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Resolving the paradox: Continuous cell-free alkaline phosphatase activity despite high phosphate concentrations
Thomson, B.; Wenley, J.; Currie, K.; Hepburn, C.; Herndl, G.J.; Baltar, F. (2019). Resolving the paradox: Continuous cell-free alkaline phosphatase activity despite high phosphate concentrations. Mar. Chem. 214: 103671. https://dx.doi.org/10.1016/j.marchem.2019.103671

Bijhorende info:
In: Marine Chemistry. Elsevier: Amsterdam. ISSN 0304-4203; e-ISSN 1872-7581, meer
Peer reviewed article  

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Author keywords
    Extracellular enzymatic activity; Cell-free enzymatic activity; Organic matter remineralisation; Phosphorus cycling

Auteurs  Top 
  • Thomson, B.
  • Wenley, J.
  • Currie, K.
  • Hepburn, C.
  • Herndl, G.J.
  • Baltar, F.

Abstract
    It is generally recognised that enzymatic activities are controlled by their end-product concentration, so that when the end-product concentration is high the enzyme activity is low and vice versa. In the marine environment, however, high alkaline phosphatase (APase) activities at high inorganic phosphorus (Pi) concentrations are commonly observed. This represents a paradox that has been unresolved for decades. This study builds on recent advances in the understanding of the role of cell-free versus cell-associated extracellular enzymatic activities (EEAs) to find an explanation for this long-time enigma. Data was collected from coastal (biweekly for 1 y), epi- and mesopelagic waters open-ocean (bimonthly for 9 months) and lab experiments to investigate the relation between APase (cell-free and cell-associated) and Pi. The proportion of cell-free APase was consistently high across all these marine environments (ranging from 65 to 100% of the total activity), irrespective of the Pi concentration. A residence times experiment performed with APase enzymes from the same marine environment confirmed that cell-free APase maintained most of its activity (ca. 75% of the initial in situ activity) over a 16-day incubation. This demonstrates the long lifetime of APase in the marine environment, and the potential for spatial and temporal decoupling between the Pi concentrations and the total APase activity. Collectively, these results together with previous evidence, suggest that the persistent presence of cell-free APase activity provides a reasonable explanation for the paradox of high potential APase activities at high end-product (Pi) concentration in the marine environment. Ultimately, this finding also serves as an example of the potential critical role that cell-free enzymes play in the ecology and biogeochemistry of the ocean.

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