Effects of CO2 concentration on a late summer surface sea ice community

Annual fast ice at Scott Base (Antarctica) in late summer contained a high biomass surface community of mixed phytoflagellates, dominated by the dinoflagellate, Polarella glacialis. At this time of the year, ice temperatures rise close to melting point and salinities drop to less than 20. At the same time, pH levels can rise above 9 and nutrients can become limiting. In January 2014, the sea ice microbial community from the top 30 cm of the ice was exposed to a gradient of pH and CO₂ (5 treatments) that ranged from 8.87 to 7.12 and 5–215 µmol CO₂ kg⁻¹, respectively, and incubated in situ. While growth rates were reduced at the highest and lowest pH, the differences were not significant. Likewise, there were no significant differences in maximum quantum yield of PSII (F_v/F_m) or relative maximum electron transfer rates (rETR_max) among treatments. In a parallel experiment, a CO₂ gradient of 26–230 µmol CO₂ kg⁻¹ (5 treatments) was tested, keeping pH constant. In this experiment, growth rates increased by approximately 40% with increasing CO₂, although differences among treatments were not significant.. As in the previous experiment, there was no significant response in F_v/F_m or rETR_max. A synchronous grazing dilution experiment found grazing rates to be inconclusive These results suggest that the summer sea ice brine communities were not limited by in situ CO₂ concentrations and were not adversely affected by pH values down to 7.1.