Cell volume and community structure related to temperature.

Sjösted, Hagström and Zweifel analysed cultures grown at different temperatures for cell volume, morphology and phylotypes, to determine the potential role ambient temperatures in community succession and cell morphology.

Continuous cultures were grown for two weeks. The cultures were kept at 10 ⁰C, 15 ⁰C, 20 ⁰C and 25 ⁰C , with samples being taken on day 7, 10 and 14. Using samples taken, Sjösted et al (2012) analysed them for differences in phylotypes using DGGE, and epifluorecent microscopy. They found distinct differences between the different temperatures, as well as between different days in the same temperature. There appeared to be a distinct shift in dominance of phylotypes. The dominant phylotypes found in the 10 ⁰C culture were found to be larger rod- and vibrio-shaped bacteria. The samples at 15 ⁰C showed a shift between day 7 and 10, from larger rod- and vibrio- shaped bacteria to smaller coccoid bacteria. In the samples taken at 20 ⁰C and 25 ⁰C, coccoid bacteria appeared to be the dominant phylotype aswell. The average decrease in biovolume was found to be 39% between 10⁰C and 20⁰C.

Sjörsted et al (2012) make a link between cell size, growth rate and nutrient uptake. Because enzymatic reactions are faster at higher temperatures, less functional ribosomes are needed to achieve the same growth rate at a lower temperature. This means that the cell can decrease the amount of “baggage” it has to carry around, which in turn reduces the cell size. An added effect of this decrease in cell size, is that it increases nutrient uptake, as the surface to volume ratio increases. Conversely it may be expected that cell size decreases due to nutrient limitations at higher temperatures. It is well documented that colder waters are relatively nutrient rich, compared to warmer waters. This has as implication that cells need a larger surface to volume ratio to obtain the nutrients needed for growth. These factors combined would imply that cell size should always decrease drastically as water temperatures rise.  An additional explanation for the 39% decrease Sjörsted et al found, may be inherent to de community structure. Sjörsted et al suggest that the coccoid species found at higher temperature may be inherently smaller than the rod- and vibrio- species found at the lower temperatures.

However these results appear inconsistent between in-situ studies. Several have reported the same decrease in cell sizes over periods of warmth, whereas others have reported an increase in cell size, and others have shown that cell size is independent of temperature. Though these studies do not appear to have made study of the community structure in these locations.

It appears that in addition to the reasons given above, the results Sjörsted et al(2012) found may be species specific. This would mean that to be able to adequately compare other studies, the community structure of those sites would need to be analysed. In addition, if species specific studies were to be conducted, it may be possible to determine if these differences in results are due to the species found in these samples or if there are environmental effects that have not been accounted for which may influence these results.

·         Shörsted ,J., Hagström, A., Zweifel, U. L. (2012). Variation in cell volume and community composition of bacteria in response to temperature. Aquatic Microbial Ecology. 66, 237-246.