Abstract
The compatible solute dimethylsulfoniopropionate (DMSP), made by many marine organisms, is one of Earth's most abundant organosulfur molecules. Many marine bacteria import DMSP and can degrade it as a source of carbon and/or sulfur via DMSP cleavage or DMSP demethylation pathways, which can generate the climate active gases dimethyl sulfide (DMS) or methanthiol (MeSH), respectively. Here we used culture-dependent and -independent methods to study bacteria catabolizing DMSP in the East China Sea (ECS). Of bacterial isolates, 42.11% showed DMSP-dependent DMS (Ddd+) activity, and 12.28% produced detectable levels of MeSH. Interestingly, although most Ddd+ isolates were Alphaproteobacteria (mainly Roseobacters), many gram-positive Actinobacteria were also shown to cleave DMSP producing DMS. The mechanism by which these Actinobacteria cleave DMSP is unknown, since no known functional ddd genes have been identified in genome sequences of Ddd+ Microbacterium and Agrococcus isolates or in any other sequenced Actinobacteria genomes. Gene probes to the DMSP demethylation gene dmdA and the DMSP lyase gene dddP demonstrated that these DMSP-degrading genes are abundant and widely distributed in ECS seawaters. dmdA was present in relatively high proportions in both surface (19.53% ± 6.70%) and bottom seawater bacteria (16.00% ± 8.73%). In contrast, dddP abundance positively correlated with chlorophyll a, and gradually decreased with the distance from land, which implies that the bacterial DMSP lyase gene dddP might be from bacterial groups that closely associate with phytoplankton. Bacterial community analysis showed positive correlations between Rhodobacteraceae abundance and concentrations of DMS and DMSP, further confirming the link between this abundant bacterial class and the environmental DMSP cycling.
Highlights
The tertiary sulfonium compound dimethylsulfoniopropionate (DMSP) is made in prodigious amounts in marine environments (Ksionzek et al, 2016)
Culture-independent methods were used to explore the spatial distribution and diversity of key DMSP catabolic genes in a transect of five East China Sea (ECS) stations from inshore to the offshore waters, and the results further demonstrate the importance of bacterial DMSP-catabolism in the ECS
Surface seawater (SW) and Bottom seawater (BW) samples were collected onboard the R/V ‘Dong Fang Hong 2’ in the ECS during two cruises from 14 July to 1 August 2013 and 19 October to 2 November 2015, respectively (Figure 1 and Table 1)
Summary
The tertiary sulfonium compound dimethylsulfoniopropionate (DMSP) is made in prodigious amounts (several petagrams, worldwide annually) in marine environments (Ksionzek et al, 2016). Heterotrophic bacteria have been reported to synthesize DMSP (Curson et al, 2017). In these organisms DMSP may function in e.g., storage of excess sulfur and carbon (Stefels, 2000), cryoprotection, oxidative damage protection (Sunda et al, 2002), signaling pathways acting as chemoattractant (Seymour et al, 2010) and enhancing the prodution of quorum-sensing molecules (Johnson et al, 2016). DMS, largely derived from DMSP catabolism, is the most significant biogenic sulfur compound transferred from oceans to the atmosphere (~3 x 108 tonnes, worldwide annually; (Andreae, 1990), where its oxidative products act as cloud condensation nuclei (CCN) affecting cloud cover and the radiation reaching Earth’s surface (Vallina and Simó, 2007)
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