production.

Materials & Methods

Algae strain and cultivation method

The diatom strain Thalassiosira weissflogii TRG10-P105 (TW P105) was obtained from a previous study by Katayama et al. (2020). The stock culture was cultivated and maintained in the exponential growth phase in 250 mL of Conway medium to ensure the cells were in a healthy growth phase prior to inoculation. Conway medium was prepared using 30 ppt sterile seawater with one mL L⁻¹ of micronutrients (100 g L⁻¹ NaNO₃, 45 g L⁻¹ EDTA, 33.6 g L⁻¹ H₃BO₃, 20 g L⁻¹ NaH₂PO₄, 1.30 g L⁻¹ FeCl₃, 0.36 g L⁻¹ MnCl₂, 0.1 mL L⁻¹ of trace metals (2.10 g L⁻¹ ZnCl₂, 2 g L⁻¹ CoCl₃, 0.90 g L⁻¹ (NH₄)₆Mo₇O₂, 2 g L⁻¹ CuSO₄⋅5H₂O)), two mL L⁻¹ of silicate solution (15 g L⁻¹ Na₂SiO₃), and 0.1 mL L⁻¹ of vitamin solution (Tompkins et al., 1995).

Erlenmeyer flasks with a total volume of 1 L were used for a one-stage cultivation system of TW P105 for seven days under white (control, 390–780 nm), red (605–700 nm), blue (450–480 nm), red-blue (combination of red and blue light throughout the cultivation period), and white-UV (UV-A introduced for 3 h per day from day 5 to 7 to apply an acute, short-term stressor) light treatments. Continuous aeration was provided, with temperature and photoperiod controlled at 25 °C and a 12-hour dark–light cycle, respectively, as described by Rahman et al. (2020). A low light intensity of 10 µmol photons m⁻² s⁻¹ was applied to optimize Fx accumulation (Gómez-Loredo, Benavides & Rito-Palomares, 2016; Wang et al., 2018; Lu et al., 2023). UV-A exposure was provided using an actinic black light tube (TL-D 15W, Philips) at 40 µmol photons m⁻² s⁻¹. Light intensity was calibrated and monitored with a light meter (AS803; Smart Sensor). Light treatments were applied from the start of the 7-day cultivation period, and the results reflect the algal response during the acclimation phase. Each treatment was conducted in triplicate. All cultures were harvested on day 7, as obtaining biomass in the exponential phase was essential for the main objectives of this study. Cultures were centrifuged at 4500 rpm for 10 min, freeze-dried for 72 h, and stored at −80 °C. Extracts were obtained via methanolic extraction of freeze-dried biomass (Foo et al., 2015).

Growth and specific growth rate

Growth of TW P105 was monitored daily by measuring the culture optical density at 750 nm (OD₇₅₀) using a microplate reader (Spark; Tecan) according to the protocol of Rahman et al. (2020). Specific growth rate (µ, % d −1) was calculated as:

$${\displaystyle \mathrm{\mu }=\frac{ln\left(N2\right)-ln\left(N1\right)}{t2-t1}\times 100\text{\%}}$$

where N1 and N2 are the biomass concentrations at t1 and t2.

Biomass productivity (P)

Biomass was quantified gravimetrically and expressed as dry cell weight (DCW), following the method of Peng et al. (2024). Cultures were harvested into pre-weighed centrifuge tubes and reweighed to determine DCW after freeze-drying. Biomass productivity (P, g L⁻¹ d⁻¹) was calculated as:

$${\displaystyle P=\frac{X2-X1}{t2-t1}}$$

where X1 and X2 are the biomass concentrations at t1 and t2.

Pigment content

TW P105 extracts were analyzed for chlorophyll a (Chl a) and chlorophyll c (Chl c) using the methods of Ritchie (2008), and for total carotenoids (Car) content using the methods of Lichtenthaler & Wellburn (1983). Chl a, Chl c, and Car were calculated as follows:

$${\displaystyle Chla=-2.0780\times O{D}_{632}-6.5079\times O{D}_{652}+16.2127\times O{D}_{665}-2.1372\times O{D}_{696}}$$

$${\displaystyle Chlc=34.0115\times O{D}_{632}-12.7873\times O{D}_{652}+1.4489\times O{D}_{665}-2.5812\times O{D}_{696}}$$

$${\displaystyle Car=\frac{\left(1000\times O{D}_{470}-1.63\times Chla\right)}{221}}$$

Fucoxanthin (Fx)

TW P105 extracts were analyzed in triplicate using ultra-high-performance liquid chromatography (UHPLC; Waters ACQUITY Arc System). Ten microliters of extract were injected onto an XBridge^® BEH Amide 2.5 µm column (4.6  × 150 mm) with a flow rate of 0.8 mL min⁻¹ (Marella & Tiwari, 2020). The mobile phase gradient consisted of 100% methanol (A) and 100% water (B), starting from 95% A and 5% B to 0% A and 100% B over 10 min. Absorbance at 450 nm was recorded. Pure fucoxanthin standards (Sigma-Aldrich, Germany) were used to generate a standard curve and calibrate retention times.

