Feeding in the Phycosphere

Dissolved organic matter (DOM) is important in the carbon cycle, but it’s distribution within the ocean and the effect of this on bacterial communities needs further research. DOM can appear to be homogenously distributed in bulk seawater, yet tiny oases in the form of phycospheres disrupt the monotony of background levels of DOM. The phycospheres are small auras of DOM which surround phytoplankton and other detrital particles, and they form a steep chemical gradient which allows chemotactic Bacteria to move towards them.

A recent study by Smriga et al (2015) attempts to predict the wider ecological consequences of microscale interactions, by using observations of bacterial movement during diatom lysis events in conjunction with a resource utilisation model. The authors induced lysis in the diatom Chaetoceros affinis and then mapped the movements of Bacteria, showing that less than one minute post-lysis the Bacteria had formed dense clusters within the phycosphere. Findings showed that at low concentrations of Bacteria (<10⁵cells per ml) the majority of the DOM diffused past the cluster and escaped the phycosphere, but at high concentrations similar to those found during plankton blooms (10⁷cells per ml) 92% of the DOM was consumed within the phycosphere.

The study also found that the typical bacterial assemblage inverts during plankton bloom and collapse simulations. Normally non-motile oligotrophs dominate because they are adapted to survive with low, but consistent, DOM levels, but during a bloom motile copiotrophic species become most prevalent in terms of biomass. Bacterial populations grow as a result of the bloom and this increases competition, but motile species gain an advantage because they can move towards higher concentrations of nutrients. The switch in biomass from oligotrophs to copiotrophs will become more pronounced as the bloom collapses and phycosphere encounters increase.

The authors pose an interesting idea about the risk that Bacteria may face from viral attack when clustering within phycospheres, since it brings together huge amounts of Bacteria into close proximity. Conversely, they also state that low diffusivity particles in the phycosphere, such as phages, are not preferentially captured by motile Bacteria so the risk may be reduced by selective feeding, but this idea would be interesting to investigate. 

I think that although the author’s findings are useful for helping to map the ‘bigger picture’ of microbial interactions, it is limited in that it only uses one species of diatom. Different types of plankton may induce different levels of chemotaxis or different behaviours in bacterial species, so this is something that could be looked into further to provide a more complete picture of bacterial assemblages and the ecological effect this has in turn.

Smriga S., Fernandez V. I., Mitchell J. G., Stocker R. (2016) Chemotaxis towards phytoplankton drives organic matter partitioning among marine bacteria. Proceedings of the National Academy of Sciences. 113(6) pp 1576-1581