Bacterial Populations in an Anthropogenically Disturbed Stream: Comparison of Different Seasons

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Abstract
To determine the effects of environmental changes on stream bacterial populations, assemblage- and population-level measurements were compared between an anthropogenically disturbed stream and an undisturbed reference stream during different seasons. Physical and chemical variables monitored at two disturbed sites from a stream affected by multiple environmental perturbations confirmed discernibly different water quality from three reference sites: two from an adjacent, undisturbed watershed and one from the headwaters of the polluted stream. Assemblage-level variables, including total number of bacteria, colony forming units, and number of Bacteria from in situ hybridization revealed only one statistically significant difference between disturbed and undisturbed sites. Population-level changes of three bacterial species, Burkholderia cepacia, Pseudomonas putida, and Acinetobacter calcoaceticus, were determined by colony hybridization with rDNA probes. Abundance of culturable A. calcoaceticus was higher at disturbed sites in November and February; B. cepacia and P. putida did not exhibit pollution-associated responses. In contrast, in situ hybridization indicated that there was more A. calcoaceticus at the reference sites in November and April, suggesting that culturability of the species increased at disturbed sites. To determine if differences among sites were attributable to changes in water quality among the streams, three bacterial strains isolated from the disturbed stream were grown for 64 h in flasks in water from disturbed and reference sites. As observed in the stream, A. calcoaceticus numbers increased in polluted stream water after an initial lag period of approximately 24 h. Our results indicate that although assemblage-level measurements of bacterial communities did not reflect environmental differences among sites, A. calcoaceticus population sizes differed between disturbed and reference sites, suggesting that anthropogenic disturbance can alter some bacterial populations and not others.

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