Total phenolic content (TPC)

Total phenolic content (TPC) was determined using the Folin-Ciocalteu method as described by Li et al. (2007). Briefly, 25 µL of extract was added to 125 µL of 1:10 diluted Folin–Ciocalteu reagent. After 4 min, 100 µL of sodium carbonate (75 g L⁻¹) was added, followed by incubation for 2 h at room temperature. The standard calibration curve was prepared with gallic acid at different concentrations (0–100 µg mL⁻¹), and the results were expressed as gallic acid equivalents (GAE g⁻¹ dw).

Statistical analysis

Statistical analysis was performed using one-way analysis of variance (ANOVA) followed by Tukey’s post hoc test to evaluate the effects of light treatments on TW P105. Normality was confirmed before analysis. Statistical significance was set at P < 0.05. Analyses were conducted using the statistical software SPSS (SPSS Inc., USA).

Results

Growth and specific growth rate, biomass productivity

As shown in Fig. 1A, TW P105 exhibited a slight decline in optical density (OD₇₅₀) on the first day after inoculation across all treatments. TW P105 showed the highest growth under white light, followed by blue light. In contrast, cultures exposed to red light and red-blue (a combination of red and blue) light experienced a noticeable adaptation delay. By day six, cultures under white and blue light reached peak absorbance values of 0.0603 ± 0.0084 A and 0.0568 ± 0.0079 A, respectively, before dropping to 0.0539  ± 0.0054 A and 0.0416 ± 0.0005 A on day seven. Meanwhile, cultures under red and red-blue light showed slower growth and had not reached a plateau by day seven. Their highest absorbance values at harvest were 0.0490 ± 0.0028 A for red light and 0.0409 ± 0.0072 A for red-blue light. Consistent with these trends, the highest specific growth rate (SGR) over seven days (Fig. 2A) occurred under white light at 30.17 ± 2.06% d⁻¹, followed by blue light, and was significantly higher than that of the red (P = 0.013) and red-blue (P = 0.003) treatment groups. The SGRs for blue, red, and red-blue treatments were 23.2 ± 2.26, 15.69 ± 1.48, and 12.35 ± 1.82% d⁻¹, respectively. White light also produced the highest biomass accumulation (0.060 ± 0.01 g L⁻¹) and productivity (0.009 ± 0.002 g L⁻¹ d⁻¹), followed by red, red-blue, and blue light, as shown in Fig. 3.

Figure 1. Growth curves of Thalassiosira weissflogii TRG10-P105.

(A) White, red, blue, red-blue treatment. (B) White and white-UV light treatment. Values are means of triplicates ± standard errors (SE, n = 3).

Figure 2. The specific growth rate (SGR) of Thalassiosira weissflogii TRG10-P105.

(A) White, red, blue, red-blue treatment. (B) White and white-UV light treatment. Values are means of triplicates ± standard errors (SE, n = 3). Data with asterisk (*) indicate statistically significant differences between treatments (P < 0.05).

Figure 3. Biomass accumulation and productivity of Thalassiosira weissflogii TRG10-P105.

(A) White, red, blue, red-blue treatment. (B) White and white-UV light treatment. Values are means of triplicates ± standard errors (SE, n = 3). Data with asterisk (*) indicate statistically significant differences between treatments (P < 0.05).

Figure 1B showed that the slight decline in optical density (OD₇₅₀) for the white-UV treatment group continued until the third day of cultivation. Notably, growth resumed from the fifth day onward with UV-A exposure, indicating a potential role of UV-A in facilitating the acclimation of TW P105. The final absorbance recorded at harvest was 0.0255 ± 0.0015 A, which was close to the control value on day three (0.0283 ± 0.0076 A). As seen in Fig. 2B, the SGR of the white-UV treatment group over seven days was significantly lower (P = 0.002) than that of the control, at 10.94 ± 3.96% d⁻¹. Figure 3B showed that TW P105 exposed to white-UV light achieved a biomass accumulation of 0.017 ± 0.00 g L⁻¹ and a productivity of 0.002  ± 0.000 g L⁻¹ d⁻¹, both significantly lower than those of the control (P = 0.008).

