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Microalgae have a growing recognition of generating biomass and capturing carbon in the form of CO2. The genus Chlorella has especially attracted scientists’ attention due to its versatility in algal mass cultivation systems and its potential in mitigating CO2. However, some aspects of how these green microorganisms respond to increasing concentrations of CO2 remain unclear. In this work, we analyzed Chlorella sorokiniana and Chlorella vulgaris cells under low and high CO2 levels. We monitored different processes related to carbon flux from photosynthetic capacity to carbon sinks. Our data indicate that high concentration of CO2 favors growth and photosynthetic capacity of the two Chlorella strains. Different metabolites related to the tricarboxylic acid cycle and ATP levels also increased under high CO2 concentrations in Chlorella sorokiniana, reaching up to two-fold compared to low CO2 conditions. The signaling molecules, inositol polyphosphates, that regulate photosynthetic capacity in green microalgae were also affected by the CO2 levels, showing a deep profile modification of the inositol polyphosphates that over-accumulated by up to 50% in high CO2 versus low CO2 conditions. InsP4 and InsP6 increased 3- and 0.8-fold, respectively, in Chlorella sorokiniana after being subjected to 5% CO2 condition. These data indicate that the availability of CO2 could control carbon flux from photosynthesis to carbon storage and impact cell signaling integration and energy levels in these green cells. The presented results support the importance of further investigating the connections between carbon assimilation and cell signaling by polyphosphate inositols in microalgae to optimize their biotechnological applications.

The characterization of the molecular mechanisms, such as high light irradiance resistance, that allowed plant terrestralization is a cornerstone in evolutionary studies since the conquest of land by plants played a pivotal role in life evolution on Earth. Viridiplantae or the green lineage is divided into two clades, Chlorophyta and Streptophyta, that in turn splits into Embryophyta or land plants and Charophyta. Charophyta are used in evolutionary studies on plant terrestralization since they are generally accepted as the extant algal species most closely related to current land plants. In this study, we have chosen the facultative terrestrial early charophyte alga Klebsormidium nitens to perform an integrative transcriptomic and metabolomic analysis under high light in order to unveil key mechanisms involved in the early steps of plants terrestralization. We found a fast chloroplast retrograde signaling possibly mediated by reactive oxygen species and the inositol polyphosphate 1-phosphatase (SAL1) and 3′-phosphoadenosine-5′-phosphate (PAP) pathways inducing gene expression and accumulation of specific metabolites. Systems used by both Chlorophyta and Embryophyta were activated such as the xanthophyll cycle with an accumulation of zeaxanthin and protein folding and repair mechanisms constituted by NADPH-dependent thioredoxin reductases, thioredoxin-disulfide reductases, and peroxiredoxins. Similarly, cyclic electron flow, specifically the pathway dependent on proton gradient regulation 5, was strongly activated under high light. We detected a simultaneous co-activation of the non-photochemical quenching mechanisms based on LHC-like stress related (LHCSR) protein and the photosystem II subunit S that are specific to Chlorophyta and Embryophyta, respectively. Exclusive Embryophyta systems for the synthesis, sensing, and response to the phytohormone auxin were also activated under high light in K. nitens leading to an increase in auxin content with the concomitant accumulation of amino acids such as tryptophan, histidine, and phenylalanine.

The optimization of the operation, under the climatic conditions of southern Spain, of an experimental plant for β-carotene production by Dunaliella has been pursued. The effects of mixing, culture depth, cell density and dilution cycles on β-carotene and biomass productivity were studied under a semicontinuous culture regime in open tanks outdoors. Using 3 m2-surface containers, the highest productivity values, for both β-carotene and biomass, were recorded with a flow rate of 0.55 m s−1; 10 cm depth; 0.7 − 0.9 × 106cell ml−1, population density; and dilution cycles of two days. An average annual productivity of 1.65 g (dry wt) m−2 d−1 was estimated for Dunaliella biomass, being that for β-carotene of about 0.1 g m−2 d−1. Under these optimized conditions, experiments have been carried out at the Cadiz Bay with 20 m2-surface tanks during a whole-year cycle. The results obtained have validated this location and the operating conditions established as being most appropriate for efficient mass production of β-carotene rich D. salina.

