Pigment-targeted light wavelength and intensity promotes efficient photoautotrophic growth of Cyanobacteria

Traditional and new trend strategies to enhance pigment contents in microalgae

Microalgae are a source of a wide variety of commodities, including particularly valuable pigments. The typical pigments present in microalgae are the chlorophylls, carotenoids, and phycobiliproteins. However, other types of pigments, of the family of water-soluble polyphenols, usually encountered in terrestrial plants, have been recently reported in microalgae. Among such microalgal polyphenols, many flavonoids have a yellowish hue, and are used as natural textile dyes. Besides being used as natural colorants, for example in the food or cosmetic industry, microalgal pigments also possess many bioactive properties, making them functional as nutraceutical or pharmaceutical agents. Each type of pigment, with its own chemical structure, fulfills particular biological functions. Considering both eukaryotes and prokaryotes, some species within the four most promising microalgae groups (Cyanobacteria, Rhodophyta, Chlorophyta and Heterokontophyta) are distinguished by their high contents of specific added-value pigments. To further enhance microalgae pigment contents during autotrophic cultivation, a review is made of the main related strategies adopted during the last decade, including light adjustments (quantity and quality, and the duration of the photoperiod cycle), and regard to mineral medium characteristics (salinity, nutrients concentrations, presence of inductive chemicals). In contrast to what is usually observed for growth-related pigments, accumulation of non-photosynthetic pigments (polyphenols and secondary carotenoids) requires particularly stressful conditions. Finally, pigment enrichment is also made possible with two new cutting-edge technologies, via the application of metallic nanoparticles or magnetic fields.

Advanced treatment of antibiotic-polluted wastewater by a consortium composed of bacteria and mixed cyanobacteria

This study constructed a cyanobacteria-bacteria consortium using a mixture of non-toxic cyanobacteria (Synechococcus sp. and Chroococcus sp.) immobilized in calcium alginate and native bacteria in wastewater. The consortium was used for the advanced treatment of sulfamethoxazole-polluted wastewater and the production of cyanobacterial lipid. Mixed cyanobacteria increased the abundances of denitrifying bacteria and phosphorus-accumulating bacteria as well as stimulated various functional enzymes in the wastewater bacterial community, which efficiently removed 70.01-71.86% of TN, 91.45-97.04% of TP and 70.72-76.85% of COD from the wastewater. The removal efficiency of 55.29-69.90% for sulfamethoxazole was mainly attributed to the upregulation of genes encoding oxidases, reductases, oxidoreductases and transferases in two cyanobacterial species as well as the increased abundances of Stenotrophomonas, Sediminibacterium, Arenimonas, Novosphingobium, Flavobacterium and Hydrogenophaga in wastewater bacterial community. Transcriptomic responses proved that mixed cyanobacteria presented an elevated lipid productivity of 33.90 mg/L/day as an adaptive stress response to sulfamethoxazole. Sediminibacterium, Flavobacterium and Exiguobacterium in the wastewater bacterial community may also promote cyanobacterial lipid synthesis through symbiosis. Results of this study proved that the mixed cyanobacteria-bacteria consortium was a promising approach for advanced wastewater treatment coupled to cyanobacterial lipid production.

Interlaboratory Reproducibility in Growth and Reporter Expression in the Cyanobacterium Synechocystis sp. PCC 6803

In recent years, a plethora of new synthetic biology tools for use in cyanobacteria have been published; however, their reported characterizations often cannot be reproduced, greatly limiting the comparability of results and hindering their applicability. In this interlaboratory study, the reproducibility of a standard microbiological experiment for the cyanobacterial model organism Synechocystis sp. PCC 6803 was assessed. Participants from eight different laboratories quantified the fluorescence intensity of mVENUS as a proxy for the transcription activity of the three promoters PJ23100, PrhaBAD, and PpetE over time. In addition, growth rates were measured to compare growth conditions between laboratories. By establishing strict and standardized laboratory protocols, reflecting frequently reported methods, we aimed to identify issues with state-of-the-art procedures and assess their effect on reproducibility. Significant differences in spectrophotometer measurements across laboratories from identical samples were found, suggesting that commonly used reporting practices of optical density values need to be supplemented by cell count or biomass measurements. Further, despite standardized light intensity in the incubators, significantly different growth rates between incubators used in this study were observed, highlighting the need for additional reporting requirements of growth conditions for phototrophic organisms beyond the light intensity and CO2 supply. Despite the use of a regulatory system orthogonal to Synechocystis sp. PCC 6803, PrhaBAD, and a high level of protocol standardization, ∼32% variation in promoter activity under induced conditions was found across laboratories, suggesting that the reproducibility of other data in the field of cyanobacteria might be affected similarly.

