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

Abstract

The structure of the bacterial population (free vs. attached bacteria), variations in bac- terial abundance and ciliate group composition were monitored as a function of transparent exopoly- meric 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.