Pigments

The pigment profiles of TW P105 under different spectral conditions are summarized in Fig. 4. Chlorophyll a (Chl a) concentration was highest under white light (5.92 ± 1.20 mg g⁻¹; P = 0.044). In contrast, red and blue light produced moderate Chl a levels, while the red-blue light treatment resulted in the lowest value at 3.22 ± 0.53 mg g⁻¹ (P = 0.131). A similar pattern was observed for chlorophyll c (Chl c), which peaked at 1.15 ± 0.67 mg g⁻¹ under white light, whereas the red-blue treatment showed a negligible content of 0.01 ± 0.05 mg g⁻¹ (P = 0.151). Total carotenoids (Car) accumulation followed this trend, with the highest concentration under white light (3.99 ± 0.38 mg g⁻¹), followed by blue light (3.72 ± 0.51 mg g⁻¹). In contrast, red and red-blue treatments showed lower Car levels of 3.34 ± 0.20 mg g⁻¹ (P = 0.733) and 2.94 ± 0.24 mg g⁻¹ (P = 0.338), respectively. Notably, as shown in Fig. 4B, the red-blue light treatment resulted in the highest (Chl c + Car) to Chl a ratio, at 0.95 ± 0.01 (P = 0.973), exceeding the ratios observed under blue (0.88 ± 0.09), white (0.87 ± 0.03), and red (0.75 ± 0.04) light.

Figure 4. Pigment content and composition of Thalassiosira weissflogii TRG10-P105.

(A) Pigment content under white, red, blue, red-blue treatment. (B) Pigment composition under white, red, blue, red-blue treatment. (C) Pigment content under white and white-UV light treatment. (D) Pigment composition under white and white-UV light treatment. Values are means of triplicates ± standard errors (SE, n = 3). Data with asterisk (*) indicate statistically significant differences between treatments (P < 0.05).

As shown in Fig. 4C, Chl a content was significantly lower under the white-UV treatment (P = 0.032). Although Chl c content also decreased, the reduction was not statistically significant (P = 0.160). Additionally, the Car content in TW P105 exposed to white-UV light was 2.13 ± 0.44 mg g⁻¹, a significant decrease compared to the control (P = 0.036). The white-UV treatment group had a slightly higher (Chl c + Car) to Chl a ratio than the control (1.01 ± 0.14), but this difference was not statistically significant (P = 0.812).

Fucoxanthin

As shown in Fig. 5A, Fx accumulation in TW P105 remained remarkably stable across different light spectra. The highest concentrations were observed under red light (18.49 ± 0.69 mg g⁻¹ dw), followed by blue (18.26 ± 0.79 mg g⁻¹ dw) and white light (18.12 ± 0.61 mg g⁻¹ dw). Cultures exposed to the red-blue light treatment had a slightly lower Fx level at 17.27 ± 0.32 mg g⁻¹ dw, though this difference was not statistically significant (P = 0.875). In contrast, a significant reduction in Fx content was found in the white-UV treated group (15.06 ± 0.69 mg g⁻¹ dw) compared to the control, as shown in Fig. 5B (P = 0.044). Although the effect of spectral quality on Fx levels was not always statistically significant, these trends suggest a subtle modulation of fucoxanthin biosynthesis or chemical stability by light quality in TW P105.

Figure 5. Fucoxanthin accumulation of Thalassiosira weissflogii TRG10-P105.

(A) White, red, blue, red-blue treatment. (B) White and white-UV light treatment. Values are means of triplicates ± standard errors (SE, n = 3). Data with asterisk (*) indicate statistically significant differences between treatments (P < 0.05).

Total phenolic content

The influence of spectral quality on the TPC of TW P105 is shown in Fig. 6. Analysis revealed that the control group (white light) had the highest TPC, 8.63 ± 0.07 g GAE g⁻¹ dw (Fig. 6A). The blue and red light treatments produced intermediate concentrations. In contrast, the red-blue light treatment group had a TPC of 4.71  ± 0.07 g GAE g⁻¹ dw, a significant reduction compared to the control (P = 0.042). Similarly, TW P105 exposed to the white-UV regime showed a TPC of 3.09 ± 0.88 g GAE g⁻¹ dw (Fig. 6B), which was significantly lower than that of the control group (P = 0.005).

Figure 6. Total phenolic compounds (TPC) of Thalassiosira weissflogii TRG10-P105.

