Initiation of efficient C4 pathway in response to low ambient CO2 during the bloom period of a marine dinoflagellate

Impacts of phosphoenolpyruvate carboxylase gene silencing on photosynthetic efficiency and carbon fixation in Skeletonema costatum

Diatoms play a crucial role in marine primary productivity through carbon fixation, which is essential for understanding the operation of marine biological pumps and carbon sinks. This study focuses on the phosphoenolpyruvate carboxylase (PEPC) gene, a key enzyme in the carbon assimilation pathway of diatoms, by investigating the consequences of its silencing in Skeletonema costatum. Through this approach, we aimed to clarify the distinct contributions of PEPC to the overall carbon fixation process. The mutant strains of S. costatum were subjected to thorough analysis to identify any shifts in physiological behavior, alterations in the gene expression of key carbon-fixing enzymes, and changes in the associated enzyme activities. Notably, the inhibition of the PEPC gene did not significantly affect the growth rate of S. costatum; however, it did have a notable impact on the photosynthetic apparatus, as evidenced by a reduction in the maximal electron transport rate and a decline in light utilization efficiency. A significant decrease was observed in both the enzymatic activity and gene expression of PEPCase. This down-regulation also affected other enzymes integral to the carbon fixation pathway, such as phosphoenolpyruvate carboxykinase and pyruvate-phosphate dikinase, indicating a wider metabolic perturbation. In contrast, the expression and activity of the Rubisco enzyme suggested that some facets of carbon fixation remained resilient. Furthermore, the substantial upregulation of carbonic anhydrase expression and activity probably represented an adaptive mechanism to sustain the inorganic carbon supply necessary for the carboxylation process of Rubisco. This research not only underscores the pivotal role of the PEPC gene in the carbon fixation of S. costatum but also expands our comprehension of carbon fixation mechanisms in diatoms.

Usage of Chlorella and diverse microalgae for CO2 capture - towards a bioenergy revolution

To address climate change threats to ecosystems and the global economy, sustainable solutions for reducing atmospheric carbon dioxide (CO2) levels are crucial. Existing CO2 capture projects face challenges like high costs and environmental risks. This review explores leveraging microalgae, specifically the Chlorella genus, for CO2 capture and conversion into valuable bioenergy products like biohydrogen. The introduction section provides an overview of carbon pathways in microalgal cells and their role in CO2 capture for biomass production. It discusses current carbon credit industries and projects, highlighting the Chlorella genus's carbon concentration mechanism (CCM) model for efficient CO2 sequestration. Factors influencing microalgal CO2 sequestration are examined, including pretreatment, pH, temperature, irradiation, nutrients, dissolved oxygen, and sources and concentrations of CO2. The review explores microalgae as a feedstock for various bioenergy applications like biodiesel, biooil, bioethanol, biogas and biohydrogen production. Strategies for optimizing biohydrogen yield from Chlorella are highlighted. Outlining the possibilities of further optimizations the review concludes by suggesting that microalgae and Chlorella-based CO2 capture is promising and offers contributions to achieve global climate goals.

Multifaceted Dinoflagellates and the Marine Model Prorocentrum cordatum

BACKGROUND

Dinoflagellates are a monophyletic group within the taxon Alveolata, which comprises unicellular eukaryotes. Dinoflagellates have long been studied for their organismic and morphologic diversity as well as striking cellular features. They have a main size range of 10-100 µm, a complex "cell covering", exceptionally large genomes (∼1-250 Gbp with a mean of 50,000 protein-encoding genes) spread over a variable number of highly condensed chromosomes, and perform a closed mitosis with extranuclear spindles (dinomitosis). Photosynthetic, marine, and free-living Prorocentrum cordatum is a ubiquitously occurring, bloom-forming dinoflagellate, and an emerging model system, particularly with respect to systems biology.

