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Heterosigma akashiwo is a raphidophyte known for forming ichthyotoxic blooms. In order to predict the potential impacts of rising CO 2 on H. akashiwo it is necessary to understand the factors influencing growth rates over a range of CO 2 concentrations. Here we examined the physiology and gene expression response of H. akashiwo to concentrations from 200 to 1000 ppm CO 2. Growth rate data were combined from this and previous studies and fit with a CO 2 limitation-inhibition model that revealed an apparent growth optimum around 600–800 ppm CO 2. Physiological changes included a significant increase in C:N ratio at ∼800 ppm CO 2 and a significant decrease in hydrogen peroxide concentration at ∼1000 ppm. Whole transcriptome sequencing of H. akashiwo revealed sharp distinctions in metabolic pathway gene expression between ∼600 and ∼800 ppm CO 2. Hierarchical clustering by co-expression identified groups of genes with significant correlations to CO 2 and growth rate. Genes with significant differential expression with CO 2 included carbon concentrating mechanism genes such as beta-carbonic anhydrases and a bicarbonate transporter, which may underpin shifts in physiology. Genes involved in cell motility were significantly changed by both elevated CO 2 and growth rate, suggesting that future ocean conditions could modify swimming behavior in this species. [ABSTRACT FROM AUTHOR]

Microalgae can tolerate different CO 2 concentrations and its mechanism of rapid CO 2 fixation is key to understanding the relationship between intracellular substance and energy conversion, as well as to optimize the production of microalgae bioproducts. The present study conducted a transcriptome-based analysis of gene expression of an industrial Chlorella pyrenoidosa strain, FACHB-9, with high oil production ability under different CO 2 concentrations. Transcriptome-based gene expression analysis indicated the redirection of central metabolism under CO 2 deprivation. The C3 pathway served as the main metabolic pathway for C. pyrenoidosa , which was subjected to a high CO 2 environment in the present study. Similar to C4 plants wherein limited CO 2 activates CO 2 -concentrating mechanism to compensate for the low activity of RuBisCO in the Calvin cycle, C. pyrenoidosa undergoes CO 2 compensation by active transport to ensure sufficient amounts of Ci for its growth and metabolism. Comparative analysis has allowed the identification of several candidate genes for further strain improvement. These genes encode proteins that might be CAs, as indicated by its localization to the cell membrane or chloroplast membrane, or act as Ci transporters that assist Ci transmembrane transportation. Certain types of ABC transport proteins and CCCH-type zinc finger proteins also showed significant changes in gene expression. These findings indicate that these may be promising targets for functional and genetic modification studies. The results not only reveal the significance of the mechanism of carbon sequestration to eukaryotic green algae, but also provide a basis for further construction of the energy microalgae cell factory with high efficiency carbon biofixation capacity. [ABSTRACT FROM AUTHOR]

Carbon dioxide (CO 2) and light are essential for high photosynthetic rates of microalgal cultures. Microalgal photosynthetic behavior at low CO 2 concentrations has not been revealed yet at the same level of detail as leaf photosynthesis. In the present study, we investigated the short-term photosynthetic response of suspended Chlorella sorokiniana to limiting light intensity and CO 2 concentration. We used a novel CO 2 -based photosynthesis monitor originating from leaf research but equipped with an aquatic chamber sparged with CO 2 enriched air. Photosynthesis was measured by employing a steady-state CO 2 mass balance over the chamber. Light and carbon response curves were determined under constant pH, temperature, dissolved oxygen, light intensity or dissolved carbon dioxide. We determined the volumetric mass transfer coefficient of the aquatic chamber to accurately convert gaseous CO 2 partial pressure into aqueous CO 2 concentration to evaluate the CO 2 response measurements. Light response measurements on dilute algal cultures revealed a high photosynthetic capacity on a time scale of minutes which by far exceeded the average CO 2 uptake on a time scale of hours (cell growth). Light response measurements with dense and fully absorbing cultures provided accurate insight into the response of microalgal mass culture to changing incident irradiance, including the efficiency of photosynthesis of the algal culture as a whole. CO 2 response measurements demonstrated severe CO 2 limitation at dissolved CO 2 levels of <20 μM giving a concrete target for optimization of CO2 supply in large-scale cultivation systems. Measurements at a background of either 21 % O 2 (air) or 2 % O 2 indicated the existence of photorespiration in Chlorella. Moreover, the magnitude of photorespiration first increased and then decreased again while reduced dissolved CO 2 levels, indicating the activation of a carbon concentrating mechanism at low CO 2. Simultaneous measurement of the quantum yield of PSII linear electron transport (ϕ PSII) and CO 2 uptake revealed the transition of CO 2 limited photosynthesis towards light-limited photosynthesis. • Algal photosynthetic response to limiting light intensity and CO 2 concentration (81 characters) • Photosynthetic response measurement requires apt choice of biomass concentration (82 characters). • CO 2 response at different O 2 concentrations suggested photorespiration in Chlorella (85 characters). • Activation of carbon concentrating mechanism amid measurements was indicated (78 characters). • Monod equation is too simple to describe algal photosynthetic responses precisely (83 characters). [ABSTRACT FROM AUTHOR]

