Experiment design and bacterial abundance control extracellular H 2 O 2 concentrations during four series of mesocosm experiments

• Published in: Biogeosciences
• Main Authors: M. J. Hopwood, N. Sanchez, D. Polyviou, Ø. Leiknes, J. A. Gallego-Urrea, E. P. Achterberg, M. V. Ardelan, J. Aristegui, L. Bach, S. Besiktepe, Y. Heriot, I. Kalantzi, T. Terbıyık Kurt, I. Santi, T. M. Tsagaraki, D. Turner
• Format: Article in Journal/Newspaper
• Language: English
• Published: Copernicus Publications 2020
• Subjects: Ecology; QH540-549.5; Life; QH501-531; Geology; QE1-996.5; Svalbard; Patagonia
• Online Access: https://doi.org/10.5194/bg-17-1309-2020; https://doaj.org/article/c89650c285674a39b20c53bcc5360baa

The extracellular concentration of H 2 O 2 in surface aquatic environments is controlled by a balance between photochemical production and the microbial synthesis of catalase and peroxidase enzymes to remove H 2 O 2 from solution. In any kind of incubation experiment, the formation rates and equilibrium concentrations of reactive oxygen species (ROSs) such as H 2 O 2 may be sensitive to both the experiment design, particularly to the regulation of incident light, and the abundance of different microbial groups, as both cellular H 2 O 2 production and catalase–peroxidase enzyme production rates differ between species. Whilst there are extensive measurements of photochemical H 2 O 2 formation rates and the distribution of H 2 O 2 in the marine environment, it is poorly constrained how different microbial groups affect extracellular H 2 O 2 concentrations, how comparable extracellular H 2 O 2 concentrations within large-scale incubation experiments are to those observed in the surface-mixed layer, and to what extent a mismatch with environmentally relevant concentrations of ROS in incubations could influence biological processes differently to what would be observed in nature. Here we show that both experiment design and bacterial abundance consistently exert control on extracellular H 2 O 2 concentrations across a range of incubation experiments in diverse marine environments. During four large-scale ( >1000 L) mesocosm experiments (in Gran Canaria, the Mediterranean, Patagonia and Svalbard) most experimental factors appeared to exert only minor, or no, direct effect on H 2 O 2 concentrations. For example, in three of four experiments where pH was manipulated to 0.4–0.5 below ambient pH, no significant change was evident in extracellular H 2 O 2 concentrations relative to controls. An influence was sometimes inferred from zooplankton density, but not consistently between different incubation experiments, and no change in H 2 O 2 was evident in controlled experiments using different densities of the copepod ...