(A) White, red, blue, red-blue treatment. (B) White and white-UV light treatment. Values are means of triplicates ± standard errors (SE, n = 3). Data with asterisk (*) indicate statistically significant differences between treatments (P < 0.05).

Discussion

The initial lag phase observed during the transition from pre-culture to fresh medium reflects the metabolic plasticity required for adjustment to a new photo-environment (Rolfe Matthew et al., 2012). The growth attenuation shown in Fig. 1 was likely a result of dilution stress and cellular recalibration to experimental conditions. Some cells may die or temporarily undergo metabolic arrest during this period, leading to a transient decrease in biomass before exponential growth resumes.

In diatoms, as in other photosynthetic organisms, light-dependent photosynthetic reactions occur within the thylakoid membranes and involve the coordinated activity of Photosystem II (PSII), Photosystem I (PSI), and the subsequent Calvin-Benson-Bassham cycle (Satpati & Pal, 2020). As shown in Fig. 7, PSII captures light energy and transfers excited electrons through the electron transport chain (ETC) to PSI, generating adenosine triphosphate (ATP) via chemiosmosis. PSI, with additional light energy, reenergizes the electrons and reduces NADP+ to NADPH. Both ATP and nicotinamide adenine dinucleotide phosphate (NADPH) produced by these processes supply the Calvin cycle, where carbon dioxide is fixed and converted into glucose, which is then used for growth. Unlike higher plants or green algae, diatoms exhibit minimal differences in the light-harvesting pigments associated with PSI and PSII (Lepetit et al., 2012; Schellenberger Costa et al., 2012). Consequently, monochromatic wavelengths are unlikely to induce preferential excitation of either photosystem. This suggests that the spectral effects on the growth and specific growth rate (SGR) of TW P105 are driven not by selective photosystem excitation, but by the overall efficiency of energy transduction (ATP and NADPH production) and the activation of photoprotective mechanisms such as non-photochemical quenching (NPQ).

Figure 7. The short flowchart-based depiction of the major photosynthetic pathways in diatoms.

In diatom thylakoid membranes, PSII and PSI perform light-dependent reactions that produce adenosine triphosphate (ATP) and nicotinamide adenine dinucleotide phosphate (NADPH), which drive the Calvin cycle. Diatom photoprotection occurs mainly through energy-dependent quenching (qE), largely mediated by the diatoxanthin (Dtx)-dependent NPQ pathway.

In this study, white light supported superior growth and specific growth rate (SGR), likely due to its balanced spectral composition that optimizes the quantum yield of both photosystems (Su, Lundholm & Ellegaard, 2017). This spectral equilibrium enables an efficient electron transport chain, providing the energetic flux required to drive the Calvin cycle and subsequent biomass synthesis, thereby supporting the enhanced growth observed under white light conditions. Conversely, blue light, though narrower in spectrum, is efficiently absorbed by photosynthetic pigments and plays an important role in activating non-photochemical quenching (NPQ). NPQ dissipates excess excitation energy as heat, protecting the photosystems from damage and helping maintain stable photosynthetic performance (Schellenberger Costa et al., 2012; Havurinne & Tyystjärvi, 2017). The robust growth observed under blue light reflects a balance between effective photoprotection and sustained photosynthetic efficiency.

In contrast, red and red-blue spectra appeared insufficient to fully energize the photosynthetic machinery of TW P105. Because diatoms lack strong preferential excitation of PSI or PSII (Lepetit et al., 2012), red-enriched spectra may not provide the energy balance required for optimal electron flow and ATP/NADPH production. In diatoms, NPQ is predominantly mediated by the energy-dependent quenching (qE) component, a process governed almost exclusively by the diatoxanthin (Dtx)-dependent pathway (Cruz et al., 2010; Derks & Bruce, 2018). State-transition quenching (qT), which redistributes excitation energy between PSII and PSI via movement of light-harvesting complexes, has not been observed in diatoms (Owens, 1986; Depauw et al., 2012). Meanwhile, photoinhibitory quenching (qI) is generally reduced as diatoms actively transport and accumulate inorganic carbon inside their cells (Cruz et al., 2010). Because red light carries lower photon energy, it generally induces weaker NPQ (Su, 2019). This weakened photoprotective state potentially increases the susceptibility of the photosystems to damage, resulting in slower growth (Havurinne & Tyystjärvi, 2017). This effect is consistent with the observed lag in growth and reduced biomass yields under red and red-blue light treatment.