Abstract: Coccolithophorid algae (Haptophycea) are mainly marine unicellular phytoplankton. The coccolithophorids are of global interest as they can fix carbon by photosynthesis as well as in calcium carbonate (coccoliths). They are the largest carbon sinks and one of the largest primary producers on the planet. They can also produce high amounts of lipids which have a high potential application as a renewable fuel and alternative food source. This paper reviews current knowledge on coccolithophorid algae photosynthesis and calcification and their potential industrial applications. [Copyright &y& Elsevier]

The availability of inorganic carbon (C i) as the source for photosynthesis is fluctuating in aquatic environments. Despite the involvement of transcriptional regulators CmpR and NdhR in regulating genes encoding C i transporters at limiting CO 2 , the C i -sensing mechanism is largely unknown among cyanobacteria. Here we report that a cAMP-dependent transcription factor SyCRP1 mediates C i response in Synechocystis. The mutant ∆ sycrp1 exhibited a slow-growth phenotype and reduced maximum rate of bicarbonate-dependent photosynthetic electron transport (V max) compared to wild-type at the scarcity of CO 2. The number of carboxysomes was decreased significantly in the ∆ sycrp1 at low CO 2 consistent with its reduced V max. The DNA microarray analysis revealed the upregulation of genes encoding C i transporters in ∆sycrp1. The membrane-localized SyCRP1 was released into the cytosol in wild-type cells shifted from low to high CO 2 or upon cAMP treatment. Soluble His-tagged SyCRP1 was shown to target DNA-binding sites upstream of the C i -regulated genes sbtA and ccmK 3. In addition, cAMP enhanced the binding of SyCRP1 to its target sites. Our data collectively suggest that the C i is sensed through the second messenger cAMP releasing membrane-bound SyCRP1 into cytoplasm under sufficient CO 2 conditions. Hence, SyCRP1 is a possible regulator of carbon concentrating mechanism, and such a regulation might be mediated via sensing C i levels through cAMP in Synechocystis. • It has been known that cAMP receptor proteins (CRP) play a crucial role in regulation of gene expression in bacteria. • Transcription regulators of CCM genes were decribed previously, however C i -sensing mechanism in cyanobacteria is unclear. • A membrane associated Synechocystis CRP (SyCRP1), releases to cytosol when bound to CAMP to regulate CCM. • Thus, SyCRP1 mediates C i response through the signalling molecule cAMP in Synechocytis. [ABSTRACT FROM AUTHOR]

Microalgae are promising alternative feedstocks that can be used to produce biofuels and replace conventional fossil fuels. In order to increase the lipid production of microalgae, strain development via genetic engineering is required. To date, metabolic engineering studies have mainly focused on lipid synthesis pathways. However, the complicated metabolic pathways that exist in many organelles along with limitations of genetic engineering tools have made it difficult to obtain desirable microalgal strains. As an alternative strategy, we tried to increase carbon flux into fatty acid synthesis by overexpressing the NADP-dependent malic enzyme, NsME1, in Nannochloropsis salina. We found that the biomass and fatty acid methyl ester (FAME) contents were increased in NsME1 overexpressing transformants, such that the FAME yield of the top-producing NsME1 transformant was 53% higher than the wild type. To understand the effects of NsME1 on lipid production, we analyzed the total carbon concentration and NADPH/NADP ratio, which were found to be enhanced in the transformants. We also investigated mRNA expression levels of genes involved in C4-like carbon concentrating mechanism and fatty acid synthesis by quantitative real-time PCR, and confirmed their positive contribution to fatty acid production. Taken together, our results demonstrate that overexpression of NsME1 could simultaneously improve the carbon concentration and reducing power in cells, thereby increasing the lipid and FAME yields of Nannochloropsis salina. We suggest that NsME1 can serve as a promising genetic target for enhancing lipid-based bio-products in Nannochloropsis and potentially other industrial microalgae. • NsME1 overexpression affected total carbon concentration in N. salina culture. • NsME1 overexpression increased NADPH/NADP ratio. • NsME1 overexpression increased dry cell weight and fatty acid contents. [ABSTRACT FROM AUTHOR]