Short-Term Physiological and Transcriptomic Analysis of Saxitoxin-Producing and Nonproducing Raphidiopsis raciborskii (Cyanobacteria) under an Extremely High CO2 Environment

Findings about CO2 dynamics in the Earth’s ancestral atmosphere have suggested much higher concentrations in past eras. Along this line, cyanobacteria are an early evolved photosynthetic group that is suggested to have experienced both high and low CO2 availability since their Precambrian origin, and therefore, it is reasonable to assume that these microbes have the potential to cope with these scenarios by rapidly adjusting to various carbon dioxide levels. Thus, in this work, we performed a short-term (72-h) investigation of the physiological parameters (cell growth, photosynthesis and saxitoxin production) of toxic and nontoxic strains of the cyanobacterium Raphidiopsis raciborskii challenged by an extremely high pCO2 (40,000 ppm). Additionally, the transcriptomic profile (regarding the carbon concentrating mechanism and photosynthesis) of selected toxic and nontoxic strains is also presented. We found that short-term exposure to extremely elevated CO2 concentrations did not affect R. raciborskii physiology regardless of toxin production. However, transcripts related to bicarbonate transporters and the RuBisCO enzyme indicated the upregulation of CCM and downregulation of the Calvin cycle, respectively. According to our findings, at least at the initial growth phase, R. raciborskii was able to cope with a very high CO2 level, which shed light on the understanding that this species might have the potential to cope with carbon dioxide in water above the predicted levels.