Carbon concentrating mechanisms (CCMs) are used by photoautotrophs to overcome possible limitations in carbon acquisition but the competitive strategies and efficiencies of these mechanisms among photosynthesizers can be variable. The diversity in carbon acquisition abilities establishes the potential for alterations in community structure with shifting carbon concentrations. Given the role of phytoplankton and benthic microalgae (BMA) in the trophodynamics of estuaries, understanding the mechanisms of carbon acquisition in these systems is important in predicting how primary productivity and nutrient cycling might change in response to increasing concentrations of atmospheric CO2. Our approach to investigate whether induced carbon limitation would show predictable shifts in microalgal community structure and production was conducted through the inhibition of an enzyme used in CCMs, carbonic anhydrase (CA). CA catalyzes the rates of interconversion between CO2 and HCO3- to facilitate transport of inorganic carbon into the cell and trap that carbon there. Although CA has the potential to help mitigate increasing CO2 levels in the atmosphere, evaluations on how different species use CA and the physiological roles it may perform in microalgae are needed. We show phytoplankton communities from different environments are altered when a CA inhibitor (i.e. ethoxyzolamide, EZ) is present and CA activity is suppressed. Diatoms remained the dominant taxonomic group in all samples following a 3-day inhibition of CA but there were lower-level community shifts. These shifts in community structure suggest that phytoplankton composition is affected by carbon acquisition using CA, and some diatom genera may depend on the competitive advantage of this enzyme for their CCMs to maintain high abundances in estuarine environments. Most of the diatom genera had strong growth limitation and cell mortality without active CA, however, some pennate diatoms like Cylindrotheca persisted with positive growth rates. All four of our cultured diatoms experienced a decrease in gross primary production (GPP) and relative electron transport rate at high irradiance levels indicating that some other physiological traits were giving Cylindrotheca the competitive benefit. Decreased GPP was similarly observed in the BMA communities as well with CA inhibition. However, this limitation in carbon acquisition drove motile benthic microalgae to make use of a smaller vertical profile closer to the sediment surface rather than exhibit the mortality seen in most of our cultured diatom genera. Predicting marine microalgal responses to changes in CO2 availability requires further characterization of other physiological traits across a higher diversity of growth conditions and taxa. Our research demonstrates that there can be wide variability in carbon acquisition strategies within the diatom genera and that the competitive advantage provided by CA and efficiency of their CCMs may be dependent on the environment’s carbon availability.
Continued mechanistic approaches are needed to recognize the impacts of CA activity on microalgal communities with respect to their assemblage, cell-size fractions, primary production rates, and physiological performance
