Chemotactic response to extracellular products of Cyanobacteria
Dissolved organic carbon (DOC) is released into the water column through extracellular exudation and cell lysis by cyanobacteria, such as the two important marine groups Synechocococcus and Prochlorococcus, which often constitute the bulk of photosynthetic biomass and are therefore responsible a significant proportion of primary production in oligotrophic waters. DOC is essential to the growth of heterotrophic bacteria and while they are expected to cluster around living phytoplankton cells, using chemotaxis to take advantage of the exuded DOC, yet they had not previously been examined quantitatively in association with prokaryotic phytoplankton. Seymour et al. (2010) tested whether the exuded chemical products of Synechococcus and Prochlorococcus are chemoattractants for three heterotrophic marine bacterium strains; Pseudoalteromonas haloplanktis, Silicibacter TM1040 and Vibrio alginolyticus. Global Ocean Sampling data from 15 open ocean sites showed that they occurred in 100%, 100% and 93% of samples, respectively, and are therefore likely to co-occur with Synechococcus and Prochlorococcus.
Synechococcus exudants were found to induce a strong chemotactic response from all three strains, with P. haloplanktis accumulating in concentrations up to 9-fold higher than the background levels. While all three strain exhibited a marked response, this was far stronger in P. haloplanktis than either of the other two strains, but was significantly different from only V. alginolyticus. Response time is driven by differences in the chemotactic velocity; typically a fraction of the maximum swimming speed but it also depends on the sensitivity of their chemoreceptor’s. Similar results were found for Prochlorococcus, with P. haloplanktis exhibiting the strongest response, but again this was not significantly different from Silicibacter TM1040. Although a slight chemotactic response was observed in V. alginolyticus, it was not significantly different from the control. Interestingly, V. alginolyticus was found to have a higher mean swimming speed than Silicibacter TM1040 (54 and 52µm s-1 respectively), so perhaps it may much less sensitive chemoreceptor’s than Silicibacter TM1040. P. haloplanktis was found to have a much higher mean swimming speed than either of the other strains; 85µm s-1.
The DOC values used here were 3-5 fold higher than commonly found in the oligotrophic ocean, however they were still within an environmentally relevant range as they are within concentrations that would be observed during a bloom event or large, localised aggregation of cells. Rapid chemotactic responses to the DOC released by these cyanobacteria are likely to provide a strong competitive advantage for heterotrophic bacteria, which will gain exposure to the nutrients before competitors. The rapid response of P. haloplanktis is likely to give it a strong competitive advantage in comparison to Silicibacter TM1040 and V. alginolyticus.
The chemotactic behaviour exhibited here provides a potential mechanism for the development of associations, recently found to commonly occur in the open ocean (in a separate study), between Synechococcus and heterotrophic bacteria. These associations may have major implications for nutrient cycling rates and microbial competition.
Seymour, J., Ahmed, T., Durham, W. & Stocker, R. (2010) Chemotactic response of marine bacteria to the extracellular products of Synechococcus and Prochlorococcus. Aquatic Microbial Ecology. 59, 161-168
http://web.mit.edu/romanstocker/publications/SeymourEtAl_AME_2010.pdf
Separate study mentioned is Malfatti, F. & Azam, F. (2009) Atomic force microscopy reveals Microscale networks and possible symbioses among pelagic marine bacteria. Aquatic Microbial Ecology. 58, 1-14http://www.int-res.com/articles/feature/a058p001.pdf
Following Colins lecture this morning I thought it may be relevant to add that the Prochlorococcus strain used was Prochlorococcus marinus MED 4Ax. I have not been able to find out whether this is a shallow or a deep strain, however it is of course possible that V. alginolyticus exhibited no chemotactic response towards the extracellular products because it is possibly not found in the same environment. Again, I was unable to find out the depth at which it usually lives, but did find that it usually lives at temperatures above 5 degrees celsius, so presumably does not live particularly deep.
ReplyDeleteRobyn - this is a very interesting comment. MED4 is one of the high light strains used in the early physiological studies and genome sequencing projects. I am guessing MED4X is a variant of the original culture, so I'm pretty sure it will be found near the surface. The authors used the GOS data to show that the heterotrophs they studied were abundant; since the GOS sampled surface waters I don't think we can explain the lack of response of V. alginolyticus in the way you suggest as a possibility.
ReplyDeleteProchlorococcus cells are very, very small and one of the most exciting implications of this study is the realisation that heterotrophic bacteria can respond to such tiny point sources of nutrients.