The faster you are, the more you get to eat

The ocean is a turbulent environment and dissolved organic carbon (DOC) is quickly dissipated through diffusion and advection. The nutrient concentration is heterogeneous with patches of DOC appearing via sloppy eating, faecal pellets and cell lysis. Approximately 30% of oceanic bacteria have evolved the ability to use motility (Fenchel, 2002) and use chemotaxis to seek out patches of nutrients. But to what extent can they respond?

Previously, computer models have been used to predict bacterial response to nutrient patches, using known values for diffusion and observed bacterial motile behaviour, but are limited by being purely theoretical.

Here, Stocker et al (2008) has created new methods using microfluidics to analyse bacterial response to nutrient pulses in a micro-scale 1D environment.

Stocker et al (2008) visually examines how a known motile chemotactic bacterium (Pseudoaltermonas haloplanktis) responded to two types of nutrient pulses. He uses a small microfluidic chamber with a micro channel, which allowed flow when necessary.

A nutrient pulse mimicking a point source from a sloppy eater/cell lysis.

This was created, by adding the nutrient and bacteria simultaneously to the chamber with no flow, so that the nutrient dispersed laterally. To monitor nutrient diffusion dynamics, Stocker (2008) used a fluorescein as a proxy for a low molecular weight compound in DOM. The fluorescein is beneficial as it is not a chemoattractant to P. halopanktis and is a visible colour, being a dye. To visualise the bacterial response, Stocker et al (2008) used phase contrasting microscopy, recording numerous frames per second, and image analysis software to create trajectories of the bacteria.

Within tens of seconds the bacteria created a distinct band exactly where the nutrient was dispersing, and stayed in this band for 15minutes. When this response was compared to Escherichia coli (in the presence of two of the most chemoattractant compounds for this bacteria), P. halopanktis is ten times faster at chemotaxis.

Stocker et al (2008) mathematically simulated a 3D environment, and found that response time and swimming speeds could be even faster in 3D, and bacteria could potentially respond to patches of nutrient a fraction of the size simulated in the chamber.

A nutrient plume mimicking a sinking marine snow particle.

In a similar format to the previous, except Stocker et al (2008) used a stationary silicone particle in the chamber, and switched on the flow to create a plume projecting out behind the particle. He measured the response of P. haloplanktis to three ‘sinking speeds’, by increasing the flow from the micro channel.

The slowest sinking speed (66um/s) created the widest plume, and the bacteria aggregated in the plume right behind the particle. The concentration of bacterial cells was four-fold higher within the plume than outside. The intermediate speed (220um/s) was also rapidly aggregated, but the concentration of bacteria was slightly further back from the particle than the previous. The fastest sinking speed (660um/s) seemed to be too fast for the bacteria to aggregate, although they did swim into the nutrients, but couldn’t form an aggregation. However, this would be expected, as the swimming speed of these bacteria is 68um/s.

Next, Stocker et al (2008) used these results and calculated the nutrient exposure experienced by motile chemotactic bacteria compared to non-motile bacteria. The conclusion being that chemotaxis creates a considerable advantage when compared to non-motile bacteria. Motile bacteria utilising particles sinking at 66um/s will be exposed to nutrients by 4-fold compared to non-motile bacteria.

This paper was able to provide evidence to support theoretical models of how quickly motile bacteria respond to sudden pulses of nutrients in the ocean. It was achieved using novel methods - microfluidics. The results could then be mathematically simulated to give figures for 3D environment, which has improved the knowledge of bacteria’s ability to seek out nutrients in the ocean.

Primary reference:

Stocker, R., Seymour, J. R., Samadani, A., Hunt, D. E., & Polz, M. F. (2008). Rapid chemotactic response enables marine bacteria to exploit ephemeral microscale nutrient patches. Proceedings of the National Academy of Sciences, 105(11), 4209-4214.

Link: http://www.pnas.org/content/105/11/4209.full

Other reference:

 Fenchel, T. (2002). Microbial behavior in a heterogeneous world. Science,296(5570), 1068-1071.