AME 82:73-85 (2018)  -  DOI: https://doi.org/10.3354/ame01879

ABSTRACT: Carbon concentrating mechanisms (CCMs) are used by phytoplankton to concentrate dissolved inorganic carbon within their cells for use in photosynthesis. However, CCMs which involve carbonic anhydrase (CA) may become redundant in the future due to increasing surface water dissolved CO₂ (CO_2(aq)) concentrations. Most of our knowledge of the CA enzyme is based on single-species phytoplankton cultures or oligotrophic water samples. Few studies have examined the consequences of CA activity on competitive interactions in estuarine phytoplankton communities or measured the long-term effects on community composition. Using bioassays of natural phytoplankton communities, we explored 2 different estuarine systems and determined how community composition was altered when the CA enzyme was removed. Using the CA inhibitor ethoxyzolamide (EZ), our results demonstrate that communities are altered when the inhibitor is present and CA activity is suppressed. Diatoms were the dominant taxonomic group in all samples following a 3 d exposure of the community to EZ. However, our findings suggest that diatom growth was both stimulated and inhibited, depending on the salinity of the location where samples were collected. Furthermore, microscopy of the high salinity phytoplankton community indicated that centric diatom genera (e.g. Skeletonema, Rhizosolenia) were severely reduced in treatments that removed the competitive advantage of CA, while pennate diatom genera (e.g. Asterionellopsis, Cylindrotheca) dominated these same treatments. 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 CCM to maintain high abundances in estuarine environments.

KEY WORDS: Carbon acquisition · Carbon concentrating mechanisms · Community structure · Diatoms · Estuary · Phytoplankton