AME 41:115-124 (2005)  -  doi:10.3354/ame041115

ABSTRACT: We conducted a short-term course experiment using a size-fractionation approach to manipulate grazers and test for their effects on viral and prokaryotic standing stocks and activities and on prokaryotic community composition, as assessed by the FISH (fluorescence in situ hybridisation) method. Experimental samples were collected in the Sep Reservoir during severe P-limiting conditions. The presence of grazers in microcosms appeared to be a stimulating factor for prokaryotic growth and viral proliferation, likely through the related nutrient and substrate enrichments. About 60% of the total prokaryote abundance was detected by FISH, with a dominance of Eubacteria and beta-Proteobacteria throughout the experiment, i.e. in the presence and in the absence of grazers. The relative abundances of the minor phylotypes remained unchanged in the absence of grazers, but significantly increased with time when grazers were present, indicating a grazer-mediated reduction in resource competition in prokaryotic assemblages. However, grazer effects resulted in an apparently greater alteration of the bacterial size structure (i.e. the occurrence of large, grazing-resistant cells) than of the relative abundances of the phylotypes analysed. The main findings suggest that, at least on a short-term scale (i.e. ≤1 prokaryotic net generation time): (1) There is a synergy between grazer, prokaryotic and viral activities in oligotrophic conditions, through a cascading effect from grazer-mediated resource enrichment, with (2) a higher impact on the prokaryotic size structure than on the relative abundances of the major phylotypes, and (3) a significant acquisition of competitive advantages by the less adundant phylotypes. These findings are discussed in light of the recent ‘phage kills winner’ theory.

KEY WORDS: Plankton · Lakes · Microcosms · Viruses · Prokaryotes · Community composition · Grazer effects · Microbial ecology