Blue Light, a Positive Switch Signal for Nitrate and Nitrite Uptake by the Green Alga Monoraphidium braunii

Impact of green and blue-green light on the growth, pigment concentration, and fatty acid unsaturation in the microalga Monoraphidium braunii

The spectral composition of light is an important factor for the metabolism of photosynthetic organisms. Several blue light-regulated metabolic processes have already been identified in the industrially relevant microalga Monoraphidium braunii. However, little is known about the spectral impact on this species' growth, fatty acid (FA), and pigment composition. In this study, M. braunii was cultivated under different light spectra (white light: 400-700 nm, blue light: 400-550 nm, green light: 450-600 nm, and red light: 580-700 nm) at 25°C for 96 h. The growth was monitored daily. Additionally, the FA composition, and pigment concentration was analyzed after 96 h. The highest biomass production was observed upon white light and red light irradiation. However, green light also led to comparably high biomass production, fueling the scientific debate about the contribution of weakly absorbed light wavelengths to microalgal biomass production. All light spectra (white, blue, and green) that comprised blue-green light (450-550 nm) led to a higher degree of FA unsaturation and a greater concentration of all identified pigments than red light. These results further contribute to the growing understanding that blue-green light is an essential trigger for maximized pigment concentration and FA unsaturation in green microalgae.

Adaptation versus plastic responses to temperature, light, and nitrate availability in cultured snow algal strains

Snow algal blooms are widespread, dominating low temperature, high light, and oligotrophic melting snowpacks. Here, we assessed the photophysiological and cellular stoichiometric responses of snow algal genera Chloromonas spp. and Microglena spp. in their vegetative life stage isolated from the Arctic and Antarctic to gradients in temperature (5 - 15°C), nitrate availability (1 - 10 µmol L-1), and light (50 and 500 µmol photons m-2 s-1). When grown under gradients in temperature, measured snow algal strains displayed Fv/Fm values increased by ∼115% and electron transport rates decreased by ∼50% at 5°C compared to 10 and 15°C, demonstrating how low temperatures can mimic high light impacts to photophysiology. When using carrying capacity as opposed to growth rate as a metric for determining the temperature optima, these snow algal strains can be defined as psychrophilic, with carrying capacities ∼90% higher at 5°C than warmer temperatures. All strains approached Redfield C:N stoichiometry when cultured under nutrient replete conditions regardless of temperature (5.7 ± 0.4 across all strains), whereas significant increases in C:N were apparent when strains were cultured under nitrate concentrations that reflected in situ conditions (17.8 ± 5.9). Intra-specific responses in photophysiology were apparent under high light with Chloromonas spp. more capable of acclimating to higher light intensities. These findings suggest that in situ conditions are not optimal for the studied snow algal strains, but they are able to dynamically adjust both their photochemistry and stoichiometry to acclimate to these conditions.

Light-Dependent Nitrate Removal Capacity of Green Microalgae

In the present study, Chlamydomonas sp. MACC-216 was used to investigate total nitrate removal in TAP medium with sodium nitrate as the sole nitrogen source under several light conditions made up of permuted combinations of three light colors (referred to as blue, red, and white light) and three light intensities (50 µmol m^-2 s^-1, 100 µmol m^-2 s^-1, and 250 µmol m^-2 s^-1). It was observed that nitrate removal efficiency is influenced by light color as well as light intensity. Additionally, Chlamydomonas sp. MACC-216 was cultivated in synthetic wastewater under four light conditions, namely, Blue 250, Blue 125 + Red 125, Red 250, and White 250, where it showed the highest nitrate removal efficiency and nitrate reductase activity under the Blue 125 + Red 125 light condition. To observe the impact of light color on the nitrate removal capacity of Chlamydomonas sp. MACC-216, the expression of five genes participating in nitrate transport and reduction (NRT1, NRT2.1, NRT2.2, NIA, and MCP) was also analyzed; these genes showed the highest expression under the Blue 125 + Red 125 light condition. Based on the above-mentioned findings, the blue + red light combination emerged as a promising light combination for nitrate removal. Hence, our study suggests the importance of the blue + red light combination together with high light intensity, as the optimal light condition for nitrate removal from synthetic wastewater in comparison to other monochromatic lights with high light intensity.

