50. Ocean Acidification Effects on Productivity in a Coastal Antarctic Marine Microbial Community

Stacy L Deppeler (1)*, Karen Westwood (2,3), Imojen Pearce (2), Penelope Pascoe (2), Andrew T Davidson (2,3)

1 Institute for Marine and Antarctic Studies, University of Tasmania, Private Bag 129, Hobart, Tasmania 7001, Australia
2 Australian Antarctic Division, Channel Highway, Kingston, Tasmania 7050, Australia
3 Antarctic Climate and Ecosystems Cooperative Research Centre, Private Bag 80, Hobart, Tasmania 7001, Australia


Background
The Southern Ocean is responsible for ~40% of the ocean’s uptake of anthropogenic CO2. Marine microbes (phytoplankton, protozoa and bacteria) are the base of the Antarctic food chain and critical determinants of the fixation and fate of carbon in the oceans. Few studies on ocean acidification have been performed in Antarctic waters and while its effect on marine microbes in this region are critical to ecosystem function they are poorly understood.

Methods
Six 650 L minicosm tanks were used to expose a natural community of Antarctic marine microbes from near-shore waters off Davis Station, Antarctica, to different CO2 concentrations ranging from ambient (343 ppm) to 1641 ppm. Primary and bacterial productivity was measured through the uptake of radioisotope labelled 14C-bicarbonate and 14C-Leucine, respectively, and normalised to cell abundance.

Findings
Results showed that rates of gross primary production decreased markedly with increasing CO2 concentration mainly due to the lower cell growth in the high CO2 treatments. The maximum photosynthetic rate (Pmax) and cell-specific productivity were similar across all treatments, suggesting that the cellular photosynthetic performance was not diminished by increased CO2. No marked difference was observed in rates of bacterial production amongst CO2 treatments. However, cell-specific rates of production increased substantially, despite the decline in abundance over time in all treatments.

Conclusions
This research helps establish critical thresholds of pCO2 that change microbial productivity in Antarctic waters and aids prediction of the future effects of anthropogenic CO2 on the Antarctic ecosystem.