Bacterial DMSP Metabolism Inside Zooplankton

     Osmosis regulator, precursor to the climate altering gas dimethyl sulfide, cryoprotectant and occasional chess player; behold the famous dimethylsulfoniopropionate, or DMSP, a ubiquitous component of the water column and algal cells. The organic carbon in some phytoplanktonic species can be as much as 20 percent DMSP and once outside of a cell, DMSP can undergo a myriad of biological or chemical transformations. There are many free living DMSP-consuming bacteria (DCB) which cleave and demethylate DMSP for their carbon needs; 5 percent of microbial carbon demand is estimated to be fulfilled by DMSP alone. Most DMSP research has been aimed at its metabolism, but little has been done on the DCB associated with zooplankton; phytoplankton grazing is known to accumlate DMSP in copepod guts and thriving DCB communities have been identified in the copepod Acartia tonsa. This study predicted that since copepod associated DCB abundances and composition will vary with changes in which phytoplankton species are being grazed, then they should also change in relation to DMSP content of copepod diets. DCB abundances associated with A. tonsa were tested across five phytoplankton diets varying in DMSP content. The DMSP metabolising abilities of these bacteria were tested against 4 other carbon sources by measuring their growth rates, allowing a clearer picture of how DMSP is used by microbes and what marine adventures it undertakes in the environment.

     To ensure grazing behaviour was not a confounding factor, A. tonsa diets were standardised in carbon content and consisted of 5 different phytoplankton species, all of which had similar cell sizes. Despite prior knowledge that copepod consumption of DMSP rich phytoplankton can increase DCB abundances, there was no clear correlation between dietary DMSP content and DCB abundances. For example, abundances of DCB in copepods fed with DMSP rich Alexandrium tamarense algae were similar in copepods fed DMSP deficient Dunaliella tertiolecta algae. This could be because the DCB associated with copepods might be using other carbon sources; this is supported by previous work showing that bacteria associated with copepod faeces, bodies and habitats are usually from the same metabolically functional groups. DCB in this study were also shown to be able to grow on all the other substrates tested, with varying successes at assimilating either methyl or carboxyl groups from each carbon source treatment.

     Overall it appeared that these DCB associated with copepods were very metabolically flexible and whist not dependent on DMSP, they would still make opportunistic use of it when it became available, so this bacteria can maintain their populations even when DMSP is scarce. This link between DCB and copepods highlights a potentially large sink for DMSP spanning across all oceans, given how both copepods and DMSP have cosmopolitan distributions. Further studies should give attention to larger scale temporal and spatial variations in phytoplankton communities and how they influence the communities of DMSP metabolising bacteria which are a crucial part of how DMSP moves and changes through ecosystems. These findings are especially interesting because they establish that there is direct link in the DMSP cycle between the zooplankton and marine bacteria; they also show that DMSP has more to offer than just being a chemotactic signal.

Yuan Dong , Gui-Peng Yang & Kam W. Tang (2013) Dietary effects on abundance and carbon utilization ability of DMSP-consuming bacteria associated with the copepod Acartia tonsa Dana. Marine Biology Research, 9:8, 809-814.