Blue-green light is required for a maximized fatty acid unsaturation and pigment concentration in the microalga Acutodesmus obliquus

Blue-green light is known to maximize the degree of fatty acid (FA) unsaturation in microalgae. However, knowledge on the particular waveband responsible for this stimulation of FA desaturation and its impact on the pigment composition in microalgae remains limited. In this study, Acutodesmus obliquus was cultivated for 96 h at 15°C with different light spectra (380-700 nm, 470-700 nm, 520-700 nm, 600-700 nm, and dark controls). Growth was monitored daily, and qualitative characterization of the microalgal FA composition was achieved via gas chromatography coupled with electron impact ionization mass spectrometry (GC-EI/MS). Additionally, a quantitative analysis of microalgal pigments was performed using high-performance liquid chromatography with diode array detection (HPLC-DAD). Spectra that included wavelengths between 470 and 520 nm led to a significantly higher percentage of the polyunsaturated fatty acids (PUFA) 18:3 and 16:4, compared to all other light conditions. However, no significant differences between the red light cultivations and the heterotrophic dark controls were observed for the FA 18:3 and 16:4. These results indicate, that exclusively the blue-green light waveband between 470 and 520 nm is responsible for a maximized FA unsaturation in A. obliquus. Furthermore, the growth and production of pigments were impaired if blue-green light (380-520 nm) was absent in the light spectrum. This knowledge can contribute to achieving a suitable microalgal pigment and FA composition for industrial purposes and must be considered in spectrally selective microalgae cultivation systems.

Growth and fatty acid composition of Acutodesmus obliquus under different light spectra and temperatures

The combined impact of temperature and light spectra on the fatty acid (FA) composition in microalgae has been sparsely investigated. The aim of this study was to investigate the interactions of light and temperature on the FA composition in Acutodesmus obliquus. For this purpose, A. obliquus was cultivated with different temperatures (20, 30, and 35°C), as well as broad light spectra (blue, green, and red light). Growth and FA composition were monitored daily. Microalgal FA were extracted, and a qualitative characterization was done by gas chromatography coupled with electron impact ionization mass spectrometry (GC-EI/MS). Compared to red light, green and blue light caused a higher percentage of the polyunsaturated fatty acids (PUFA) 16:4, 18:3, and 18:4, at all temperatures. The highest total percentage of these PUFA were observed at the lowest cultivation temperature and blue and green light. These data imply that a combination of lower temperatures and blue-green light (450-550 nm) positively influences the activity of specific FA-desaturases in A. obliquus. Additionally, a lower 16:1 trans/cis ratio was observed upon green and blue light treatment and lower cultivation temperatures. Remarkably, green light treatment resulted in a comparably high growth under all tested conditions. Therefore, a higher content of green light, compared to blue light might additionally lead to a higher biomass concentration. Microalgae cultivation with low temperatures and green light might therefore result in a suitable FA composition for the food industry and a comparably high biomass production.

Transcriptomic and proteomic choreography in response to light quality variation reveals key adaption mechanisms in marine Nannochloropsis oceanica

Photosynthetic organisms need to respond frequently to the fluctuation of light quality and light quantity in their habitat. In response to the fluctuation of different single wavelength lights, these organisms have to adjust and optimize the employment of light energy by redistributing excitation energy and remodeling photosystem stoichiometry or light complex structure. However, the response of whole cellular processes to fluctuations in single wavelength light is mostly unknown. Here, we report the transcriptomic and proteomic dynamics and metabolic adaptation mechanisms of Nannochloropsis oceanica to blue and red light. Preferential exposure to different light spectra induces massive reprogramming of the Nannochloropsis transcriptome and proteome. Combined with physiological and biochemical investigation, the rewiring of many cellular processes was observed, including carbon/nitrogen assimilation, photosynthesis, chlorophyll and cartenoid biosynthesis, reactive oxygen species (ROS) scavenging systems, and chromatin state regulation. A strong and rapid regulation of genes or proteins related to nitrogen metabolism, photosynthesis, chlorophyll synthesis, ROS scavenging system, and carotenoid metabolism were observed during 12 h and 24 h of exposure under red light. Additionally, two light harvesting complex proteins induced by blue light and one by red light were observed. The differential responses of N. oceanica to red and blue irradiation reveal how marine microalgae adapt to change in light quality and can be exploited for biofuel feedstock development.

The effect of light quality and quantity on carbon allocation in Chromera velia

Chromera velia is a marine photosynthetic relative of human apicomplexan parasites. It has been isolated from coral reefs and is indicted for being involved in symbioses with hermatypic corals. C. velia has been subject to intensive research, but still very little is known of its response to light quality and quantity. Here, we have studied the growth and compositional responses of C. velia to culture under monochromatic light (blue, green or red), at two photon flux densities (PFD, 20 and 100 μmol photons m^-2 s^-1). Our results show that C. velia growth rate is unaffected by the quality of light, whereas it responds to PFD. However, light quality influenced cell size, which was smaller for cells exposed to blue monochromatic light, regardless of PFD. PFD strongly influenced carbon allocation: at 20 μmol photons m^-2 s^-1, carbon was mainly allocated into proteins while at 100 μmol photons m^-2 s^-1, carbon was allocated mainly into carbohydrate and lipid pools. The blue light treatment caused a decrease in the lipids and carbohydrates to proteins and thus suggested to affect nitrogen metabolism in acclimated cells. Whole-cell absorption spectra revealed the existence of red-shifted chlorophyll a antenna not only under red light but in all low PFD treatments. These findings show the ability of C. velia to successfully adapt and thrive in spectrally very different environments of coral reefs.

