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

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 (csGPP_¹⁴C), causing significant reductions in gross primary production (GPP_¹⁴C), 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 (GBP_¹⁴C) remained unchanged and cell-specific bacterial productivity (csBP_¹⁴C) was reduced. Towards the end of the experiment, GBP_¹⁴C and csBP_¹⁴C 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.