Blue light, characterized by its high-energy photons, can induce greater oxidative stress in microalgal cells compared to other spectral qualities (Li et al., 2023b). This oxidative stress may compromise cellular membranes and other vital components, leading to reduced biomass concentration. Additionally, blue light strongly activates NPQ in diatoms, diverting a significant portion of absorbed energy to thermal dissipation instead of carbon fixation, which limits biomass accumulation even under favorable growth conditions (Schellenberger Costa et al., 2012). Together, the combined effects of oxidative stress, the metabolic cost of increased phenolic compound production (Fig. 6A), and high NPQ activation provide a robust explanation for the lower biomass yields observed under both blue and white light treatments.

The introduction of UV-A on day five likely activated protective pathways, including the xanthophyll cycle and the synthesis of mycosporine-like amino acids (MAA), facilitating stress recovery. This is consistent with findings by Zudaire & Roy (2001), where UV-A initially induced cellular stress but subsequently triggered photoprotective responses, allowing growth to resume. However, applying UV-A midway through the experiment may introduce a temporal treatment switch that could contribute to the observed stress–recovery pattern. To address this limitation, future studies should use continuous UV-A treatments from inoculation and vary onset times. As stated by Turcsányi & Vass (2000), the damaging efficiency of UV-A on PSII is approximately 45–60 times higher than that of photosynthetically active radiation (PAR) when normalized for energy dose or photon flux. The lower growth and SGR observed under white-UV conditions could be attributed to the damaging effects of UV-A radiation on the PSII complex, which impairs ETC, leading to reduced ATP and NADPH production and consequently lower glucose synthesis.

The highest Chl a content observed under white light aligns with its ability to maintain a balanced photosynthetic apparatus. Similarly, Prins et al. (2020) showed that natural microphytobenthos-dominated biofilms containing diatom species such as Navicula meulemansii, Navicula spartinetensis, Gyrosigma limosum, Pleurosigma angulatum, and others exhibited higher Chl a under blue light at low light intensity, but higher Chl a under red light at high light intensity. This suggests that light quality and intensity interact to determine pigment production in these diatom communities. While Chl a is highly responsive to blue wavelengths (Kuczynska, Jemiola-Rzeminska & Strzalka, 2015) and typically increases in diatoms such as Chaetoceros sp., Skeletonema costatum, and Thalassiosira pseudonana under blue light (Sánchez-Saavedra & Voltolina, 2002), the slightly lower Chl a observed in blue light cultures in the present study may be due to the onset of the senescence phase by day seven (Fig. 1A). During this phase, natural chlorophyll degradation typically accelerates. In contrast, red light cultures remained in the exponential phase, characterized by active photosynthesis and sustained Chl a synthesis (Sánchez-Saavedra & Voltolina, 2002). Furthermore, the activation of NPQ under blue light may divert metabolic resources away from pigment biosynthesis, contributing to the lower Chl a content. Notably, the red–blue combination yielded the lowest Chl a concentration, even at the low light intensity of 10 µmol photons m⁻² s⁻¹. Similar results were reported for Phaeodactylum tricornutum, where red–blue light suppressed Chl a accumulation compared to red light alone, even at much higher light intensities (102–255 µmol photons m⁻² s⁻¹). This indicates that red–blue light can activate photoprotective pathways that reduce Chl a accumulation, regardless of light intensity. To clarify the underlying mechanisms, further studies using pulse–amplitude–modulated (PAM) fluorometry and gene expression analysis are recommended. Chl c also functions as an accessory pigment in diatoms, binding alongside Chl a and various carotenoids such as fucoxanthin, diadinoxanthin, and violaxanthin to support light harvesting and photoprotection (Büchel, 2020). Its concentration often parallels that of Chl a, which is consistent with our study. White light treatment, which yielded the highest Chl a level, also produced the highest Chl c content. In contrast, all other light treatments exhibited consistently lower levels of both pigments, highlighting their co-regulation in response to light quality.

Carotenoids are essential lipophilic accessory pigments that protect cells from photo-oxidative damage (Zarekarizi, Hoffmann & Burritt, 2023). Under white light, the balanced spectrum likely upregulates pigment biosynthesis to expand antenna size, enhancing light capture and energy transfer. This promotes increases in Chl a, Chl c, and Fx, which are tightly integrated into fucoxanthin–chlorophyll protein complexes (FCPs) (Büchel, 2020), resulting in high total carotenoid content. Blue light produces the second highest pigment yields, likely because it triggers the xanthophyll cycle and stimulates the synthesis of photoprotective carotenoids such as Fx, diadinoxanthin, and diatoxanthin, which are integral to the NPQ mechanism (Prins et al., 2020). In contrast, red light primarily supports photosynthesis with less activation of photoprotective mechanisms, leading to lower carotenoid accumulation (Yi et al., 2019). The reduced carotenoid levels under red and red–blue light may reflect either lower oxidative stress or decreased pigment biosynthesis. Red photons (∼660 nm) provide lower excitation energy, generating fewer ROS and reducing the need for photoprotective pigments. Additionally, the limited activation of blue-light-sensitive photoreceptors, such as cryptochromes, under red-dominant spectra may further downregulate carotenoid biosynthesis via transcriptional control (Bertrand, 2010).

