Thursday, 21 November 2013

Microbial chemoattraction to DMSP: the shaping of planktonic food webs and influencing global climate change

Dimethylsulfoniopropionate (DMSP) is solute produced by phytoplankton, and released into the surrounding water by grazing, exudation and cell lysis. The nature of these point source events results in spatiotemporal DMSP pulses. Such pulses have proved to be important underwater foraging cues for many species yet previous suggestions of the ecological function of DMSP amongst marine microorganisms have been contradictory to one another, and to date are largely unresolved. Though produced by microalgal species, many autotrophs that don’t produce DMSP have the capacity to uptake and assimilate it suggesting differing physiological needs for DMSP. 

The behaviour of seven marine microbial species were studied in response in to submillimeter diffusing pulses of DMSP, glycine betaine (GBT) and DMSP related degradation products: dimethylsulfide (DMS) and dimethylsulfoxide (DMSO). Though differences were found between chemicals, DMSP and its related products proved to the powerful chemoattractants with 74% of tested cases, across multiple trophic levels, resulting in positive chemotactic behaviour.  Five organisms displayed strong chemotactic responses to both DMSP and GBT (likely due to analogous physiological function and chemical structure between both chemicals). Weaker attraction was also shown to DMS and DMSO though that is congruent with the lower biological liability of these products. Relative response was quantified and stronger responses showed a 65% enhancement in chemical exposure.

Unlike most motile phytoplankton and despite possessing the ability to uptake DMSP cyanobacterium Synechoccus exhibited no chemotaxis. On the other hand Chlorophyte, Dunaliella tertiolecta, showed a positive response to DSMP despite not uptaking or assimilating it. Its strong attraction to DMS, supported by trials showing its capacity for extracellular transformation of DMSP to DMS could perhaps infer an ecophysiological requirement for DMS .

30-90% of oceanic DMSP is metabolised by heterotrophic bacteria. Two species, of which are known to demethylate DMSP, Silicibacter sp. and Pseudoalteromonas haloplanktis, exhibited strong chemotaxis to DMSP. Highly directional swimming and high chemotactic migration rates produced a rapid response to DMSP patches and resulted in a 66% exposure increase providing a substantial advantage over non-motile competitors.

Prey ingestion and osmotrophic uptake are two ways in which microzooplankton achieve their reduced sulphur supply. Strong positive responses to DMSP and related products were observed in both Oxyrrhis marina and Neobodo designis, herbivorous and bacterivorous flagellates respectively. A pronounced shift in swimming behaviour and swim velocities, consistent with that exhibited by bacteria, suggest that for both zooplankton and bacteria DMSP can be used as a resource and an infochemical.  However, with predators being attracted to DMSP patches the ecological benefits of DMSP chemotactic responses of phytoplankton must outweigh the increased risk of grazing; perhaps explaining why Synechoccus exhibited no chemotaxis.

DMS is the principal natural source of sulphur gas, and is a significant factor affecting the climate system. Microbial exposure to DMSP and grazing on DMSP-producing prey are key processes involved in regulating ocean-atmosphere DMS flux. These demonstrated chemotactic responses could in fact enhance DMS production by increasing such processes, mediating a potentially large influence on global sulphur biogeochemistry.

Seymour J. R., Simó R., Ahmed T. and Stocker R. (2010) Chemoattraction to Dimethysulfoniopropionate Throughout the Marine Microbial Food Web. Science, 329, 342-345


  1. Did the authors sequence the genomes of the bacteria? Is it possible that Synechoccus didn't exhibit any chemotaxis because it lacked the genes to express chemotactic ability?

    1. The authors didn't in fact sequence the genome of Synechoccus, however chemotaxis towards nitrogen compounds has been observed before in Synechoccus spp. so i believe they do possess the required genes (unless the genes are different for responses to different chemicals?). After reading around, it appears that coastal isolates of Synechoccus are non-motile, only open-ocean isolates have swimming capacity. It is not stated in the paper, but perhaps in this study coastal isolates were used which would explain the lack of chemotaxis.

  2. Did they mention where they collected their samples from? I'm just wondering whether the abundance (and dominance), of the different microbial groups differs between different regions of the oceans. For example, there tends to be higher concentration of phytoplankton around coastal areas, but less so in the open ocean...if most of the microbial species in this study showed chemotaxis towards DMSP, could there be a greater degree of uptake from a particular species depending on its environmental conditions, or geographical location?

    1. As in my reply to Rachel, the authors didn't state where they collected their samples from. As their is a difference in the swimming ability of coastal and open-ocean isolates it seems quite reasonable to believe that their degree of uptake and availability could be geographically or environmentally dependant too. Equally more phytoplankton (DMSP-producing) in coastal areas could correlate to an increased amount and availability of DMSP, though perhaps the amount of other microbes may also increase in response to this.

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