Printed in Great Britain 25 A Quantitative Study of the Bacteria of a Temporary Pond

To examine the role of bacteria in the life history of a temporary pond, nine physiological groups of bacteria were determined by plate counts and enrich-ment culture techniques in pond water, in soil from the pond basin, and in soil from the pinewoods area surrounding the basin. Sampling was begun when the pond was dry and continued through the period when it contained water, into the next dry period. Low counts were obtained for sulphur-, ammonia-, and nitrite-oxidizing autotrophs; iron-oxidizing autotrophs were not detected. This indicates that bacteria did not make a major contribution to the ecosystem as primary producers. Soil samples contained millions to hundreds of millions (106-108) of aerobic nitrogen-fixers and millions to tens of millions ( 10g-107) of urea-utilizing organisms per gramme, suggesting that they may be of significance in the nitrogen cycle in the ecosystem. Hundreds of thousands to tens of millions ( lo5-lo7) of cellulose-decomposers, and tens of millions to hundreds of millions (107-108) of aerobic and of anaerobic heterotrophs were present per gramme in soil samples. The several hetero-trophic types were also present in pond water in maximum numbers ranging from tens of thousands per millitre for nitrogen-fixers to millions per milli-litre for anaerobic heterotrophs. As the pond dried the numbers of bacteria in its water decreased. These data suggest that bacteria in the pond ecosystem play a role in the nitrogen, carbon and energy cycles as decomposers and transformers, as a source of nutrilites and as members of the food chain