Under white-UV exposure, the reduction in Chl a and Chl c was likely a result of stress and photoinhibition, which can impair pigment synthesis and photosynthetic efficiency in diatoms (Zudaire & Roy, 2001). Additionally, since the culture was exposed to UV-A only from the fifth to the seventh day, TW P105 may not have had sufficient time to stabilize or recover through activation of photoprotective mechanisms. However, the elevated ratio of (Car + Chl c) to Chl a suggests a relative increase in accessory pigments compared to primary Chl a, potentially reflecting a shift toward enhanced photoprotection in response to UV-A exposure. Both red-blue and UV-A light have been shown to regulate the expression of carotenoid biosynthesis genes, contributing to photoprotection and modulation of light-harvesting complexes (Bertrand, 2010).

Fucoxanthin (Fx), the primary light-harvesting carotenoid in diatoms, is functionally integrated with Chl a and Chl c binding proteins in diatom light-harvesting complexes (FCPs), making its concentration a reliable indicator of antenna size and composition (Wang et al., 2019). Even subtle changes in Fx levels can indicate adjustments in antenna complex structure, reflecting a dynamic photoacclimation strategy that optimizes light capture under varying light conditions. Fx also efficiently absorbs blue–green light (450–550 nm) and transfers that energy to Chl a for photosynthesis (Bertrand, 2010; Lepetit et al., 2012). In the present study, Fx concentrations in TW P105 remained remarkably stable across white, red, blue, and red-blue spectra (17.27–18.49 mg g⁻¹ dw). This stability deviates from previous reports of pronounced spectral effects on carotenoid accumulation in diatoms (Schellenberger Costa et al., 2012; Yang & Wei, 2020), and is likely due to the high antioxidant nature of this specific strain. Previous bioprospecting of over 125 indigenous tropical marine microalgal strains identified TW P105 as a superior carotenoid producer, yielding approximately 20.2 mg g⁻¹ dw total carotenoids, including 11.5 mg g⁻¹ dw Fx, under a light intensity of 150 µmol photons m⁻² s⁻¹ (Rahman et al., 2020; Katayama et al., 2022).

Remarkably, under the lower light intensity used in this study (10 µmol photons m⁻² s⁻¹), Fx content nearly doubled, suggesting that low light conditions strongly favor Fx accumulation in this strain. This is consistent with previous studies showing that Fx production in diatoms generally declines under higher light intensities. Yang & Wei (2020) reported a significant reduction in Fx content in Phaeodactylum tricornutum when light intensity exceeded 30 µmol photons m⁻² s⁻¹. Similarly, Wang et al. (2018) observed the highest Fx levels in P. tricornutum and Cylindrotheca fusiformis at 30 µmol photons m⁻² s⁻¹, with a marked decrease as light intensity increased to 180 µmol photons m⁻² s⁻¹. Gómez-Loredo, Benavides & Rito-Palomares (2016) also demonstrated that low light (13.5 µmol photons m⁻² s⁻¹) maximized Fx production in Isochrysis galbana and Phaeodactylum tricornutum. Once Fx synthesis reaches near-maximal levels under these favorable conditions, spectral quality appears to exert only minor regulatory influence. Notably, although Fx production under different light spectra was not significantly affected when cultivated at 10 µmol photons m⁻² s⁻¹, TW P105 under white light still achieved Fx yields 1.9–13.2 times those reported for other Thalassiosira species (Table 1). For example, Li et al. (2023a), Guo et al. (2016), and Marella & Tiwari (2020) obtained maximum Fx yields of 1.37, 2, and 9.56 mg g⁻¹ from T. weissflogii, respectively. Meanwhile, the highest Fx yields of T. pseudonana were 2 and 4.73 mg g⁻¹ under 30 µmol m⁻² s⁻¹ light and 90.75 µmol m⁻² s⁻¹ blue light, respectively (Guo et al., 2016; Peng et al., 2024). Because cultivation conditions vary widely among studies, the values in Table 1 are provided solely for comparison of reported Fx levels across diatom species. This consistently high productivity highlights TW-P105 as a robust candidate for sustainable Fx production, with direct relevance for food and pharmaceutical applications where natural antioxidants are in growing demand.