Light signalling-induced regulation of nutrient acquisition and utilisation in plants

Light is the foremost regulator of plant growth and development, and the critical role of light signalling in the promotion of nutrient uptake and utilisation was clarified in recent decades. Recent studies with Arabidopsis demonstrated the molecular mechanisms underlying such promotive effects and uncovered the pivotal role of the transcription factor ELONGATED HYPOCOTYL5 (HY5) whose activity is under the control of multiple photoreceptors. Together with a recent finding that phytochrome B, one of photoreceptors, is activated in subterranean plant parts, the discovery that HY5 directly promotes the transcription of genes involved in nutrient uptake and utilisation, including several nitrogen and sulphur assimilation-related genes, expands our understanding of the ways in which light signalling effectively and co-ordinately modulates uptake and utilisation of multiple nutrients in plants. This review presents a summary of the current knowledge regarding light signalling-induced regulation of nutrient uptake and utilisation.

NH4+ enrichment and UV radiation interact to affect the photosynthesis and nitrogen uptake of Gracilaria lemaneiformis (Rhodophyta)

Solar ultraviolet radiation (UVR, 280-400 nm) is known to inhibit the photosynthesis of macroalgae, whereas nitrogen availability may alter the sensitivity of the algae to UVR. Here, we show that UV-B (280-315 nm) significantly reduced the net photosynthetic rate of Gracilaria lemaneiformis. This inhibition was alleviated by enrichment with ammonia, which also caused a decrease in dark respiration. The presence of both UV-A (315-400 nm) and UV-B stimulated the accumulation of UV-absorbing compounds. However, this stimulation was not affected by enrichment with ammonia. The content of phycoerythrin (PE) was increased by the enrichment of ammonia only in the absence of UVR. Ammonia uptake and the activity of nitrate reductase were repressed by UVR. However, exposure to UVR had an insignificant effect on the rate of nitrate uptake. In conclusion, increased PE content associated with ammonia enrichment played a protective role against UVR in this alga, and UVR differentially affected the uptake of nitrate and ammonia.

MACRONUTRIENT UTILIZATION BY PHOTOSYNTHETIC EUKARYOTES AND THE FABRIC OF INTERACTIONS

Organisms acclimate to a continually fluctuating nutrient environment. Acclimation involves responses specific for the limiting nutrient as well as responses that are more general and occur when an organism experiences different stress conditions. Specific responses enable organisms to efficiently scavenge the limiting nutrient and may involve the induction of high-affinity transport systems and the synthesis of hydrolytic enzymes that facilitate the release of the nutrient from extracellular organic molecules or from internal reserves. General responses include changes in cell division rates and global alterations in metabolic activities. In photosynthetic organisms there must be precise regulation of photosynthetic activity since when severe nutrient limitation prevents continued cell growth, excitation of photosynthetic pigments could result in the formation of reactive oxygen species, which can severely damage structural and functional features of the cell. This review focuses on ways that photosynthetic eukaryotes assimilate the macronutrients nitrogen, sulfur, and phosphorus, and the mechanisms that govern assimilatory activities. Also discussed are molecular responses to macronutrient limitation and the elicitation of those responses through integration of environmental and cellular cues.

Acclimation of Chlamydomonas reinhardtii to its nutrient environment

To cope with low nutrient availability in nature, organisms have evolved inducible systems that enable them to scavenge and efficiently utilize the limiting nutrient. Furthermore, organisms must have the capacity to adjust their rate of metabolism and make specific alterations in metabolic pathways that favor survival when the potential for cell growth and division is reduced. In this article I will focus on the acclimation of Chlamydomonas reinhardtii, a unicellular, eukaryotic green alga to conditions of nitrogen, sulfur and phosphorus deprivation. This organism has a distinguished history as a model for classical genetic analyses, but it has recently been developed for exploitation using an array of molecular and genomic tools. The application of these tools to the analyses of nutrient limitation responses (and other biological processes) is revealing mechanisms that enable Chlamydomonas to survive harsh environmental conditions and establishing relationships between the responses of this morphologically simple, photosynthetic eukaryote and those of both nonphotosynthetic organisms and vascular plants.

Oxidoreductases in plant plasma membranes

Electron transporting oxidoreductases at biological membranes mediate several physiological processes. While such activities are well known and widely accepted as physiologically significant for other biological membranes, oxidoreductase activities found at the plasma membrane of plants are still being neglected. The ubiquity of the oxidoreductases in the plasma membrane suggests that the activity observed is of major importance in fact up to now no plant without redox activity at the plasmalemma is known. Involvement in proton pumping, membrane energization, ion channel regulation, iron reduction, nutrient uptake, signal transduction, and growth regulation has been proposed. However, positive proof for one of the numerous theories about the physiological function of the system is still missing. Evidence for an involvement in signalling and regulation of growth and transport activities at the plasma membrane is strong, but the high activity of the system displayed in some experiments also suggests function in defense against pathogens.