Complex Interactions Within Microbial Communities

Interactions between micro-organisms are an important feature of marine environments, and have a significant role in shaping communities and ecosystems. Previous studies have revealed some of the mechanisms of these interactions, which include the facilitation of vitamin and mineral uptake, inter- and intra-specific communication and allelopathy. Results have shown that heterotrophic bacteria can have a positive or negative effect on growth of algae and cyanobacteria, and more recent studies have investigated the effects of heterotrophic bacteria on one of the most abundant phototrophs in temperate and tropical oceans, Prochlorococcus.

The paper expands upon this previous work, and details the effects of 344 strains of heterotrophic bacteria on the growth of two Prochlorococcus strains; a low light adapted strain (MIT9313) isolated from the Gulf Stream and a high light adapted strain (MED4) isolated from the Mediterranean Sea. Two ecologically distinct strains were chosen in order to determine whether the same heterotroph would have different effects on each strain. The heterotrophic bacteria, consisting of gamma-proteobacteria and alpha-proteobacteria, were isolated from the Hawaii Ocean Time Series (HOT) station ALOHA, which has an extensively studied Prochlorococcus-dominated community.

The heterotrophs and the Prochlorococcus strains were co-cultured in 96-well plates on Pro99 media with pyruvate, acetate, lactate, glycerol and a vitamin mix added. Plates were kept at 20°C and 27µE constant cold white illumination for 42 days, and almost every day a reading of the bulk chlorophyll fluorescence (FL) was taken to produce a growth curve for each strain.

The authors discovered that the growth of strain MIT9313 was affected more than that of strain MED4, with ninety-eight per cent of the heterotrophs having no clear effect on MED4, compared to only 6% which had no effect on MIT9313. Other heterotrophs caused the MIT9313 strain to enter exponential growth earlier or later, or caused growth to arrest early. Partial 16S ribosomal sequencing was used to define closely related bacteria to determine whether they had similar effects on the growth of the strains. The results revealed that bacteria whose sequences differed by 1-2% did have similar effects on MIT9313.

The authors also sought to find out whether the interactions required cell-to-cell contact, by co-culturing MIT9313 with five bacterial strains from different clades, and separating them with a 0.4 µm membrane. Those heterotrophs which caused early exponential growth of strain MIT9313 in the original experiment had the same effect when separated by the membrane, indicating that diffusible soluble compounds mediated this response. Interestingly, Alteromonas strain HOTo1A3 only caused late growth when mixed with MIT9313, and not when separated by the membrane.

The relevance of this study could be called into question as the authors were unable to replicate the natural environment of Prochlorococcus, and did not isolate the strains from the same location as the heterotrophs. However the paper does lay the foundations for further research to take place, involving a more diverse range of bacteria, to further our understanding of these complex interactions which are so important within communities.

A review of: Sher, D., Thompson, J.W., Kashtan, N., Croal, L. and Chisholm, S.W. (2011) Response of Prochlorococcus ecotypes to co-culture with diverse marine bacteria, The ISME Journal, 5, 1125-1132.