Exploitation of nutrient plumes and pulses by Marine Bacteria

Fig 1.

A review of :Roman Stocker, Justin R. Seymour, Azadeh Samadani, Dana E. Hunt, and Martin F. Polz, 2008, Rapid chemotactic response enables marine bacteria to exploit ephemeral microscale nutrient patches

The ocean is not a homogeneous medium; it contains patches, namely pulses and plumes, rich in nutrients that are available only for a short time before they become dissipated throughout the sea. These nutrient sources are important to certain motile microbes that are able to take advantage of them thanks to their fast chemotactic response. Bacteria, like Pseudoalteromonas haloplanktis (used in this paper), require a biological response which is shorter than the time it takes for physical effects to homogenize the nutrients allowing them to exploit the patches. This is represented in the text as T_(BIO) < T_(PHYS). The stationary pulses in our ocean are created by lysing cells or left over nutrients from a feeding event and are readily dispersed by diffusion, plumes of nutrients however are moving or sinking particles where advection (horizontal ocean current) also plays its part in dispersal. The authors have managed to set up an experiment using microchannels which create both sinking plumes and purely diffusive pulses at a spatiotemporal scale that is consistent with the actual environment. This allows realistic quantitative results to be taken, something which has not yet been achieved. The journal focuses on exposure time of P. haloplanktis to the nutrients (longer exposure suggests increased exploitation) and suggests marine microbes could have an important role in the biogeochemical alteration of our seas.

The Diffusing Nutrient Pulse – A pulse of width 300µm was created using a microinjector to simulate lysis of a cell. This band reached a width of approximately 1mm within 3 minutes due to diffusion leaving only 40% of the nutrients within the original 300µm. The microbial cells strongly aggregated at the centre of the nutrient band once it had been simulated creating a hotspot (H). It took the motile marine cells just 2.9 minutes to reach H_MAX = 2.8 thanks to their strong chemostatic response. To compare, the non – motile cell Escherichia coli was used in a conduction of the same experiment. In near optimum conditions the chemotactic response of this bacterium was much slower taking 7.0 minutes to reach an H_MAX = 1.9, by which time much of the nutrients would have been homogenized giving E. coli no chance of exploiting the nutrients. It was found that P. haloplanktis was ten times faster chemotaxing into the nutrient pulse thanEscherichia coli, giving the former an extreme advantage over other bacteria when it comes to exposure to dissolved organic matter.

Nutrient Plumes – Fig 1a shows the response of microbacteria to a nutrient plume. This is created using a stationary particle in a constant stream of which rate can be changed and monitored. The stream runs from the bottom to the top in the diagram, and accumulation of the bacteria was monitored at three different speeds 66, 220 and 660 µm S^-1. You can see a dark line in the middle of the nutrient plume showing how chemotaxis brings the majority of microbacteria to the centre of the nutrient plume. At the slowest stream speed the microbacteria captured the entire plume, with accumulation occurring immediately behind the particle. An intense hot spot was also created at 4 – 8mm upstream. In comparison, at a stream speed of 660 µm s ^-1 there was no immediate accumulation and there was no hotspot even though the bacteria had 36 seconds to respond. Some bacteria did however enter the plume. Further experiments measured Tc, or nutrient exposure, with a 4 -fold advantage observed in motile cells thanks to their chemotactic response, compared to non - motile cells.

The aim of this journal was to investigate whether marine microbes can fully take advantage of nutrient patches found throughout the ocean. They can move faster than non – motile cells and the creation of hotspots in both plumes and pulses show that motile microbes, with a suitable chemotactic response, are exposed to nutrients long enough to influence carbon turnover in our seas. This is an important discovery and will help in the understanding of the carbon cycle. Maybe other marine microbes could be investigated to see if they are able to respond even faster than the one used in this experiment.