MEPS 279:23-32 (2004)  -  doi:10.3354/meps279023

Role of TEP in the microbial food web structure. II. Influence on the ciliate community structure

Xavier Mari1,3,*, Fereidoun Rassoulzadegan1, Corina P. D. Brussaard2

1Marine Microbial Ecology Group, Observatoire Océanologique, Laboratoire d¹Océanographie de Villefranche, UMR 7093, BP 28, 06234 Villefranche-sur-Mer Cedex, France
2Royal Netherlands Institute for Sea Research, Department of Biological Oceanography, PO Box 59, 1790 AB Den Burg, Texel, The Netherlands
3Present address: Institut de Recherche pour le Développment (IRD), Centre de Nouméa, BP A5, NC-98848 Noumea, New Caledonia

ABSTRACT: The structure of the bacterial population (free vs. attached bacteria), variations in bacterial abundance and ciliate group composition were monitored as a function of transparent exopolymeric particle (TEP) concentration during Phaeocystis globosa blooms that developed in mesocosms. Two ciliate groups dominated at different stages of the blooms. The oligotrichous ciliate Strombidium spp. were dominant during the growth phase of the blooms, when TEP volume concentration was lower than 20 ppm. The hypotrichous ciliate Euplotes spp. emerged and became dominant after blooms peaked, when TEP concentration reached values between 20 and 50 ppm. The succession from Strombidium spp. to Euplotes spp. was closely related to TEP variation. Strombidium spp. depletion was apparently not caused by a reduction of prey availability due to TEP aggregation, as the bacterial concentration increased over time and the fraction of available bacteria for Strombidium spp. remained close to 90%, irrespective of variations in TEP concentration. Instead, the results of incubations conducted by adding S. sulcatum to seawater collected in the mesocoms suggest that Strombidium spp. disappearance may have been caused by direct TEP-mediated aggregation of the ciliates. The emergence of Euplotes spp. coincided with the formation of macroaggregates (favored by high TEP concentration), which provide them with the physical support required for feeding on attached bacteria. Our results suggest that variations of the size in the TEP pool may induce a ciliate population succession, modify the size distribution of the bacterial population and ultimately control the microbial food web structure and function.

KEY WORDS: Transparent exopolymeric particles · Phaeocystis globosa · Ciliates · Strombidium · Euplotes · Succession · Microbial food web · Bacteria

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