Abstract
Increasing anthropogenic CO2 emissions in recent decades cause ocean acidification (OA), affecting carbon cycling in oceans by regulating eco-physiological processes of plankton. Heterotrophic bacteria play an important role in carbon cycling in oceans. However, the effect of OA on bacteria in oceans, especially in oligotrophic regions, was not well understood. In our study, the response of bacterial metabolic activity and community composition to OA was assessed by determining bacterial production, respiration, and community composition at the low-pCO2 (400 ppm) and high-pCO2 (800 ppm) treatments over the short term at two oligotrophic stations in the northern South China Sea. Bacterial production decreased significantly by 17.1–37.1 % in response to OA, since bacteria with high nucleic acid content preferentially were repressed by OA, which was less abundant under high-pCO2 treatment. Correspondingly, shifts in bacterial community composition occurred in response to OA, with a high fraction of the small-sized bacteria and high bacterial species diversity in a high-pCO2 scenario at K11. Bacterial respiration responded to OA differently at both stations, most likely attributed to different physiological responses of the bacterial community to OA. OA mitigated bacterial growth efficiency, and consequently, a larger fraction of DOC entering microbial loops was transferred to CO2.
Highlights
Twenty-three percent of atmospheric CO2 released by anthropogenic activities is absorbed by oceans (Le Quere et al, 2018), leading to ocean acidification (OA)
Our results indicated that these active bacteria with high nucleic acid, primarily Rhodobacteraceae at both stations and Bacteroidetes at K11, were preferentially affected by ocean acidification, resulting in shifts in bacterial community composition and a decrease in Bacterial production (BP)
The OA-induced decrement of bacterial growth efficiency (BGE) at both stations indicated that a greater fraction of Dissolved organic carbon (DOC) was transferred to CO2 in oligotrophic oceans in the OA scenario, rather than higher trophic level
Summary
Twenty-three percent of atmospheric CO2 released by anthropogenic activities is absorbed by oceans (Le Quere et al, 2018), leading to ocean acidification (OA). PH of surface seawater would fall by 0.4–0.5, and the carbonate system would be affected (Riebesell and Gattuso, 2015). Ocean acidification is reported to modulate the cell morphology (Yu and Chen, 2019), metabolism (Westwood et al, 2018; Vaque et al, 2019), and community structure (Grossart et al, 2006; James et al, 2019) of marine bacteria, affecting the carbon flow of the marine microbial loop. The effect of ocean acidification on bacterial metabolic activity and community composition has been recognized in the last decades (Riebesell and Gattuso, 2015), contrasting findings are achieved (Weinbauer et al, 2011), likely due to different nutritional conditions or experimental designs
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