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Trophic interactions in the microbial food web at a coastal station in the Alboran Sea (Western Mediterranean) in winter. (II): Size selective flagellate feeding on bacteria and its implication on the microbial loop size-structure
Francisco Jiménez-Gómez, Valeriano Rodríguez, Begoña Bautista

We report the changes in abundance, mean cell size, total biovolume and size structure of bacteria (prey) and flagellates (predator) following 24h incubation of a set of dilution mixtures of natural seawater. Epifluorescence microscopy and image analysis were used to analyze the changes in each dilution sample. The abundance and total biovolume of heterotrophic bacteria increased exponentially related to the decline of predator pressure that took place in the diluted samples. In the most diluted sample, bacteria number and biomass increased by factors of llx and 18x respectively, and mean cell biovolume increased from 0.07 mm3 to 0.11 mm3 (0.5 to 0.6 mm ESD). The bacterial assemblage in the natural seawater was characterized by lower total biovolume and smaller mean cell size compared with that of the diluted samples. Heterotrophic nanoflagellates were mainly responsible for the changes in bacterial cell size, biovolume and size structure due to selective feeding on larger bacterial cells. The biomass of heterotrophic nanoflagellates increased markedly (2.3x) only in the undiluted sample. The prey:predator size ratio was 1:5 (ESD:ESD). No appreciable changes were observed in the phototrophic flagellates. Ciliates were of minor importance as predators of bacteria and flagellates. These results support the hypothesis of size-selective grazing on larger bacteria by nanoflagellates in marine ecosystems.

Keywords: Nanoflagellates, bacteria, size-selective feeding, size-structure, epifluorescence.
Contents of this volume Sci. Mar. 58(1-2) : 153-159 Back PDF
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