Table 1. Overview of light treatment impact on microalgae growth, biomass, carotenoids and fucoxanthin production.

Summary of maximum biomass and pigment (carotenoids and fucoxanthin) production of selected marine algae under different light conditions, highlighting species-specific responses and comparison with previous studies.

Species	Light treatment	Max. Biomass (g L−1)	Max. Carotenoids (mg g−1)	Max. Fucoxanthin (mg g−1)	References
T. weissflogii TRG10-P105	White light 10 μ mol m−2 s−1	0.06	1.00	18.12	This study
T. weissflogii	Blue light, 300 μ mol m−2 s−1	3.2	n/a	9.56	Marella & Tiwari (2020)
T. weissflogii	30 μ mol m−2 s−1	0.76	2.91	1.372	Li et al. (2023a) and Li et al. (2023b)
T. weissflogii (CCMP 1051)	30 μ mol m−2 s−1 light	n/a	n/a	2	Guo et al. (2016)
T. pseudonana	Blue light, 90.75 μ mol m−2 s−1	0.918 (white)	0.623 (green)	4.729 (blue)	Peng et al. (2024)
T. pseudonana	30 μ mol m−2 s−1 light	n/a	n/a	2	Guo et al. (2016)
Coccomyxa sp. KGU-D001	Fluorescent lump, 40 μ mol m−2 s−1	1.54	n/a	n/a	Aburai et al. (2020)
Gracilaria lemaneiformis	Green LED (growth), 120 μ mol m−2 s−1	n/a	0.11 fresh weight (blue)	n/a	Gong, Liu & Zou (2020)
Isochrysis galbana	Blue intermittent light (52 μ mol m−2 s−1, 10,000 Hz, duty ratio 50%)	0.155	n/a	n/a	Yoshioka et al. (2012)
Phaeodactylum tricornutum (PT 29)	Red light, 120 μ mol m−2 s−1 (Phase 1), then Blue-Green 2:1, 60 μ mol m−2 s−1 (Phase 2)	1.12 (Phase 1), 1.07 (Phase 2)	n/a	11.5	Song et al. (2025)
Phaeodactylum tricornutum	Red-Blue 6:1, 20 μ mol m−2 s−1 (Phase 1), Red-Blue 5:1, 25 μ mol m−2 s−1 + tryptone (Phase 2)	6.52	n/a	13.26	Yang & Wei (2020)
Ulva lactuca	Blue LED (growth), 120 μ mol m−2 s−1	n/a	1.15 fresh weight (Red LED)	n/a	Gong, Liu & Zou (2020)

As shown in Table 1, the preference and sensitivity to light quality for Fx accumulation were species-specific. The relative stability of Fx content observed in TW P105 likely reflects its dual role as both a light-harvesting pigment and a structural component of the FCP complexes (Wang et al., 2019). Because Fx is integral to these complexes, its biosynthesis and turnover are less flexible than those of other pigments, such as chlorophylls and xanthophyll cycle carotenoids, which can be more readily adjusted according to the light environment (Bertrand, 2010). In contrast, the variation in total carotenoid (Car) content under different light conditions likely reflects the dynamic photoprotective responses and metabolic flexibility of diatoms. These Cars, including diadinoxanthin and diatoxanthin, respond to light-induced oxidative stress and can adjust rapidly to protect the photosystems by dissipating excess energy and scavenging reactive oxygen species (Lavaud, Rousseau & Etienne, 2004). The slight reduction in Fx under red–blue light likely represents a minor adjustment in pigment allocation rather than stress, as Fx levels remained within a narrow range across all treatments. This stability aligns with Fx’s essential role in maintaining the structure and function of the light-harvesting antenna (Wang et al., 2019). Other carotenoids, however, are more dynamically regulated and act as flexible photoprotective pigments, adjusting under varying light conditions to optimize photosynthetic efficiency (Lavaud, Rousseau & Etienne, 2004; Bertrand, 2010). Taken together, these findings suggest that TW P105 maintains high, stable fucoxanthin levels to safeguard core photosynthetic processes while modulating accessory carotenoids in response to changing light conditions. To more precisely understand these dynamics, further investigations at the molecular level, such as quantifying gene expression and enzyme activity related to Fx biosynthesis, including fucoxanthin synthase and carotenoid isomerases, or profiling all relevant carotenoids, are recommended.

