Food run! The concept of speed velocity vs. nutrient uptake.

It is well documented that nutrient availability and consumption rates determine the energy balance of an organism and in turn may determine the future of a species in any given environment. In oligotrophic habitats, such as the open ocean, many microorganisms move with the assistance of the currents whilst microorganisms have the ability to swim independently, allowing them to find nutrients more easily. However, this is resulted in being forced to transform the energy they absorb efficiently into motional energy and is not without cost (Di Salvo & Condat, 2012). Taylor & Stocker (2012) noted that motile marine bacteria spend a considerable fraction of their metabolic budget on locomotion and that they swim at an optimal swimming speed that takes into account the uptake benefit of chemotaxis and the cost of locomotion.

For their study, Di Salvo and Condat (2012) based their work on the Brownian motion concept (also known as pedesis; the random motion of particles suspended in a fluid resulting from their collision with the quick atoms or molecules in the gas or liquid) and that of Schweitzer's et al., (1998) concept that Brownian particles have the ability to take up energy from the environment to store it in an internal depot and to convert internal energy into kinetic energy. However, as they were only interested in the relation between energy absorption and speed, variables were slightly altered to consider only the microorganism’s trajectory and run-phase, neglecting the effect of Brownian noise (as it does not affect the speed of the microorganism; Schweitzer's et al., 1998).

It was noted that various chemotactic patterns have been identified in the last decade with swimming speeds of several hundreds of μm/s being recorded in such bacteria as Thiovulum majus, Shewanella putrefaciens and Pseudoalteromonas haloplanktis (Fenchel, 1994; Barbara & Mitchell, 2003), with the majority of speeds being discovered in hyperthermophilic archaea such as Methanocaldococcus jannaschii and M. villosus (Herzog & Wirth, 2012). More recently the discovery of Vibrio alginolyticus responding to chemical gradients by executing a three-step swimming pattern (forward, reverse, and flick) has been responded with raising interest in this area of research (Xie et al., 2011).

Di Salvo and Condat (2014) hypothesised that because a sizable part of the microorganisms’ energetic budget must be allocated to locomotion, they assumed that some fast-swimming microorganisms may increase their nutrient absorption by increasing their speed velocity.

Results showed that even modest increases in nutrient absorption lead to a significant increase of the speed velocity, summarising that their conclusion had shown that fast-moving microorganisms can considerably increase its swimming speed by taking advantage of the advective (horizontal movement) uptake of nutrients. Even though the advection-dependent uptake may be relatively small compared to diffusive uptake, the additional energy can be more efficiently converted into mechanical power (Di Salvo & Condat, 2014).

References

Barbara, G.M., & Mitchell, J.G. (2003). 'Marine bacterial organisation around point-like sources of amino acids' FEMS Microbiol. Ecol. 43, 99–109.

Di Salvo, M.E., & Condat, C.A. (2014). 'Enhancement of microbial motility due to speed-dependent nutrient absorption' Phys. Biol. 11 016004 doi:10.1088/1478-3975/11/1/016004.

Fenchel, T. (1994). 'Motility and chemosensory behavior of the sulfur bacterium Thiovulum majus' Microbiol. 140, 3109–3116.

Herzog, B., & Wirth, R. (2012). 'Swimming behavior of selected species of archaea' Appl. Environ. Microbiol. 78, 1670–1674.

Institute of Physics. (2007). Einstein Year – Brownian motion [on-line]. Available at: http://www.einsteinyear.org/facts/brownian_motion/ [15.10.2014].

Schweitzer, F., Ebeling, W., & Tilch, B. (1998). 'Complex Motion of Brownian Particles with Energy Depots' Phys. Rev. Lett. 80, 5044.

Taylor, J.R., & Stocker, R. (2012). 'Trade-Offs of Chemotactic Foraging in Turbulent Water' Science 338, 675; DOI: 10.1126/science.1219417DOI: 10.1126/science.1219417.

Xie. L., Altindal, T., Chattopadhyay, S., & Wu, X.L. (2011) 'Bacterial flagellum as a propeller and as a rudder for efficient chemotaxis' Proc. Natl Acad. Sci. USA. 108, 2246–2251.