Chemoattraction to DMSP throughout the marine microbial food web!

DMSP (Dimethylsulphoniopriopionate) has many roles in the environment, from chemical cues on a microscale to cloud formation, and therefore possible influence on climate on a global scale. 
 I have looked at a 2010 paper by Justin Seymour et al on the chemoattraction to DMSP throughout the marine microbial food web by using microfluidics and image analysis of swimming behaviour.

 DMSP is produced by phytoplankton (the two main contributors of DMSP being the dinoflagellates and prymnesiophytes). When the phytoplankton die, for example, by predation or grazing, DMSP is released into the water column. This release of DMSP acts as a chemical hotspot to many organisms in the immediate surroundings which use DMSP as a carbon and/or sulphur source. They are deemed hotspots because the concentrations of DMSP in bulk seawater are typically in the nanomolar range, whereas phytoplankton internal concentrations can exceed 100mM.

 In this experiment, the chemotactic behavioural response of 7 species of marine microbes (Pseudoaltermonal haloplanktis, Silicibacter TM1040, Synechococcus WH8102, Micromonas pusilla, Dunaliella tertiolecta, Neobodo designis and Oxyrrhis marina) were observed when exposed to ephemeral patches of DMSP, DMS (Dimethyl sulphide), DMSO (Dimethylsulphoxide), and GBT (Glycine betaine). DMS, and DMSO are degradations of DMSP, and GBT is analogous to DMSP in terms of structure and function).

 A 300µm wide band of the chosen chemoattractant was released by a microinjector into a 3mm wide microchannel containing the micro organisms. This was filmed over a period of 6-12 hours, after which, cell positions could be observed in each frame using image analysis, therefore providing spaciotemporal distributions throughout the experiments. The paths the cells followed and the trajectory allowed the swimming speeds, rate of change of direction, and inward migration speed to be computed.
 In 74% of the tested cases, positive chemotaxis was observed, indicating that DMSP and its related chemicals are chemoattractants across multiple trophic levels in the marine microbial food web.

 The relative strength of the responses was then measured using a chemotaxis index (the ratio of mean cell concentration within the central 300µm of the microchannel). The strong responses led to >65% enhancement in exposure to these compounds, which in the ocean is very advantageous to chemotactic foragers. 

 Motile phytoplankton can use chemotaxis to find DMSP patches, eg Dunaliella tertiolecta and Micromonas pusilla were both substantially attracted to DMSP, and D. tertiolecta also showed attraction towards DMS. The attraction of D. tertiolecta is interesting as it doesn't uptake or assimilate DMSP, it actually extracellularly cleaves it to produce DMS, for which, it is believed to have an ecophysiological requirement.

 Between 30-90% of oceanic DMSP is metabolised by bacteria, which either demethylate, or cleave it. The chemotactic response of Silicibacter spp. & P. haloplanktis was quantified. Both of these were shown to demethylate DMSP. P. haloplanktis showed strong chemotaxis towards high concentrations of DMSP, using highly directional swimming to get to the patch, with a chemotactic velocity of up to 44% of the mean swimming speed (bacterial chemotactic migration rates are usually less than 10% of swimming speed), indicating that DMSP is a strong chemoattractant for this particular strain. 

 DMSP can be a resource and an infochemical for microzooplankton. DMSP uptake has been shown to supply reduced sulphur to a dinoflagellate grazer by prey ingestion, and osmotrophic uptake. It can also provide a chemical indication of prey for foragers, however it is also known that DMSP inhibits grazing by microzooplankton. 
 To test this, the foraging response of herbivorous dinflagellate Oxyrrhis marina, and the bacterivorous heterotrophic nanoflagellate Neobodo designis to DMSP patches were measured. N. designis showed highest levels of chemoattraction to low DMSP levels, and O. marina showed chemoattraction to all concentrations of DMSP, DMS, DMSO and GBT, with it's response to DMSP being the highest. O. marina showed shifts in swimming behaviour, getting to speeds of up to 35% of the mean swimming speed, and then doubling in turning rates once inside the patch to stay in there.

I think this was an interesting read as it has lots of small experiments which all linked up to DMSP, showing different results. 

Here's the link to the main bit: http://web.mit.edu/romanstocker/publications/SeymourSimoAhmedStocker_Science2010.pdf

The methods and materials were in a separate document but that was easily accessible (Here's the link:  http://www.sciencemag.org/content/329/5989/342/rel-suppl/167519cd186b24bf/suppl/DC1 )

Seymour, J. et al. (2010). Chemoattraction to Dimethylsulfoniopriopionate Throughout the Marine Microbial Food Web.Science. 329, p342-345.