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.
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