Articles | Volume 14, issue 1
Biogeosciences, 14, 1–15, 2017

Special issue: Effects of rising CO2 on a Baltic Sea plankton...

Biogeosciences, 14, 1–15, 2017

Research article 02 Jan 2017

Research article | 02 Jan 2017

Ocean acidification impacts bacteria–phytoplankton coupling at low-nutrient conditions

Thomas Hornick1, Lennart T. Bach2, Katharine J. Crawfurd3, Kristian Spilling4,5, Eric P. Achterberg2,6, Jason N. Woodhouse1, Kai G. Schulz2,7, Corina P. D. Brussaard3,8, Ulf Riebesell2, and Hans-Peter Grossart1,9 Thomas Hornick et al.
  • 1Leibniz Institute of Freshwater Ecology and Inland Fisheries (IGB), Experimental Limnology, 16775 Stechlin, Germany
  • 2GEOMAR Helmholtz Centre for Ocean Research Kiel, Düsternbrooker Weg 20, 24105 Kiel, Germany
  • 3NIOZ Royal Netherlands Institute for Sea Research, Department of Marine Microbiology and Biogeochemistry, and Utrecht University, P.O. Box 59, 1790 AB Den Burg, Texel, the Netherlands
  • 4Marine Research Centre, Finnish Environment Institute, P.O. Box 140, 00251 Helsinki, Finland
  • 5Tvärminne Zoological Station, University of Helsinki, J. A. Palménin tie 260, 10900 Hanko, Finland
  • 6National Oceanography Centre Southampton, European Way, University of Southampton, Southampton, SO14 3ZH, UK
  • 7Southern Cross University, P.O. Box 157, Lismore, NSW 2480, Australia
  • 8Aquatic Microbiology, Institute for Biodiversity and Ecosystem Dynamics, University of Amsterdam, P.O. Box 94248, 1090 GE Amsterdam, the Netherlands
  • 9Potsdam University, Institute for Biochemistry and Biology, Maulbeerallee 2, 14469 Potsdam, Germany

Abstract. The oceans absorb about a quarter of the annually produced anthropogenic atmospheric carbon dioxide (CO2), resulting in a decrease in surface water pH, a process termed ocean acidification (OA). Surprisingly little is known about how OA affects the physiology of heterotrophic bacteria or the coupling of heterotrophic bacteria to phytoplankton when nutrients are limited. Previous experiments were, for the most part, undertaken during productive phases or following nutrient additions designed to stimulate algal blooms. Therefore, we performed an in situ large-volume mesocosm ( ∼  55 m3) experiment in the Baltic Sea by simulating different fugacities of CO2 (fCO2) extending from present to future conditions. The study was conducted in July–August after the nominal spring bloom, in order to maintain low-nutrient conditions throughout the experiment. This resulted in phytoplankton communities dominated by small-sized functional groups (picophytoplankton). There was no consistent fCO2-induced effect on bacterial protein production (BPP), cell-specific BPP (csBPP) or biovolumes (BVs) of either free-living (FL) or particle-associated (PA) heterotrophic bacteria, when considered as individual components (univariate analyses). Permutational Multivariate Analysis of Variance (PERMANOVA) revealed a significant effect of the fCO2 treatment on entire assemblages of dissolved and particulate nutrients, metabolic parameters and the bacteria–phytoplankton community. However, distance-based linear modelling only identified fCO2 as a factor explaining the variability observed amongst the microbial community composition, but not for explaining variability within the metabolic parameters. This suggests that fCO2 impacts on microbial metabolic parameters occurred indirectly through varying physicochemical parameters and microbial species composition. Cluster analyses examining the co-occurrence of different functional groups of bacteria and phytoplankton further revealed a separation of the four fCO2-treated mesocosms from both control mesocosms, indicating that complex trophic interactions might be altered in a future acidified ocean. Possible consequences for nutrient cycling and carbon export are still largely unknown, in particular in a nutrient-limited ocean.

Final-revised paper