Marine snow, organic solute plumes and optimal chemosensory behaviour of bacteria

Aggregates of detritus known as marine snow are abundant and essential aspects of the water column. Marine snow provides the main transport mechanism for organic carbon from surface waters down to deep ocean, it also creates plumes of organic particles. Organisms associated with marine snow include microbes including bacteria, both living and dead zooplankton and various phytoplankters. The plumes ejected by marine snow are caused by enzymes breaking down the particle by colonising marine bacteria, this plume creates a nutrient gradient which attracts organisms associated with the organic detritus. This paper looks into the impact of the sinking aggregate and its association to the behaviour of marine bacteria.

Kiørboe and Jackson’s work adapted previous models of measuring the impact of sinking aggregate and the chemokinetic (the motile response to a chemical gradient) swimming behaviour of marine bacteria to understand the utilization of aggregate solute plumes on bacterial growth, behaviour and production.

By using amino acids and nitrogen as the aggregate the adapted model involved 10 000 bacteria starting in a horizontal row, starting point was determined by swimming speed. Two swimming speeds were measured for the bacterial chemosensory behaviour; 10 µm s~¹ (used in previous studies) and 100 µm s~¹ (representative of swimming speeds in marine bacteria). Simulations were run for both small and large aggregates and until most or all bacteria has left the plume.

Both swimming behaviours; ‘back and forth’ (characteristic of some marine bacteria) and ‘run and tumble’ were examined. However, was it was found that both run and tumble and back and forth mechanisms, did not influence either colonisation rate or plume utilization, both followed noticeably similar trends. Likewise, sheer alignment had little effect on efficiency. So run and tumble was assumed for the rest of the experiment.

When investigating the optimal chemokinetic behaviour for bacterial colonisation and utilization of the plume, the model previously used in studies were estimated for E. coli but were not known for marine bacteria. This paper uses knowledge and previous work to design for marine bacteria by assuming that marine bacteria can survive in a much more nutritionally dilute environment than E. coli, due to its free-living nature and living in open waters. They allowed for lower sensitivity by allowing for longer tumbling times with the marine bacteria. It soon became apparent from the paper there was a clear trade-off between long tumbling periods and high sensitivity against regions of high concentration. However, allowing to longer tumbling periods may result in the bacteria moving off the concentration region. The paper found that the positioning of this optimum trade off depends on sinking velocity of aggregate and bacterial swimming velocity.

For slower bacteria, residence time in the plume was less due to swimming speed. Chemosensory response did not allow slower swimming bacteria to utilize the plumes from small or large aggregates better than slow swimming bacteria without chemosensory response. Faster swimming bacteria were able to use the plumes much more due to their motility ability, optimum efficiency occurs with shortest possible ‘run’ length. Colonisation rate decreased slightly with increasing length of tumbling intervals. Therefore, the smallest tumbling interval with optimal sensitivity generated the highest colonisation rate.

The rate at which bacterial colonised with the aggregate was dependent on the motility of the bacterial and its chemosensitivity. This paper highlighted this importance of a reliable size spectra, sizing seems fairly inconsistent across papers whereas this paper has taken sizing into account, although when comparing with other papers this should be kept in mind. It was suggested that the marine bacteria that do not colonise the aggregate may utilize the enhanced DOM concentration in the plume generating a larger opportunity of area to exploit. The potential significance of aggregate-solute-plumes for the growth of free marine bacteria was assessed. Excitingly, it was found that bacteria with optimal chemokenetic sensitivity estimated growth rates were substantial, however, the plumes also proved highly useful for bacteria without high sensitivity as growth rates were still significant. Suggesting that plumes can be significant growth habitats for suspended free-living bacteria.

The aggregates simulating marine snow in this experiment have highlighted plumes as an important source for DOM and pinpointed the interaction between marine bacteria and the uneven distribution of organic matter. All highlighting how sinking marine snow has a big impact on the world’s oceans carbon cycling and much of this influence is dependent on the sensory characteristics and response from marine bacteria to the nutrient gradient.

Additional papers have highlighted the influence this research has established on the food web. Because these plumes attract bacteria-eating protozoa this attracts larger organisms and fish that feed on the protozoa. With this research providing knowledge of bacterial growth, this could result in a boost of marine bacteria population size which could in turn greatly influence the food web.

One area for moving forward with this research would be looking into the particle size and properties of the aggregates being used. We understand from this paper that marine bacteria growth is significant within the plumes, however more of a sizing spectrum and distribution of particle sizes would offer more understanding of aggregate use for marine bacteria populations. It is also important to understand that this paper only used one source of organic material, we understand that marine systems contain many dissolved or simply, fine particulates that could offer even wider influences on this process.

Keeping in mind of other potential work to wider understand our knowledge of the crucial processes, this paper is key to understanding the associations of sinking aggregates with marine bacteria.


Kiørboe, T. and Jackson, G. (2001). Marine snow, organic solute plumes, and optimal chemosensory behavior of bacteria. Limnology and Oceanography, 46(6), pp.1309-1318.