Ocean acidification changes the structure of an Antarctic coastal protistan community

Abstract

<p><strong>Abstract.</strong> Antarctic near-shore waters are amongst of the most vulnerable in the world to ocean acidification. Microbes occupying these waters are critical drivers of ecosystem productivity, elemental cycling and ocean biogeochemistry, yet little is known about their sensitivity to ocean acidification. An unreplicated, six-level dose-response experiment was conducted using 650 L incubation tanks (minicosms) adjusted to fugacity of carbon dioxide (ƒCO<sub>2</sub>) from 343 to 11 641 μatm. The minicosms were filled with near-shore water from Prydz Bay, East Antarctica and the protistan composition and abundance was determined by microscopy analysis of samples collected during the 18 day incubation. No CO<sub>2</sub>-related change in the protistan community composition was observed during the initial 8 day acclimation period under low light. Thereafter, the response of protists to ƒCO<sub>2</sub> were species-specific for both heterotrophic and autotrophic protists. The response by diatoms was related to cell size, large cells increasing in abundance with low to moderate ƒCO<sub>2</sub> (634–953 μatm). Similarly, the abundance of Phaeocystis antarctica increased with increasing ƒCO<sub>2</sub> peaking at a ƒCO<sub>2</sub> of 634 μatm. Above this threshold the abundances of large diatoms and Phaeocystis antarctica fell dramatically, and small diatoms dominated, therefore culminating in a significant shift in the composition of the protistan community. The threshold CO<sub>2</sub> level at which the composition changed agreed with that previously measured at this location, indicating it remains consistent among seasons. This suggests that near-shore microbial communities are likely to change significantly near the end of this century if anthropogenic CO<sub>2</sub> release continues unabated, with profound ramifications for near-shore Antarctic ecosystems.</p>

Ocean acidification of a coastal Antarctic marine microbial community reveals a critical threshold for CO<sub>2</sub> tolerance in phytoplankton productivity

“…9a), it was possible that this response was driven by a decline in grazing by heterotrophs Westwood et al, 2018) instead of a direct CO 2 -related promotion of bacterial growth. The subsequent decline in abundance was likely due to topdown control from the heterotrophic nanoflagellate community, which displayed an increase in abundance at this time (Hancock et al, 2017). Bacterial tolerance to high CO 2 has been reported previously in this region Westwood et al, 2018) and has also been reported in numerous studies in the Arctic (Grossart et al, 2006;Allgaier et al, 2008;Paulino et al, 2008;Baragi et al, 2015;Wang et al, 2016), suggesting that the marine bacterial community will be resilient to increasing CO 2 .…”

“…Numerous photophysiological investigations on individual phytoplankton species also report species-specific tolerances to increased CO 2 (Gao et al, 2012a;Gao and Campbell, 2014;Trimborn et al, 2013Trimborn et al, , 2014, and a general trend toward smaller-celled communities with increased CO 2 has been reported in ocean acidification studies globally . Changes in community structure were observed with increasing CO 2 , with taxon-specific thresholds of CO 2 tolerance (Hancock et al, 2017). Within the diatom community, the response was also related to size, leading to an increase in abundance of small (< 20 µm) diatoms in the higher CO 2 treatments (≥ 953 µatm).…”

“…9b). This promotion of growth may be linked to an increase in diatom abundance observed in this treatment (Hancock et al, 2017). The coupling of bacterial growth with phytoplankton productivity has been reported by numerous studies on natural marine microbial communities (Allgaier et al, 2008;Grossart et al, 2006;Engel et al, 2013;Piontek et al, 2013;Sperling et al, 2013;Bergen et al, 2016).…”

“…Despite this, the community displayed the ability to adapt to these high CO 2 conditions by down-regulating CCMs and likely adjusting other intracellular mechanisms to cope with the added stress of low pH. However, the lag in growth and subsequent acclimation to high CO 2 conditions allowed for more tolerant species to thrive (Hancock et al, 2017).…”

“…These differing responses may be due to differences in community composition, nutrient supply, or ecological adaptations of the phytoplankton community in the region studied. They may also be due to differences in the experimental methods, especially the range of CO 2 concentrations employed (Hancock et al, 2017), the mechanism used to manipulate CO 2 concentrations, the duration of the acclimation and incubation, the nature and volume of the mesocosms used, and the extent to which higher trophic levels are screened from the mesocosm contents (see Davidson et al, 2016).…”