Ocean acidification impacts primary and bacterial production in Antarctic coastal waters during austral summer

Polar waters may be highly impacted by ocean acidification (OA) due to increased solubility of CO₂ at colder water temperatures. Three experiments examining the influence of OA on primary and bacterial production were conducted during austral summer at Davis Station, East Antarctica (68°35′ S, 77°58′ E). For each experiment, six minicosm tanks (650 L) were filled with 200 μm filtered coastal seawater containing natural communities of Antarctic marine microbes. Assemblages were incubated for 10 to 12 days at CO₂ concentrations ranging from pre-industrial to post-2300. Primary and bacterial production rates were determined using NaH¹⁴CO₃ and ¹⁴C-leucine, respectively. Net community production (NCP) was also determined using dissolved oxygen. In all experiments, maximum photosynthetic rates (P_max, mg C mg chl a^− 1 h^− 1) decreased with elevated CO₂, clearly reducing rates of total gross primary production (mg C L^− 1 h^− 1). Rates of cell-specific bacterial productivity (μg C cell^− 1 h^− 1) also decreased under elevated CO₂, yet total bacterial production (μg C L^− 1 h^− 1) and cell abundances increased with CO₂ over Days 0–4. Initial increases in bacterial production and abundance were associated with fewer heterotrophic nanoflagellates and therefore less grazing pressure. The main changes in primary and bacterial productivity generally occurred at CO₂ concentrations > 2 × present day (> 780 ppm), with the same responses occurring regardless of seasonally changing environmental conditions and microbial assemblages. However, NCP varied both within and among experiments, largely due to changing nitrate + nitrite (NOx) availability. At NOx concentrations < 1.5 μM photosynthesis to respiration ratios showed that populations switched from net autotrophy to heterotrophy and CO₂ responses were suppressed. Overall, OA may reduce production in Antarctic coastal waters, thereby reducing food availability to higher trophic levels and reducing draw-down of atmospheric CO₂, thus forming a positive feedback to climate change. NO_X limitation may suppress this OA response but cause a similar decline.