Ocean acidification causes a community-wide bacterial pH stress response

Abstract

Anthropogenic CO2 emissions threaten the structure and function of marine food-webs by lowering oceanic pH. Despite their importance for biogeochemical cycling, physiological responses of bacterioplankton to ocean acidification remain largely unknown. We conducted a mesocosm experiment with Mediterranean seawater treated with CO2 or/and nutrient enrichment and analyzed microbial responses. While community composition remained stable, bacterial community-wide expression analysis revealed that genes involved in respiration, photosynthesis and proton transport, e.g. proteorhodopsin, were significantly enriched in acidified mesocosms. In total, 40% of the transcripts enriched upon acidification were assigned to mechanisms maintaining pH homeostasis. This suggests that acidification-induced proton stress is counteracted by energetically costly proton extrusion mechanisms, purportedly leaving less energy for other metabolic processes. Taxonomic assignment of transcripts showed distinct gene expression patterns among dominant bacterial groups, but also viruses, caused by acidification. These previously unrecognized responses were not visible under phytoplankton bloom conditions that strongly stimulated bacterial growth. Hence, effects of ocean acidification could have particularly pronounced consequences for the energetics and ecosystem functioning of bacterioplankton in oligotrophic conditions