Light Modulation for Bioactive Pigment Production in Synechocystis salina

Cyanobacteria are microorganisms that are well-adapted to sudden changes in their environment, namely to light conditions. This has allowed them to develop mechanisms for photoprotection, which encompass alteration in pigment composition. Therefore, light modulation appears to be a suitable strategy to enhance the synthesis of specific pigments (e.g., phycocyanin) with commercial interest, in addition to conveying a more fundamental perspective on the mechanisms of acclimatization of cyanobacterium species. In this study, Synechocystis salina was accordingly cultivated in two light phase stages: (i) white LED, and (ii) shift to distinct light treatments, including white, green, and red LEDs. The type of LED lighting was combined with two intensities (50 and 150 µmol_photons·m⁻²·s⁻¹). The effects on biomass production, photosynthetic efficiency, chlorophyll a (chl a) content, total carotenoids (and profile thereof), and phycobiliproteins (including phycocyanin, allophycocyanin, and phycoerythrin) were assessed. White light (under high intensity) led to higher biomass production, growth, and productivity; this is consistent with higher photosynthetic efficiency. However, chl a underwent a deeper impact under green light (high intensity); total carotenoids were influenced by white light (high intensity); whilst red treatment had a higher effect upon total and individual phycobiliproteins. Enhanced PC productivities were found under modulation with red light (low intensities), and could be achieved 7 days earlier than in white LED (over 22 days); this finding is quite interesting from a sustainability and economic point of view. Light modulation accordingly appears to be a useful tool for supplementary studies pertaining to optimization of pigment production with biotechnological interest.

Blue light irradiation increases the relative abundance of the diatom Nitzschia palea in co-culture with cyanobacterium Microcystis aeruginosa

Lake eutrophication is associated with cyanobacterial blooms. The pennate diatom Nitzschia palea (N. palea) inhibits the growth of the cyanobacterium Microcystis aeruginosa (M. aeruginosa); therefore, increasing the relative abundance of N. palea may contribute to the inhibition of Microcystis blooms. Several studies have demonstrated that blue light irradiation promotes diatom growth and inhibits cyanobacterial growth. In this study, we evaluated the effects of blue light irradiation on N. palea and M. aeruginosa abundance. Monocultures and co-cultures of N. palea and M. aeruginosa were exposed to blue light and fluorescent light at 32 μmol photons m-2 s-1. The relative abundance of N. palea under fluorescent light decreased gradually, whereas the abundance under blue light was relatively higher (approximately 74% and 98% under fluorescent light and blue light, respectively, at the end of the experiment). The inhibition efficiency of blue light on the growth rate of M. aeruginosa was related to the light intensity. The optimal light intensity was considered 20 μmol photons m-2 s-1 based on the inhibition efficiency of 100%. Blue light irradiation can be used to increase the abundance of N. palea to control Microcystis blooms. PRACTITIONER POINTS: The effects of blue light irradiation on N. palea abundance was discussed. Monocultures and co-cultures of N. palea and M. aeruginosa were exposed to blue light and to fluorescent light. The relative abundance of N. palea increased upon irradiation with blue light in co-culture with M. aeruginosa.

Pilot scale production, extraction and purification of a thermostable phycocyanin from Synechocystis sp. PCC 6803

Phycocyanin (PC) is a soluble blue pigment-protein primarily harvested from the cyanobacterium Arthrospira platensis. PC is in high demand from several industries, but its narrow stability range limits potential applications. Here, a pilot scale (120 L total) batch production, extraction and purification process for thermostable PC (Te-PC) from a Synechocystis sp. PCC 6803 'Olive' strain expressing the PC operon cpcBACD from Thermosynechococcus elongatus BP-1 on a self-replicating vector is presented. Batch cultivation without antibiotics had no impact on growth or Te-PC production and optimisation of growth conditions resulted in Te-PC contents of 75.3 ± 1.7 mg g DW^-1. Wet biomass was harvested following chitosan-based flocculation with a 97 ± 2% efficiency, and Te-PC was extracted by high pressure homogenisation. Subsequent purification by heat-treatment and two-step ammonium sulfate precipitation removed chlorophyll and allophycocyanin contamination, resulting in Te-PC purities of 2.9 ± 0.7 and a mean Te-PC recovery of 84 ± 12%.

