Elevated CO2 evokes pH Homeostasis Gene Expression Response in Marine Bacteria

Chair: Vonda Cummings

Carina Bunse (1)*, Daniel Lundin (1), Christofer M.G. Karlsson (1), Neelam Akram (1,8), Maria Vila-Costa (2,5), Joakim Palovaara (1,7), Lovisa Svensson (1), Karin Holmfeld (1), José M. González (4), Eva Calvo (3), Carles Pelejero (3,6), Cèlia Marrasé (3), Mark Dopson (1), Josep M. Gasol (3), Jarone Pinhassi (1)

1 Centre for Ecology and Evolution in Microbial Model Systems, EEMiS, Linnaeus University, Kalmar, Sweden
2 Group of Limnology, Department of Continental Ecology, Centre d’Estudis Avançats de Blanes-CSIC, Catalonia, Spain
3 Departament de Biologia Marina i Oceanografia, Institut de Ciències del Mar – CSIC, Barcelona, Spain
4 Department of Microbiology, University of La Laguna, La Laguna, Spain 5 Department of Environmental Chemistry, IDAEA-CSIC, Catalunya, Spain 6 Institució Catalana de Recerca i Estus Avançats (ICREA), Barcelona, Spain 7 Department of Agrotechnology and Food Sciences, Wageningen, The Netherlands 8 Department of Biosciences, COMSATS Institute of information technology, Islamabad, Pakistan

Background
Marine bacteria are highly abundant in the oceans where they are important drivers of biogeochemical processes and nutrient cycles. Despite the importance of marine bacteria they are often disregarded in models describing ocean acidification. As a result, while the impact of ocean acidification on phytoplankton is beginning to be unraveled, corresponding insights into how marine bacteria alter their gene expression and activity to elevated CO2 is essentially unknown.

Methods
We conducted a mesocosm experiment, to study the metatranscriptional response of marine bacteria to elevated CO2 under two different trophic stages; unaltered, natural seawater from the Northwestern Mediterranean Sea (low chlorophyll) and phytoplankton bloom conditions induced by addition of inorganic nutrients (high chlorophyll).

Findings
Marine bacteria responded to passive proton influx through the cell membrane (induced by elevated CO2) by enhancing the expression of genes encoding proton pumps such as proteorhodopsin, the electron transport chain, and membrane transporters. While these pH-homeostatic gene expression patterns were found under low chlorophyll and low energetic conditions, the magnitude of changes in gene expression under high chlorophyll conditions was low and masked by photosynthetic activities of phytoplankton. Distinctive bacterial taxa used different gene expression strategies to counter the effects of elevated CO2.

Conclusions
Since the observed pH homeostatic reactions under these sub-optimal environmental conditions are energetically costly, bacteria possibly have to allocate more energy to maintain pH homeostasis instead of using the energy to grow. Such processes could evoke changes in long-term bacterial growth efficiencies and ultimately biogeochemical process rates and ocean ecosystems.