Abstract
Marine microorganisms and their extracellular enzymes (ECEs) play an important role in the remineralization of organic material by hydrolyzing high-molecular-weight substrates to sizes sufficiently small to be transported through cell membrane, yet the diversity of the enzyme-producing bacteria and the types of ECEs involved in the degradation process are largely unknown. In this work, we investigated the diversity of cultivable bacteria and their ECEs and the potential activities of aminopeptidase in the water column at eight different depths of the New Britain Trench. There was a great diversity of cultivable bacteria and ECEs, and depth appears an important driver of the diversity. The 16S rRNA sequence analysis revealed that the cultivable bacteria were affiliated mostly with the phyla Proteobacteria and Actinobacteria, and the predominant genera were Pseudoalteromonas (62.7%) and Halomonas (17.3%). Moreover, 70.7% of the isolates were found to produce hydrolytic zone on casein and gelatin plates, in which Pseudoalteromonas was the predominant group, exhibiting relatively high protease production. Inhibitor analysis showed that the extracellular proteases from the isolated bacteria were serine proteases in the surface water and metalloproteases in the deep water. Meanwhile, the Vmax and Km of aminopeptidase exhibited a maximum in the surface water and low values in the deep bathy- and abyssopelagic water, indicating lower rates of hydrolysis and higher substrate affinity in the deeper waters. These results shed new insights into the diversity of the cultivable bacteria and bacterial ECEs and their likely biogeochemical functions in the trench environment.
Highlights
Organic matter in the ocean’s photic zone is mostly remineralized in the upper layers of the water column, and an estimated 0.1% of the production exported downward through the water column and buried in deep-sea sediments (Hedges, 1992)
The particulate organic matter (POM)-dissolved organic matter (DOM) Piezophilic Microorganism Continuum (PDPMC) model proposed by Fang et al (2015) suggests that microorganisms in the deep ocean play a more important role in mineralization of marine organic matter than hitherto recognized (Nagata, 2000; Tamburini et al, 2003, 2009, 2013; Glud et al, 2013)
Bacterial counts were higher in surface water (75 and 200 m), ranging from 1.23 × 105 to 0.68 × 105 cells ml−1, and low in the deep water (1000–6000 m), ranging from 4.5 × 103 to 13.7 × 103 cells ml−1 (Table 1, Figure S3)
Summary
Organic matter in the ocean’s photic zone is mostly remineralized in the upper layers of the water column, and an estimated 0.1% of the production exported downward through the water column and buried in deep-sea sediments (Hedges, 1992). Heterotrophic microbial communities decompose particles and aggregates of marine organic matter and shape the nature and quantity of the released carbon and nutrients that pass through the water column from surface water to the deep ocean (Azam, 1998). Recent investigations on the role of marine bacteria in carbon cycling in the mesopelagic and bathypelagic waters have greatly contributed to our understanding of the importance of microbial ECEs in the biogeochemical processes of the global ocean (Zoppini et al, 2005; Ziervogel et al, 2007; Baltar et al, 2009; Steen et al, 2010; Arnosti, 2011, 2014; Williams et al, 2013). The POM-DOM Piezophilic Microorganism Continuum (PDPMC) model proposed by Fang et al (2015) suggests that microorganisms in the deep ocean play a more important role in mineralization of marine organic matter than hitherto recognized (Nagata, 2000; Tamburini et al, 2003, 2009, 2013; Glud et al, 2013)
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