A mathematical model for the growth of bacterial microcolonies on marine sediment

Counts of bacterial microcolonies attached to deep-sea sediment particles showed 4-, 8-, 16-, and 32-celled microcolonies to be very rare. This was investigated with a mathematical model in which microcolonies grew from single cells at a constant growth rate (μ), detached from particles at constant rate (λ), and reattached as single cells. Terms for attachment of foreign bacteria (a) and death of single cells (d) were also included. The best method of fitting the model to the microcolony counts was a weighted least-squares approach by whichλ(0.83 hour(-1)) was estimated to be about 20 times greater thanμ(0.038 hour(-1)). This showed that the bacteria were very mobile between sediment particles and this mobility was explained in terms of attachment by reversible sorption. The implications of the results for the frequency of dividing cell method for estimating growth rates of sediment bacteria are discussed. The ratio ofλ andμ was found to be very robust both in terms of the errors associated with the microcolony counts and the range of microcolony sizes used to obtain the solution.