Effects of mercury stress on the ecology of culturable heterotrophic bacteria in aquatic microcosms

Abstract

The objective of this research was to study the development of communities of culturable heterotrohic bacterial communities in aquatic microcosms. Natural and gnotobiotic communities of culturable heterotrophs were disrupted with additions of inorganic mercury (Hg) and the re-establishment of stable communities studied. Principles governing development of these communities which were studied included carrying capacity of the environment interactions between member species, population limitation and sensitivity of the species to Hg. Also, the effect of Hg on the development of these communities was investigated. Results indicated that the culturable heterotrophic bacterial populations were resource-limited and that different species were limited by different resources. The concentration of nutrients in the system also affected the outcome of competition in gnotobiotic communities. Some species exhibited significant changes in population densities in axenic and mixed cultures and changes in the growth rate when other species were present, indicating that competition was occurring. Competition between some species appeared to be based on growth rates. However in other combinations of isolates in source water microcosms, no interaction could be detected. Interactions between the species was highly complex and was dependent on the prevailing environmental conditions such as nutrient concentration, and the presence of Hg. Additions of Hg to stable source water microcosms resulted in decreased culturable counts, with the amount of decrease proportional to quantity of Hg added. Following Hg stress, a stable culturable community was re-established within 4 days with the highest concentration of Hg used. Results indicated that Hg can influence the final community structure by altering growth rates of organisms remaining, and therefore, changing their competitive abilities.