Blue light induces major changes in the gene expression profile of the cyanobacterium Synechocystis sp. PCC 6803

Although cyanobacteria absorb blue light, they use it less efficiently for photosynthesis than other colors absorbed by their photosynthetic pigments. A plausible explanation for this enigmatic phenomenon is that blue light is not absorbed by phycobilisomes and, hence, causes an excitation shortage at photosystem II (PSII). This hypothesis is supported by recent physiological studies, but a comprehensive understanding of the underlying changes in gene expression is still lacking. In this study, we investigate how a switch from artificial white light to blue, orange or red light affects the transcriptome of the cyanobacterium Synechocystis sp. PCC 6803. In total, 145 genes were significantly regulated in response to blue light, whereas only a few genes responded to orange and red light. In particular, genes encoding the D1 and D2 proteins of PSII, the PSII chlorophyll-binding protein CP47 and genes involved in PSII repair were upregulated in blue light, whereas none of the photosystem I (PSI) genes responded to blue light. These changes were accompanied by a decreasing PSI:PSII ratio. Furthermore, many genes involved in gene transcription and translation and several ATP synthase genes were transiently downregulated, concurrent with a temporarily decreased growth rate in blue light. After 6-7 days, when cell densities had strongly declined, the growth rate recovered and the expression of these growth-related genes returned to initial levels. Hence, blue light induces major changes in the transcriptome of cyanobacteria, in an attempt to increase the photosynthetic activity of PSII and cope with the adverse growth conditions imposed by blue light.

Sulfamethoxazole and tetracycline induced alterations in biomass, photosynthesis, lipid productivity, and proteomic expression of Synechocystis sp. PCC 6803

Since antibiotics show hormesis effects in cyanobacteria at the nanogram per liter concentration level, the possibility for two commonly used antibiotics (sulfamethoxazole and tetracycline) to increase lipid productivity in Synechocystis sp. PCC 6803 was assessed in the present study. The two target antibiotics significantly promoted (p < 0.05) the biofuel productivity of Synechocystis sp. PCC 6803 through the increase of both biomass and lipid content. Sulfamethoxazole and tetracycline significantly stimulated (p < 0.05) cyanobacterial growth by upregulating proteins related to cell differentiation, cell division, and gene expression; significantly enhanced (p < 0.05) the photosynthetic activity by upregulating photosynthesis-related proteins; and significantly increased (p < 0.05) the lipid content in cyanobacterial cells by downregulating carbohydrate catabolic proteins and carbohydrate transport proteins. Due to the altered expression pattern of biosynthesis-related proteins, the two antibiotics increased the proportion of monounsaturated fatty acids, while tetracycline reduced the proportions of saturated and polyunsaturated fatty acids. The changes in fatty acid composition may improve the combustion performance of biofuel. This study provided insights into the application of antibiotics in cyanobacteria-based biofuel production. Graphical abstract.

White and red LEDs as two-phase batch for cyanobacterial pigments production

Carotenoids and phycobiliproteins have a high economic value, due to their wide range of biological and industrial applications. The implementation of strategies to increase their production, such as the application of two-phase light cultivation systems, can stimulate pigments production, increasing economic turnover. In this sense, Cyanobium sp. was grown in seven different two-phase white/red cultivation arrangements, varying the time of each light from 0 to 21 days. Biomass, photosynthetic activity, pigments profile and antioxidant capacity were measured along time. Red light increased photosynthetic activity and pigments content (ca. 1.8-fold), and the use of a two-phase cultivation system generally raised bioactivity and production of phytochemicals. Among the studied, the optimal cultivation condition was found with 10 days of white followed by 4 days of red light. The optimized growth led to a productivity of 137.4 ± 0.8 mg L^-1 d^-1 of biomass, 17.0 ± 0.2 mg L^-1 d^-1 of total phycobiliproteins and 4.5 ± 0.2 mg L^-1 d^-1 of carotenoids.

Comparative study on attached-growth photobioreactors under blue and red lights for treatment of septic tank effluent

Attached-growth photobioreactors (AG-PBRs) employing low-cost attached-growth media were applied to treat septic tank effluent which contained abundant organic and nutrient matters as well as pathogenic microorganisms. This study investigated effects of blue and red LED lights on organic, nutrient and pathogenic removals, biomass productivity and compositions of microbial community in the AG-PBR system. The experimental results showed the blue AG-PBR to be more effective in removing chemical oxygen demand (COD), total nitrogen (TN) and ammonia nitrogen (NH₃-N) and generating biomass productivity than those of the red AG-PBR (P < 0.05). Mass balance analysis indicated that the TN and total phosphorus (TP) were removed mainly by assimilation into the biomass. The TN removal rates via nitrification and denitrification processes in the blue AG-PBR were found to be higher than that of the red AG-PBR, corresponding to the observed results of bacterial biomass and abundances of nitrifying and denitrifying bacterial species in the treatment systems. The maximal areal algal biomass productivity of 47 gDW/(m². d) in the blue AG-PBRs was found to be higher than those of other algal attached-growth systems. Although, the red and blue AG-PBR systems could effectively treat the septic tank effluent to meet the national and international discharge standards, based on treatment efficiencies and biomass productivity, the blue AG-PBR is recommended for treatment of septic tank effluent.

