Cell-free extracellular enzymatic activity is linked to seasonal temperature changes: a case study in the Baltic Sea

Federico Baltar,Catherine Legrand,Jarone Pinhassi

doi:10.5194/bg-13-2815-2016

Federico Baltar, Catherine Legrand + Show 1 more

Open Access

https://doi.org/10.5194/bg-13-2815-2016

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Journal: Biogeosciences (Online)	Publication Date: May 13, 2016
Citations: 41	License type: CC BY 3.0

Affiliation: Linnaeus University, University of Otago

Abstract

Abstract. Extracellular enzymatic activities (EEAs) are a crucial step in the degradation of organic matter. Dissolved (cell-free) extracellular enzymes in seawater can make up a significant contribution of the bulk EEA. However, the factors controlling the proportion of dissolved EEA in the marine environment remain unknown. Here we studied the seasonal changes in the proportion of dissolved relative to total EEA (of alkaline phosphatase (APase), β-glucosidase (BGase), and leucine aminopeptidase (LAPase)), in the Baltic Sea for 18 months. The proportion of dissolved EEA ranged between 37 and 100, 0 and 100, and 34 and 100 % for APase, BGase, and LAPase, respectively. A consistent seasonal pattern in the proportion of dissolved EEA was found among all the studied enzymes, with values up to 100 % during winter and < 40 % during summer. A significant negative relation was found between the proportion of dissolved EEA and temperature, indicating that temperature might be a critical factor controlling the proportion of dissolved relative to total EEA in marine environments. Our results suggest a strong decoupling of hydrolysis rates from microbial dynamics in cold waters. This implies that under cold conditions, cell-free enzymes can contribute to substrate availability at large distances from the producing cell, increasing the dissociation between the hydrolysis of organic compounds and the actual microbes producing the enzymes. This might also suggest a potential effect of global warming on the hydrolysis of organic matter via a reduction of the contribution of cell-free enzymes to the bulk hydrolytic activity.

Highlights

Prokaryotes play a central role in the marine biogeochemical cycles by transforming dissolved organic matter (DOM) into living particulate organic matter (Azam and Cho, 1987)
A high proportion of dissolved Extracellular enzymatic activities (EEAs) could indicate a greater importance of the history of the water mass than of the actual processes occurring at the time of sampling (Arnosti, 2011; Baltar et al, 2010; Baltar et al, 2013; Karner and Rassoulzadegan, 1995)
EE lifetimes of surface waters were longer when incubated in the dark at 4 ◦C than at in situ conditions of light and temperature (Li et al, 1998). This is consistent with the only available study comparing cold deep versus warm surface waters EE lifetimes, where EE lifetimes were about 1 order of magnitude longer in the deep waters (Baltar et al, 2013). These results suggest that temperature could be a critical factor preserving the activity of cell-free EEA and thereby controlling the proportion of dissolved EEA in the marine environment

Summary

Introduction

Prokaryotes play a central role in the marine biogeochemical cycles by transforming dissolved organic matter (DOM) into living particulate organic matter (Azam and Cho, 1987). Other reports suggested a major contribution of dissolved EEA to the total oceanic EEA pool (Baltar et al, 2010; Baltar et al, 2013, 2010; Karner and Rassoulzadegan, 1995; Keith and Arnosti, 2001; Obayashi and Suzuki, 2008a) This high proportion of cell-free EEA is important because it can decouple hydrolysis rates of organic material from microbial dynamics. A high proportion of dissolved EEA could indicate a greater importance of the history of the water mass than of the actual processes occurring at the time of sampling (Arnosti, 2011; Baltar et al, 2010; Baltar et al, 2013; Karner and Rassoulzadegan, 1995)

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