The influence of nitrate and ammonium assimilation on glycogen metabolism has been determined in intact Anacystis nidulans cell actively fixing CO2. Assimilation of nitrate or ammonium resulted in significant decreases in both the incorporation into glycogen of newly fixed carbon and the accumulation of glycogen by the cells, the magnitude of these effects depending on the light intensity. The depression in glycogen synthesis induced by nitrogen assimilation was more marked at low light intensity, and especially when ammonium was the nitrogen source. Under these conditions, specific radioactivity of the glycogen pool was particularly high, indicating enhanced turnover of glycogen. Thus, in addition to a more general depressing effect of nitrogen assimilation on the carbon flow to glycogen, degradation of glycogen appears to be stimulated by ammonium assimilation at low (but not at high) light intensity.

Summary The effect of free Cd 2+ ions on growth, photosynthesis, photosynthetic pigments (chlorophyll and phycobiliproteins), nitrate and ammonium uptake and divalent cation (Mg 2+ , Ca 2+ ) content in Anabaena PCC 7119 grown in a medium containing nitrate or ammonium was investigated. After 96 h of exposure, the metal causes an inhibition of photosynthesis in cells of Anabaena PCC 7119 growing in NH 4 + and NO 3 - media and a decrease in the photosynthetic pigments. Likewise, Cd 2+ toxicity elicited a reduction in the intracellular concentrations of Mg 2+ and Ca 2+ and an inhibition in ammonium uptake, resulting in changes in the cellular structure. The possible relation between the differential response in each culture medium and pH is discussed.

Nitrogenase activity of Anabaena PCC 7119 is inhibited under conditions of boron deficiency. To elucidate the mechanisms of this inhibition, this study examined how the deficiency of boron affected photosynthesis, photosynthetic pigments, the enzymes of the oxidative pentose phosphate pathway, and respiration of Anabaena PCC 7119 cultures. After 24 to 48 hours of boron deficiency, reductions in photosynthetic O(2) evolution and in CO(2) fixation were observed. At the same time, the activities of oxidative pentose phosphate pathway enzymes and respiration increased significantly with boron deficiency. No change was observed in these processes when assays were performed after 4 to 6 hours of deficiency, a time at which nitrogenase activity was severely inhibited. These results suggest that the requirement for boron in N(2) fixation is independent of its effects on photosynthesis and reductant supply.

The effect of boron on heterocystous and nonheterocystous dinitrogen fixing Cyanobacteria was examined. The absence of boron in culture media inhibited growth and nitrogenase activity in Nodularia sp., Chlorogloeopsis sp., and Nostoc sp. cultures. Examinations of boron-deficient cultures showed changes in heterocyst morphology. However, cultures of nonheterocystous Cyanobacteria, Gloeothece sp. and Plectonema sp., grown in the absence of boron did not show any alteration in growth or nitrogenase activity. These results suggest a requirement of boron only by heterocystous Cyanobacteria. A possible role for this element in the early evolution of photosynthetic organisms is proposed.

Summary The requirement of sodium for photosynthesis and nitrate assimilation in a mutant of Nostoc muscorum, unable to fix dinitrogen, was examined. After 24 h of culture, sodium deficient cells showed changes in the contents of photosynthetic pigments as well as in photosynthesis and nitrate assimilation. Both CO2 consumption and photosynthetic oxygen evolution were reduced when cells were cultured for 24 h in absence of sodium. Similarly nitrate uptake was inhibited in sodium deficient cells. Nitrate reductase activity however was increased by 37% under these conditions. When assays were performed after 2 h of deficiency only the photosynthetic activities (both photofixation of CO2 and photoevolution of O2) were altered by the absence of sodium. These results suggest that the impairment of nitrate assimilation is a secondary effect of sodium deficiency and that the requirement of sodium for photosynthesis is independent of its effects on nitrate assimilation.

The effect of boron on nitrogenase activity has been studied. When cells were dependent on N(2) fixation, the lack of boron inhibited nitrogenase activity. However, under anaerobic conditions or in the presence of Na-dithionite this effect was not observed. Nitrogenase synthesis was not affected by boron deficiency. Similarly, the heterocyst number was not altered. Examination of boron-deficient cultures showed, however, some dramatic changes in heterocyst morphology. The increased activity of those enzymes related to the maintaining of the low intracellular level of toxic oxygen species (superoxide dismutase, catalase, and peroxidase) support our hypothesis of the role of boron in heterocyst envelope stabilization.