Effects of nitrogen and phosphorus on the bacterial dimethylsulfoniopropionate production and catabolism

Abstract:

[Objective] Dimethylsulfoniopropionate (DMSP) is one of the main organic sulfides in the ocean, and the main source of sulfur for marine bacteria. DMSP is catabolized by bacteria into dimethylsulfide (DMS), driving the sulfur cycle of the Earth. This study simulated the effect of seawater eutrophication on DMSP/DMS, DMSP producing genes (dsyB and mmtN), catabolising genes (dddP and dmdA) and corresponding functional bacteria through the mesocosm. [Methods] We used flow cytometry to quantify the abundance of microplankton of 92 water samples. We sequenced bacterial 16S rRNA gene of seawater samples by high-throughput sequencing. Then we quantified the abundance of 16S rRNA gene, DMSP producing and catabolising genes by quantitative PCR. [Results] Adding nitrate (6.00 μmol/L) and phosphate (0.375 μmol/L) simultaneously increased the concentration of chlorophyll a, DMSP and DMS. For DMSP producing genes, phosphate enriched the abundance of dsyB and some DMSP producing genera, such as Phaeobacter. Adding nitrate and phosphate could enrich dsyB simultaneously, inhibit the growth of Alteromonas and the enrichment of mmtN gene, but the effects of phosphate on dsyB enrichment was better than nitrate. In terms of DMSP catabolising genes, adding nitrate and phosphate simultaneously promoted the enrichment of dddP and the DMSP catabolising genera like Thalassococcus, Thalassobius, Loktanella and Shimia, but inhibited the enrichment of SAR11, Sulfitobacter and other species, resulting in the failure of enrichment of dmdA. Nitrogen restriction could better promote the abundance of DMSP producing gene, resulting in the increasing of bacterial DMSP production to cope with the insufficient nutritional conditions, and rising the proportion of DMSP demethylation to provide more energy for bacteria. However, in the case of nitrate and phosphate abundance, bacteria were apt to reduce the synthesis of DMSP and was more inclined to lyse DMSP to produce DMS in order to reduce the ratio of sulfur assimilation. [Conclusion] The results of this study emphasized the effect of seawater eutrophication on the bacterial DMSP production and catabolism.