Articles | Volume 14, issue 17
Biogeosciences, 14, 3971–3977, 2017
Biogeosciences, 14, 3971–3977, 2017

Research article 13 Sep 2017

Research article | 13 Sep 2017

Temperature and UV light affect the activity of marine cell-free enzymes

Blair Thomson1, Christopher David Hepburn1, Miles Lamare1, and Federico Baltar1,2 Blair Thomson et al.
  • 1Department of Marine Science, University of Otago, Dunedin, New Zealand
  • 2NIWA/University of Otago Research Centre for Oceanography, Dunedin, New Zealand

Abstract. Microbial extracellular enzymatic activity (EEA) is the rate-limiting step in the degradation of organic matter in the oceans. These extracellular enzymes exist in two forms: cell-bound, which are attached to the microbial cell wall, and cell-free, which are completely free of the cell. Contrary to previous understanding, cell-free extracellular enzymes make up a substantial proportion of the total marine EEA. Little is known about these abundant cell-free enzymes, including what factors control their activity once they are away from their sites (cells). Experiments were run to assess how cell-free enzymes (excluding microbes) respond to ultraviolet radiation (UVR) and temperature manipulations, previously suggested as potential control factors for these enzymes. The experiments were done with New Zealand coastal waters and the enzymes studied were alkaline phosphatase (APase), β-glucosidase, (BGase), and leucine aminopeptidase (LAPase). Environmentally relevant UVR (i.e. in situ UVR levels measured at our site) reduced cell-free enzyme activities by up to 87 % when compared to controls, likely a consequence of photodegradation. This effect of UVR on cell-free enzymes differed depending on the UVR fraction. Ambient levels of UV radiation were shown to reduce the activity of cell-free enzymes for the first time. Elevated temperatures (15 °C) increased the activity of cell-free enzymes by up to 53 % when compared to controls (10 °C), likely by enhancing the catalytic activity of the enzymes. Our results suggest the importance of both UVR and temperature as control mechanisms for cell-free enzymes. Given the projected warming ocean environment and the variable UVR light regime, it is possible that there could be major changes in the cell-free EEA and in the enzymes contribution to organic matter remineralization in the future.

Short summary
Recent evidences suggest that the proportion of cell-free relative to the total EEA is usually comparable or larger than to the cell-associated. Yet, it is unknown what is the fate of those cell-free enzymes in ocean (which are still active) nor what controls their activities. We found that the activity of cell-free enzymes is affected by both temperature and UV light and that this effect was enzyme specific, suggesting a link between warming and the degradation of organic matter in the sea.
Final-revised paper