Articles | Volume 15, issue 1
Biogeosciences, 15, 209–231, 2018

Special issue: The Ocean in a High-CO2 World IV

Biogeosciences, 15, 209–231, 2018

Research article 11 Jan 2018

Research article | 11 Jan 2018

Ocean acidification of a coastal Antarctic marine microbial community reveals a critical threshold for CO2 tolerance in phytoplankton productivity

Stacy Deppeler1, Katherina Petrou2, Kai G. Schulz3, Karen Westwood4,5, Imojen Pearce4, John McKinlay4, and Andrew Davidson4,5 Stacy Deppeler et al.
  • 1Institute for Marine and Antarctic Studies, University of Tasmania, Private Bag 129, Hobart, Tasmania 7001, Australia
  • 2School of Life Sciences, University of Technology Sydney, 15 Broadway, Ultimo, New South Wales 2007, Australia
  • 3Centre for Coastal Biogeochemistry, Southern Cross University, Military Rd, East Lismore, NSW 2480, Australia
  • 4Australian Antarctic Division, Department of the Environment and Energy, 203 Channel Highway, Kingston, Tasmania 7050, Australia
  • 5Antarctic Climate and Ecosystems Cooperative Research Centre, Private Bag 80, Hobart, Tasmania 7001, Australia

Abstract. High-latitude oceans are anticipated to be some of the first regions affected by ocean acidification. Despite this, the effect of ocean acidification on natural communities of Antarctic marine microbes is still not well understood. In this study we exposed an early spring, coastal marine microbial community in Prydz Bay to CO2 levels ranging from ambient (343 µatm) to 1641 µatm in six 650 L minicosms. Productivity assays were performed to identify whether a CO2 threshold existed that led to a change in primary productivity, bacterial productivity, and the accumulation of chlorophyll a (Chl a) and particulate organic matter (POM) in the minicosms. In addition, photophysiological measurements were performed to identify possible mechanisms driving changes in the phytoplankton community. A critical threshold for tolerance to ocean acidification was identified in the phytoplankton community between 953 and 1140 µatm. CO2 levels  ≥ 1140 µatm negatively affected photosynthetic performance and Chl a-normalised primary productivity (csGPP14C), causing significant reductions in gross primary production (GPP14C), Chl a accumulation, nutrient uptake, and POM production. However, there was no effect of CO2 on C : N ratios. Over time, the phytoplankton community acclimated to high CO2 conditions, showing a down-regulation of carbon concentrating mechanisms (CCMs) and likely adjusting other intracellular processes. Bacterial abundance initially increased in CO2 treatments  ≥ 953 µatm (days 3–5), yet gross bacterial production (GBP14C) remained unchanged and cell-specific bacterial productivity (csBP14C) was reduced. Towards the end of the experiment, GBP14C and csBP14C markedly increased across all treatments regardless of CO2 availability. This coincided with increased organic matter availability (POC and PON) combined with improved efficiency of carbon uptake. Changes in phytoplankton community production could have negative effects on the Antarctic food web and the biological pump, resulting in negative feedbacks on anthropogenic CO2 uptake. Increases in bacterial abundance under high CO2 conditions may also increase the efficiency of the microbial loop, resulting in increased organic matter remineralisation and further declines in carbon sequestration.

Short summary
We combined productivity and photophysiology measurements to investigate the effects of ocean acidification on a natural Antarctic marine microbial community. Our study identifies a threshold for CO2 tolerance in the phytoplankton community between 953 and 1140 μatm of CO2, above which productivity declines. Bacteria were tolerant to CO2 up to 1641 μatm. We identify physiological changes in the phytoplankton at high CO2 that allowed them to acclimate to the high CO2 treatment.
Final-revised paper