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

• Published in: Biogeosciences
• Main Authors: Deppeler, Stacy, Petrou, Katherina, Schulz, Kai G., Westwood, Karen, Pearce, Imojen, McKinlay, John, Davidson, Andrew
• Format: Text
• Language: English
• Published: 2018
• Subjects: Antarctic; Prydz Bay; The Antarctic; Antarc*; Ocean acidification
• Online Access: https://doi.org/10.5194/bg-15-209-2018; https://www.biogeosciences.net/15/209/2018/

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 CO 2 levels ranging from ambient (343 µatm) to 1641 µatm in six 650 L minicosms. Productivity assays were performed to identify whether a CO 2 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. CO 2 levels ≥ 1140 µatm negatively affected photosynthetic performance and Chl a -normalised primary productivity (csGPP 14 C ), causing significant reductions in gross primary production (GPP 14 C ), Chl a accumulation, nutrient uptake, and POM production. However, there was no effect of CO 2 on C : N ratios. Over time, the phytoplankton community acclimated to high CO 2 conditions, showing a down-regulation of carbon concentrating mechanisms (CCMs) and likely adjusting other intracellular processes. Bacterial abundance initially increased in CO 2 treatments ≥ 953 µatm (days 3–5), yet gross bacterial production (GBP 14 C ) remained unchanged and cell-specific bacterial productivity (csBP 14 C ) was reduced. Towards the end of the experiment, GBP 14 C and csBP 14 C markedly increased across all treatments regardless of CO 2 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 CO 2 uptake. Increases in bacterial abundance under high CO 2 conditions may also increase the efficiency of the microbial loop, resulting in increased organic matter remineralisation and further declines in carbon sequestration.