Gel Organic Matter: A Sticky Situation

It is predicted that around 10% of surface Dissolved Organic Matter (DOM) is in the form of Gel Organic Matter (GOM), this equates to roughly 70x10¹⁵g of organic carbon, which exceeds the global biomass of marine organisms by a factor of 50 (Verdugo et al. 2004). These gels are composed of polysaccharides, are found in multiple marine environments and are thought to play a significant role in the global carbon cycle.

Particle coagulation is mainly affected by collision rate constants of particles, particle stickiness and particle abundance (Jackson & Burd 2015), but it has been thought that bacteria help the formation of these gels by excreting sticky extracellular polymers. Yamada et al (2016) set out to see how much of a role Bacteria play in the formation of these gels. Seawater, DOM and bacteria were added to test tubes and left for an incubation period of 96 hours to see how gels were formed out of the DOM.

The results found that bacterial assemblages strongly enhanced the coagulation of GOM in comparison with the control, but they say due to the relevant volumes that the Bacteria alone cannot account for the 10⁵-fold increase in mean particle volume and this may be due to the alteration of surface properties of the gels. One thing I would have to question is the number of replicates for each experiment as surely three is not enough to gain reliable results even if there is significant differences in the statistical analysis.

They have also isolated the types of Bacteria that are thought to have played a role, with the most abundant group being the Bacteroidetes. They also found that the highest growth rate was from the Pseudoalteromonas spp. group which account for 2% at the start but increased to 28% after 24 hours. The enhancement from this particular Bacteria varies significantly between species and suggests that it is a species specific trait. The importance of this is not overlooked as they go on about this in their discussion and I agree with their recommendation of the further research into the genetic traits of this Bacteria to try to isolate what helps with the formation of the gels.

I think more research needs to be carried out in finding the exact mechanisms that Bacteria use to help the coagulation of GOM and how it may be of use to them.

References

Jackson, G.A. and Burd, A.B., 2015. Simulating aggregate dynamics in ocean biogeochemical models. Progress in Oceanography.133. pp.55-65.

Verdugo, P., Alldredge, A.L., Azam, F., Kirchman, D.L., Passow, U. and Santschi, P.H., 2004. The oceanic gel phase: a bridge in the DOM–POM continuum. Marine Chemistry.92(1). pp.67-85.

Yamada, Y., Fukuda, H., Tada, Y., Kogure, K. and Nagata, T., 2016. Bacterial enhancement of gel particle coagulation in seawater. Aquatic Microbial Ecology. 77(1). pp.11-22.