Marine Bacteria Can Act Selfishly

These are marine rod-shaped bacteria shown by their DNA (blue). They have a distinct green halo-like colouration showing that they have taken up a specific fluorescently labelled polysaccharide. This is a method which allows for the visualisation of microbial functions.

In the oceans, single cellular plants (microalgae) fix CO2 and form biomass. This fixed material (marine organic matter) is a mixture of several compounds, including large sugars (polysaccharides). Marine bacteria (heterotrophs) degrade a large amount of the produced organic matter, and this cycling of organic matter is called the microbial loop and is a key aspect of the marine and global carbon cycle.

In the publication “An alternative polysaccharide uptake mechanism of marine bacteria,” we discovered that a large number of marine bacteria use a previously unknown method to take up and degrade polysaccharides (large sugar molecules). We also present a new method (FLA-PS) which allows for the functional identification of specific polysaccharide-degrading bacteria in environmental samples.

What is this alternative mechanism?
Previously we believed that for marine bacteria to be able to use large sugars (polysaccharides), they must first make extracellular (outside of the cell) enzymes which degrade the sugars to sizes suitable for uptake. However, by degrading sugars outside of the cell, inevitably some of the sugar is lost to other organisms which steal the produced smaller sugars. The alternative mechanism which we found – selfish uptake (described by Cuskin et. al., 2015) – avoids this loss by binding the large sugars to the cell surface, partially degrading them, and then directly taking these up into the cell (see Figure). This mechanism secures a large quantity of sugar inside the cell, without loss to other organisms — a unique advantage.

Why is this important?
Firstly, this discovery changed our paradigm (model) of how marine bacteria can use large sugars in the oceans.
Secondly, our study also shows that this alternative mechanism is important in the oceans as it is both abundant and widespread.
Finally, this discovery shows that marine bacteria have more of an impact on the marine carbon cycle than previously understood.

Please note this is a simplified summary, and a better insight can be gained from reading the original publication “An alternative polysaccharide uptake mechanism of marine bacteria,”

Marine bacteria can be selfish

  • Cuskin, F., Lowe, E. C., Temple, M. J., Zhu, Y., Cameron, E. A., Pudlo, N. A., . . . Gilbert, H. J. (2015). Human gut Bacteroidetes can utilize yeast mannan through a selfish mechanism. Nature, 517(7533), 165-169. doi:10.1038/nature13995