100. Bacterial response to ocean acidification and warming varies with plankton community composition

Tim J. Burrell (1,2), Elizabeth W. Maas (1), Debbie A. Hulston (1), Cliff S. Law (1,3)*

1 National Institute of Water and Atmospheric Research, Greta Point, Kilbirnie, Wellington 6002, New Zealand
2 Victoria University of Wellington, School of Biological Sciences, Wellington 6140, New Zealand
3 Department of Chemistry, University of Otago, Dunedin, 9016, New Zealand

Background
Bacterial extracellular enzymes play an important role in the degradation of organic matter in the surface ocean, but are sensitive to changes in pH and temperature. To date there has been limited consideration of the interaction of these two stressors, and also how this response may be influenced by phytoplankton community composition.

Methods
This study tested the individual and combined effects of ocean acidification (-0.3) and warming (+3°C) projected for the year 2100 on bacterial abundance, process rates and diversity in 5-6 day incubations using four different plankton communities from surface waters east of New Zealand.

Findings
Although the magnitude and temporal response varied between phytoplankton communities, bacterial abundance and production generally increased in response to elevated temperature, whereas β-glucosidase (BG) activity showed a stronger response to low pH in some communities, and elevated temperature in others. Cell-specific and bulk leucine aminopeptidase (LAP) activity were consistently higher under low pH, and low pH and elevated temperature combined, in all phytoplankton communities. However the interactive effects of temperature and pH were variable, with an additive response in BG activity and bacterial production observed in only one community, and a synergistic response in LAP activity in two phytoplankton communities.

Conclusions
The results suggest that the effect of lower pH and higher temperature on bacterial processes may partially counteract each other in the future ocean. Warming may stimulate bacterial production, and the increase in LAP activity suggests that ocean acidification may result in differential rates of carbohydrate and amino acid remineralisation in surface waters of the South West Pacific.