Effects of light color on interspecific competition between Microcystis aeruginosa and Chlorella pyrenoidosa in batch experiment

In lakes, suspended inorganic particles and dissolved substance are able to absorb or scatter different light wavelengths, leading to the changes of underwater light spectra which are highly related to the water quality. In turn, such changes could form environmental filtering for phytoplankton community to select particular algal populations via intensive competition for light resources. As an example, eutrophic lakes where underwater light spectra changed dramatically have a result of cyanobacterial blooms. In this study, in order to test the effect of light spectrum on growth and competition of green algae and cyanobacteria, Chlorella pyrenoidosa (a common green alga) and Microcystis aeruginosa (a bloom-forming cyanobacterium) grew and competed under three light colors: white (400-700 nm), red (620-700 nm), and blue (410-490 nm) light. Mono- and co-cultured systems were designed and population dynamics of the two species were monitored. The Lotka-Volterra model was used to quantify interspecific competition. Moreover, their photosynthetic activities were measured in mono-cultures. Results showed that in mono-cultures, red light was more favorable for M. aeruginosa, while blue light promoted the growth of C. pyrenoidosa. In co-cultures, M. aeruginosa won in red light and white light, while C. pyrenoidosa dominated under blue light. Light color mainly affected the absorption flux of reaction center (ABS/RC) in photosynthetic system II (PSII) and its potential photosynthetic capacity (F_v/F_m). F_v/F_m of M. aeruginosa in red light (or C. pyrenoidosa in blue light) was significantly enhanced. This study revealed that light color showed a significant influence on interspecific competition between green algae and cyanobacteria, which offers new insights into the dominance establishment and bloom formation of Microcystis.

Exploring the low photosynthetic efficiency of cyanobacteria in blue light using a mutant lacking phycobilisomes

The ubiquitous chlorophyll a (Chl a) pigment absorbs both blue and red light. Yet, in contrast to green algae and higher plants, most cyanobacteria have much lower photosynthetic rates in blue than in red light. A plausible but not yet well-supported hypothesis is that blue light results in limited energy transfer to photosystem II (PSII), because cyanobacteria invest most Chl a in photosystem I (PSI), whereas their phycobilisomes (PBS) are mostly associated with PSII but do not absorb blue photons. In this paper, we compare the photosynthetic performance in blue and orange-red light of wildtype Synechocystis sp. PCC 6803 and a PBS-deficient mutant. Our results show that the wildtype had much lower biomass, Chl a content, PSI:PSII ratio and O₂ production rate per PSII in blue light than in orange-red light, whereas the PBS-deficient mutant had a low biomass, Chl a content, PSI:PSII ratio, and O₂ production rate per PSII in both light colors. More specifically, the wildtype displayed a similar low photosynthetic efficiency in blue light as the PBS-deficient mutant in both light colors. Our results demonstrate that the absorption of light energy by PBS and subsequent transfer to PSII are crucial for efficient photosynthesis in cyanobacteria, which may explain both the low photosynthetic efficiency of PBS-containing cyanobacteria and the evolutionary success of chlorophyll-based light-harvesting antennae in environments dominated by blue light.

Blue light reduces photosynthetic efficiency of cyanobacteria through an imbalance between photosystems I and II

Several studies have described that cyanobacteria use blue light less efficiently for photosynthesis than most eukaryotic phototrophs, but comprehensive studies of this phenomenon are lacking. Here, we study the effect of blue (450 nm), orange (625 nm), and red (660 nm) light on growth of the model cyanobacterium Synechocystis sp. PCC 6803, the green alga Chlorella sorokiniana and other cyanobacteria containing phycocyanin or phycoerythrin. Our results demonstrate that specific growth rates of the cyanobacteria were similar in orange and red light, but much lower in blue light. Conversely, specific growth rates of the green alga C. sorokiniana were similar in blue and red light, but lower in orange light. Oxygen production rates of Synechocystis sp. PCC 6803 were five-fold lower in blue than in orange and red light at low light intensities but approached the same saturation level in all three colors at high light intensities. Measurements of 77 K fluorescence emission demonstrated a lower ratio of photosystem I to photosystem II (PSI:PSII ratio) and relatively more phycobilisomes associated with PSII (state 1) in blue light than in orange and red light. These results support the hypothesis that blue light, which is not absorbed by phycobilisomes, creates an imbalance between the two photosystems of cyanobacteria with an energy excess at PSI and a deficiency at the PSII-side of the photosynthetic electron transfer chain. Our results help to explain why phycobilisome-containing cyanobacteria use blue light less efficiently than species with chlorophyll-based light-harvesting antennae such as Prochlorococcus, green algae and terrestrial plants.

Influence of culture conditions on extracellular polysaccharide production and the activities of enzymes involved in the polysaccharide synthesis of Nostoc flagelliforme

Important enzymes influencing the production ofNostoc flagelliformeEPS were investigated under different culture conditions.