Abstract
Benthic prokaryotes are the key-players in C-cycling at the sediment-seawater interface, one of the largest biologically active interfaces on Earth. Here, microbial-mediated processes, such as the degradation of organic matter and the incorporation of mobilized C into microbial biomass, depend on several factors such as environmental temperature and substrate availability, especially in shallow sediments at mid-high latitudes where seasonal fluctuations of these variables occur. In the present study, four degradative activities (β-glucosidase, lipase, chitinase and aminopeptidase), Heterotrophic C Production (HCP), Total Organic C (TOC), Total Nitrogen (TN) and Biopolymeric C (BPC) were investigated seasonally from April 2010 to April 2018 in the surface sediments of a shallow Long-Term Ecological Research (LTER) station of the northern Adriatic Sea. Significant temperature-dependences were described by Arrhenius-type equations for HCP and each of the degradative activities tested with the exception of aminopeptidase. The relatively low apparent Activation Energies suggested that these microbial-mediated processes were enhanced by the availability of palatable substrates over the study period. Nevertheless, a clear and tight dependence from such substrates was detected only for aminopeptidase, the most pronounced degradative activity observed. TN was identified by the stepwise multiple regression analysis as the environmental variable that mainly drove this exoenzymatic activity. Enhanced aminopeptidase rates mirrored peaks of TN that seemed, in turn, linked to the seasonal proliferation of benthic microalgae. By supplying prokaryotes with promptly available substrates, these autotrophs, represented mainly by diatoms, seemed to play an important role in the C-cycling regulation at the studied LTER station.
Highlights
Termed prokaryotes, bacteria and archaea play a pivotal role in C-cycling and geochemical processes as the main elements responsible for the degradation of organic matter and the incorporation of mobilized C into microbial biomass
Total Nitrogen (TN) concentrations varied between 1.11 ± 0.04 and 1.76 ± 0.06 mg N g-1 (Dec16 and Feb11, respectively) while Total Organic C (TOC) ranged from 9.85 ± 0.23 to 18.09 ± 0.14 mg C g-1
Over a time span relatively long for data of microbial-mediated processes, this study allowed us to understand how two environmental factors – i.e. temperature and substrate availability – determine the microbial C-cycling in temperate shallow marine sediments
Summary
Termed prokaryotes, bacteria and archaea play a pivotal role in C-cycling and geochemical processes as the main elements responsible for the degradation of organic matter and the incorporation of mobilized C into microbial biomass. In order to make the organic matter available, prokaryotes induce the production of non-cytoplasmatic enzymes that hydrolyze polymers and oligomers into assimilable monomers (Chróst 1992). These active molecules can be cellbound or cell-free (Baltar 2018), in the present study both kinds are inclusively referred to as exoenzymes. Their targets are specific chemical bonds that link the subunits that form organic polymers (i.e. sugars, peptides, nucleotides, structural and storage polysaccharides). The end-up products of exoenzymatic reactions are low molecular weight compounds that are incorporated into prokaryotic cells and used for growth
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