Keywords:Oyashio, DMSP, Bacteria
Bacterial community in the ocean involves in biological production of dimethylsulfide (DMS) from dimethylsulfoniopropionate (DMSP), a major osmolyte accumulated in phytoplankton cells. DMS emission from the ocean to the atmosphere plays a significant role in the production of secondary aerosols and subsequent cloud formation. Also, DMS is reportedly a signaling molecule used by some seabirds, crustaceans and marine mammals, suggesting the importance of its dynamics to understand biological interactions in the ocean ecosystems. Three metabolic pathways have been currently known as major processes of bacterial DMSP degradation. Two pathways can contribute to DMS production. The first one is the DMSP cleavage into DMS and acrylic acid and the second one is the cleavage into 3-hydroxypropionate and DMS after condensation of DMSP and acyl-CoA. The third pathway can contribute to a sink of DMS, because DMSP is degraded into methyl melcaptopropionate and then methionine by serial demethylation. The goal of this study is to reveal the relationship between DMS emission and DMSP-degrading bacteria and to determine environmental factors controlling this relationship. Here we report abundance and distribution of DMSP degradation genes and the corresponding bacterial community structure during the spring bloom of coastal Oyashio water. Seawater samples were collected from the surface and subsurface chlorophyll maximum layers at 17 stations off the east coast of Hokkaido during the KH-15-1 R/V Hakuho-maru cruise in March 2015. The water was serially passed through 3.0 μm and 0.22 μm pore-size filters to collect particle-associated and free-living bacteria. After extracting DNA, DMSP lyase genes (dddP and dddD) , DMSP demethylase gene (dmdA) and 16S rRNA gene were quantified by qPCR. As for the dmdA, 7 primer sets were used to differentiate diverse phylotypes. Also, amplicon sequencing of 16S rRNA gene was performed to determine bacterial community structure. As a result, two phylogenetic groups of dmdA, C/2 and D/1 clades were mainly detected in addition to dddP and dddD. The copy number of dmdA was higher than those of dddP and dddD in the Oyashio water, whereas dmdA copy number was lower than dddP and dddD copy numbers in the coastal Oyashio water. The dmdA copy number detected in this study was ca. 103-104 copies/ml. This value was one order lower than previously reported values (ca. 104-105 copies/ml) including our data in the tropical and subtropical Pacific Ocean and Tohoku coastal waters. Sequences of dmdA C/2 and D/1 clades were possessed by 1b and 1a subclades of Pelagibacter (SAR11) group bacteria, respectively. Although they often dominate bacterial assemblages in the ocean, they must be less abundant than usual in the Oyashio water of this study. Also, previous studies reported that dddP was the major DMSP lyase gene in the ocean and dddD was rather minor. However, we found the area where dddD was the major DMSP lyase gene. Since the dddD dominance in DMSP lyase genes was observed in the other Oyashio water off Tohoku area, this might be a specific feature of the Oyashio water. Furthermore, sequencing and phylogenetic analysis of dddD revealed that the gene originated from bacteria closely related to the genus Porticoccus (SAR92) belonging to the class Gammaproteobacteria. In this study, we found distinctive community structure of DMSP-degrading bacteria in the Oyashio water from previously reported structures. Further studies should be aimed to compare the change of bacterial communities with the change of DMSP and DMS concentrations and determine how bacterial gene type and abundance control the dynamics of these ecologically important organic sulfur compounds.