SUMMARY

Focused ion beam/scanning electron microscopy (FIB/SEM) analysis of P. cordatum recently revealed (i) a flattened nucleus with unusual structural features and a total of 62 tightly packed chromosomes, (ii) a single, barrel-shaped chloroplast devoid of grana and harboring multiple starch granules, (iii) a single, highly reticular mitochondrion, and (iv) multiple phosphate and lipid storage bodies. Comprehensive proteomics of subcellular fractions suggested (i) major basic nuclear proteins to participate in chromosome condensation, (ii) composition of nuclear pores to differ from standard knowledge, (iii) photosystems I and II, chloroplast complex I, and chlorophyll a-b binding light-harvesting complex to form a large megacomplex (>1.5 MDa), and (iv) an extraordinary richness in pigment-binding proteins. Systems biology-level investigation of heat stress response demonstrated a concerted down-regulation of CO2-concentrating mechanisms, CO2-fixation, central metabolism, and monomer biosynthesis, which agrees with reduced growth yields.

KEY MESSAGES

FIB/SEM analysis revealed new insights into the remarkable subcellular architecture of P. cordatum, complemented by proteogenomic unraveling of novel nuclear structures and a photosynthetic megacomplex. These recent findings are put in the wider context of current understanding of dinoflagellates.

Ribosome profiling in the Symbiodiniacean dinoflagellate Fugacium kawagutii shows coordinated protein synthesis of enzymes in different pathways at different times of day

Dinoflagellates respond to daily changes in light and dark by changes in cellular metabolism, yet the mechanisms used are still unclear. For example, Fugacium (previously Symbiodinium) kawagutii shows little difference in the transcriptome between day and night suggesting little transcriptional control over gene expression. Here, we have performed ribosome profiling at 2 h intervals over a daily light-dark cycle to assess the degree to which protein synthesis rates might change over the daily cycle. The number of F. kawagutii coding sequences with significant differences in the number of ribosome-protected fragments (RPF) over the 24-h cycle was 2923 using JTK_Cycle and 3655 using ECHO. The majority of the regulated transcripts showed peak translation at the onset of the dark period. The regulated sequences were assigned to different KEGG pathways and transcripts that were translated at roughly the same time were termed concurrently regulated. Both analyses revealed concurrent regulation of many transcripts whose gene products were involved in spliceosome or lysosome biogenesis with peak translation rates around the onset of the dark period, while others, involved in nitrate metabolism and ribosomal proteins, were preferentially translated around the onset of the day phase or the end of the night phase, respectively. In addition, some sequences involved in DNA synthesis were preferentially translated at the end of the day. We conclude that light-dark cycles seem able to synchronize translation of some transcripts encoding proteins involved in a range of different cellular processes, and propose that these changes may help the cells adapt and alter their metabolism as a function of the time of day.

Transcriptomic response of the harmful algae Heterosigma akashiwo to polyphosphate utilization and phosphate stress

Phosphorus (P) is one of the major macronutrients necessary for phytoplankton growth. In some parts of the ocean, however, P is frequently scarce, hence, there is limited phytoplankton growth. To cope with P deficiency, phytoplankton evolved a variety of strategies, including, utilization of different P sources. Polyphosphate (polyP) is ubiquitously present and serves an essential function in aquatic environments, but it is unclear if and how this polymer is utilized by phytoplankton. Here, we examined the physiological and molecular responses of the widely present harmful algal bloom (HAB) species, Heterosigma akashiwo in polyP utilization, and in coping with P-deficiency. Our results revealed that two forms of inorganic polyP, namely, sodium tripolyphosphate and sodium hexametaphosphate, support H. akashiwo growth as efficiently as orthophosphate. However, few genes involved in polyP utilization have been identified. Under P-deficient conditions, genes associated with P transport, dissolved organic P utilization, sulfolipid synthesis, and energy production, were markedly elevated. In summary, our results indicate that polyP is bioavailable to H. akashiwo, and this HAB species have evolved a comprehensive strategy to cope with P deficiency.