Constitutive CCMs in three Southern Ocean phytoplankton species: response to ocean acidification modulated by light availability

Dissolution of elevated atmospheric CO2 liberated through anthropogenic CO2 emission results in higher DIC content and decreased pH in marine systems. Carbon concentrating mechanisms evolved to enhance the concentration of CO2 in the vicinity of Rubisco have been postulated to be downregulated under enhanced pCO2 levels. Three ecologically relevant Southern Ocean planktonic species were grown under elevated pCO2 levels under low and high light conditions. RNA sequencing revealed that neither the haptophyte Phaeocystis antarctica nor the pennate diatom Fragilariopsis kerguelensis decreases the expression of CCM related genes under ocean acidification, whereas some of those exhibit higher abundance under ambient pCO2 and low light the centric diatom species Chaetoceros debilis. Moreover, elevated expression of fucoxanthin-chlorophyll binding genes could be observed under elevated pCO2 under high light levels in both diatoms. Whereas the diatoms appear to be more sensitive to higher light levels under OA, the presence of a constitutionally expressed CCM is indicated in all species.