Ocean acidification changes the structure of an Antarctic coastal protistan community.

Antarctic near-shore waters are amongst of the most vulnerable in the world to ocean acidification. Microbes occupying these waters are critical drivers of ecosystem productivity, elemental cycling and ocean biogeochemistry, yet little is known about their sensitivity to ocean acidification. An unreplicated, six-level dose-response experiment was conducted using 650 L incubation tanks (minicosms) adjusted to fugacity of carbon dioxide (ƒCO₂) from 343 to 11 641 μatm. The minicosms were filled with near-shore water from Prydz Bay, East Antarctica and the protistan composition and abundance was determined by microscopy analysis of samples collected during the 18 day incubation. No CO₂-related change in the protistan community composition was observed during the initial 8 day acclimation period under low light. Thereafter, the response of protists to ƒCO₂ were species-specific for both heterotrophic and autotrophic protists. The response by diatoms was related to cell size, large cells increasing in abundance with low to moderate ƒCO₂ (634-953 μatm). Similarly, the abundance of Phaeocystis antarctica increased with increasing ƒCO₂ peaking at a ƒCO₂ of 634 μatm. Above this threshold the abundances of large diatoms and Phaeocystis antarctica fell dramatically, and small diatoms dominated, therefore culminating in a significant shift in the composition of the protistan community. The threshold CO₂ level at which the composition changed agreed with that previously measured at this location, indicating it remains consistent among seasons. This suggests that near-shore microbial communities are likely to change significantly near the end of this century if anthropogenic CO₂ release continues unabated, with profound ramifications for near-shore Antarctic ecosystems. [ABSTRACT FROM AUTHOR]