Competition for inorganic substrates among chemoorganotrophic and chemolithotrophic bacteria

In aerobic enrichment experiments with a chemostat, using phosphate-limited lactate medium, aSpirillum sp. predominated at the lower range of dilution rates. At the higher dilution rates an (chemoorganotrophic) unidentified rod-shaped bacterium came to the fore. The same result was obtained in competition experiments with pure cultures of the two bacteria. Growth parameters were: Rod,Μ max=0.48 hr(-1),k s(PO4 (3-))=6.6×10(-N) M;Spirillum, Μ max=0.24 hr(-1)· ks(PO4 (3-)) =2.7×10(-8) M. TheSpirillum grew faster than the rod at low dilution rates, not only under phosphate-limitation but also in K(+)-,Mg(2+)-, NH4 (+)-, aspartate-, succinate-, and lactate-limited cultures. Both organisms showed little substrate specificity and could utilize a similar range of carbon and energy sources. The results support the view that part of the diversity among bacteria in the natural environment is based on selection toward substrate concentration. Another set of competition experiments was carried out with pure cultures of two marine obligately chemolithotrophic colorless sulfur bacteria,Thiobacillus thioparus andThiomicrospira pelophila. Tms. pelophila outgrewT. thioparus at low dilution rates under iron limitation, while the reverse was true at high dilution rates. It is concluded that the relatively fast growth ofTms. pelophila at low iron concentration may explain its higher sulfide tolerance. Organisms showing a selection advantage at very low concentrations of limiting substrates appear to have a relatively high surface to volume ratio.