A transplant experiment to identify the factors controlling bacterial abundance, activity, production, and community composition in a eutrophic canyon‐shaped reservoir

Abstract

We performed a transplant experiment in eutrophic Sau reservoir to assess the factors that control bacterial abundance, activity, growth rate, and community composition. Samples from the lacustrine and the riverine ends of the reservoir were incubated in dialysis bags placed in situ and transplanted to the other side of the reservoir and also incubated after 1 m filtration to measure predator effects. The bags were sampled at 12-h intervals to estimate bacterial abundance, whole community activity, activity structure (by flow cytometry), and phylogenetic composition (by in situ hybridization with group-specific phylogenetic probes). Bacterial production was always regulated by nutrient supply, but abundance and activity were differently regulated at both sites. The riverine bacteria were limited by predator activity, whereas the lacustrine were regulated by a combination of predation and nutrient supply. Therefore, even in the same environment, different modes of control can act simultaneously. Bacterial activity structure was also regulated in the same way. Abundance of highDNA bacteria and cells hybridizing with the universal EUB338 probe were well correlated. In the lacustrine sample, bacterial community structure did not change significantly, whereas in the riverine sample, ␣-and ␥-Proteobacteria reduced their growth when transplanted, whereas ␤-Proteobacteria were stimulated by the presence of predators. Members of the Cytophaga/Flavobacterium phylum grew only when incubated in situ in the absence of predators. This different behavior in the different bacterial groups resulted in strong changes in bacterial assemblage composition, evident already after 24 h. The experiment demonstrates that, together with the effect of predators, nutrient supply affects bacterial community properties and that a complex regulation involving both types of control can occur in a single heterogeneous planktonic system. Bacteria are relevant members of the limnetic planktonic food web, both in terms of biomass and production share (e.g., Cole and Caraco 1993;del Giorgio and Gasol 1995) and while this is very evident in oligotrophic plankton sys-1 Corresponding author

Microbial composition affects the functioning of estuarine sediments

“…To our knowledge, however, this study is only one of a few that directly demonstrates the effect of microbial composition on functioning in the field while controlling for microbial migration (Allison et al, in press;Gasol et al, 2002). Such experiments would also be useful to investigate further details of the relationship between microbial composition and ecosystem functioning.…”

“…However, two limitations with such experiments are that the soil matrix differs along with the microbial community and that the transplanted cores may not fully equilibrate with the surrounding environment. Further, such studies do not usually prevent migration of microorganisms after transplantation (but see Gasol et al, 2002), such that the effect of composition on functioning becomes more ambiguous over time; it is then unclear whether changes in microbial composition over time are because of microbial migration or growth within the soil cores.…”

“…(For simplicity, we refer to these locations as the 'freshwater' and 'saltwater' sites.) We placed the transplanted sediments into microbial 'cages' (Gasol et al, 2002;Reed and Martiny, 2007) to prevent the movement of microorganisms, while allowing the sediments to experience the abiotic conditions (such as temperature, salinity and nutrients) of the surroundings. We focused on estuarine sediments for three reasons.…”

Function-specific response to depletion of microbial diversity

“…Natural lake water contains a large variety of organic compounds (Perdue, 2009) and there is also a large variety in bacterial ability to synthesize enzymes for substrate utilization and biomass production. In mixed batch cultures, bacterial growth follows reproducible trajectories (Ammerman et al, 1984;Tranvik and Höfle, 1987) and maintains similar growth-related functions independently of the source of inoculum (Fuchs et al, 2000;Gasol et al, 2002;Langenheder et al, 2005Langenheder et al, , 2006. This can be explained either by similarly composed communities emerging in those cultures or by differently composed communities maintaining similar functions.…”

Influence of inlet bacteria on bacterioplankton assemblage composition in lakes of different hydraulic retention time

“…Experiments have shown a low survival of allochthonous bacteria in freshwater mesocosms (e.g., Höfle 1992; Leff et al 1998). In reservoirs, the influence by allochthonous bacteria has been found to be local (Simek et al 2001a;Gasol et al 2002) or low (Dumestre et al 2002). On the other hand, results from a few natural freshwater systems have indicated that allochthonous bacteria can influence the assemblage composition of the bacterioplankton (Lindström 1998(Lindström , 2001Crump et al 2003;Stepanauskas et al 2003).…”