Phenolic compounds are important contributors to the antioxidant capacity of diatoms, and TPC serves as a sensitive indicator of light-driven antioxidant and photoprotective responses in coastal diatoms (Smerilli et al., 2019). Although direct studies on diatoms are limited, Foyer & Shigeoka (2010) demonstrated that efficient antioxidant networks play a significant role in balancing redox status and protecting photosynthesis in higher plants. It is therefore reasonable to assume that similar mechanisms operate in diatoms exposed to broad-spectrum light, supporting balanced photosynthetic activity and cellular redox homeostasis. In this study, the high TPC observed under white light suggests an optimized metabolic state that supports the coordinated regulation of antioxidant pathways, including the synthesis of phenolic-like compounds. While diatoms may lack the full phenylpropanoid suite of terrestrial plants, they possess key enzymes such as 4-coumarate CoA ligase (4CL) and chalcone synthase (CHS) (Del Mondo, Sansone & Brunet, 2022). These enzymes catalyze the formation of intermediate molecules like p-coumaroyl-CoA and naringenin-chalcone, which are precursors to various phenolics, including flavonoids. Supporting this, Haoujar et al. (2019) identified several phenolic compounds in the diatom Phaeodactylum tricornutum, including protocatechuic acid, caffeic acid, caffeic acid hexoside dimer, dimethoxyflavone, and p-coumaroyl tyrosine, which contributed to a TPC of 113.83 µg g⁻¹ dw.

The moderate TPC levels under monochromatic blue and red light suggest that only limited spectral ranges may partially activate these biosynthetic pathways. Although blue and red light can stimulate photosynthetic pigments and metabolic processes, their limited spectral range may prevent the full induction of phenolic compound synthesis. Red light, while important for photosynthetic energy capture, generally has a weaker effect on secondary metabolite production compared to blue light (Goiris et al., 2012). Smerilli et al. (2017) reported that blue light strongly promotes phenolic compound accumulation in Skeletonema marinoi, whereas combined blue-red light induces a more gradual TPC response. This aligns with our observations, where blue and red–blue light treatments showed similar TPC trends, reflecting light-driven antioxidant responses in diatoms. Additionally, the presence of flavonoids such as quercetin and apigenin in microalgae further highlights their role in scavenging free radicals and absorbing UV radiation, thereby protecting cells from oxidative stress and UV-induced damage (Goiris et al., 2012).

Because only TPC was measured, the value reported in this study represents the combined pool of both primary and secondary metabolites, rather than phenolic compounds specifically associated with stress or defense. Detailed phenolic profiling would be required to distinguish these components and clarify whether white light directly supports both growth and secondary metabolite production. It should be noted that the explanations for the phenolic and pigment responses are based on findings from previous studies, as ROS, NPQ, and molecular data were not measured in this study. This remains a limitation, as clarification of the pathways involved requires ROS assays along with transcriptomic or proteomic analyses. Comprehensive characterization of phenolic compounds also requires HPLC-MS analysis.

Conclusions

This study shows that the spectral composition of light significantly affects the growth kinetics, pigment structure, and phenolic metabolism of TW P105. As an indigenous tropical diatom, TW P105 has an exceptionally high baseline antioxidant capacity. The findings reveal that broad-spectrum white light is optimal for maximizing biomass productivity, fucoxanthin (Fx) accumulation, and total phenolic content (TPC). In contrast, monochromatic and UV-A treatments induced distinct physiological shifts, yet Fx concentrations remained remarkably stable and high across all spectral regimes. This inherent metabolic stability confirms the robust carotenoid-producing potential of TW P105, which consistently outperformed other Thalassiosira sp. reported in the literature. Collectively, these results underscore the industrial suitability of this strain for high-yield production of fucoxanthin and natural antioxidants. This work provides a technical foundation for spectral optimization in controlled cultivation and suggests that tailored light regimes can be strategically used to fine-tune metabolic outputs. Future investigations focusing on molecular pathways and transcriptomic responses will further facilitate the development of efficient, scalable production systems for high-value nutraceutical and biotechnological applications.

Supplemental Information

Supplemental Information 1. Raw data.

All samples and replicates under white, red, blue, combined red–blue, and white supplemented with UV-A light treatments.

Funding Statement

This work was supported by the Fundamental Research Grant Scheme (FRGS/1/2023/WAB04/XMU/03/1) and the Xiamen University Malaysia Research Fund (XMUMRF/2022-C10/ICAM/0010). The funders had no role in study design, data collection and analysis, decision to publish, or preparation of the manuscript.