Ocean acidification changes the structure of an Antarctic coastal protistan community

Antarctic near-shore waters are amongst the most sensitive 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. A six-level, dose–response experiment was conducted using 650L incubation tanks (minicosms) adjusted to a gradient in fugacity of carbon dioxide (fCO₂) from 343 to 1641µatm. The six minicosms were filled with near-shore water from Prydz Bay, East Antarctica, and the protistan composition and abundance was determined by microscopy during 18 days of 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 both autotrophic and heterotrophic protists to fCO₂ was species-specific. The response of diatoms was mainly cell size related; microplanktonic diatoms ( > 20µm) increased in abundance with low to moderate fCO₂ (343–634µatm) but decreased at fCO₂  ≥ 953µatm. Similarly, the abundance of Phaeocystis antarctica increased with increasing fCO₂ peaking at 634µatm. Above this threshold the abundance of micro-sized diatoms and P. antarctica fell dramatically, and nanoplanktonic diatoms ( ≤ 20µm) dominated, therefore culminating in a significant change in the protistan community composition. Comparisons of these results with previous experiments conducted at this site show that the fCO₂ thresholds are similar, despite seasonal and interannual differences in the physical and biotic environment. 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 ecosystem food webs and biogeochemical cycling.