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JPR Advance Access published online on August 17, 2006
Journal of Plankton Research, doi:10.1093/plankt/fbl034
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© The Author 2006. Published by Oxford University Press. All rights reserved. For Permissions, please email: journals.permissions@oxfordjournals.org
Received February 24, 2006
Accepted August 10, 2006

Article

Potential grazing impact of the mixotrophic flagellate Ochromonas sp. (Chrysophyceae) on bacteria in an extremely acidic lake

Andrea Schmidtke 1 *, Elanor M. Bell 1, and Guntram Weithoff 1

1 University of Potsdam, Institute for Biochemistry & Biology, Dept. of Ecology and Ecosystem Modelling, Maulbeerallee 2, D-14469 Potsdam, Germany

* To whom correspondence should be addressed.
Andrea Schmidtke, E-mail: andrschm{at}uni-potsdam.de


  Abstract

Flagellates are important bacterial grazers in most planktonic food webs. The prey size preference of the mixotrophic flagellate, Ochromonas sp. (Chrysophyceae), isolated from an extremely acidic lake, Lake 111 (pH 2.6), was determined using fluorescently labelled microspheres (beads). According to grazing experiments with cultured bacteria, also isolated from Lake 111, the potential grazing impact on Lake 111's single celled bacterial production was calculated. Ochromonas sp. ingested the smallest beads offered (0.5 µm diameter) at the highest rate. Ingestion rate declined with increasing bead size. The highest prey-volume specific ingestion was measured for Ochromonas sp. feeding on intermediate-sized beads (1.9 µm). Ingestion rates were low due in part to the large fraction of inactive flagellates observed. According to the bacterial ingestion rate a mean of 88% (epilimnion) and 68% (hypolimnion) of in situ single celled bacterial production is potentially grazed daily by Ochromonas sp. In the epilimnion of Lake 111, the heterotrophic carbon gain is 3-times higher than the autotrophic production. Alongside carbon uptake Ochromonas sp. also benefits from ingesting bacteria through the uptake of phosphorus. A biovolume minimum corresponding to the prey size at which Ochromonas sp. feeds most efficiently occurred in the Lake 111 epilimnetic bacterial community, implying top-down control of the bacterial community by Ochromonas sp.

Communicating Editor: A.Mitra


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