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 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.

Bacterial colonization of transparent exopolymeric particles in mesocosms under different turbulence intensities and nutrient conditions

“…through an increase in the growth rates of diatoms that actively exude TEP precursors, particularly when nutrients become exhausted. Variations in microbial colonization of TEP have been explained by variability in the composition of TEP precursors, in degradability of TEP (Kepkay & Johnson 1988 and in grazing pressure (Mari et al 2004). In the present experiment, the increase in the rate of bacterial colonization of TEP appeared linked to the rapid formation of TEP after the diatom bloom in enriched turbulent tanks.…”

Seasonal variation in marine-snow-associated and ambient-water prokaryotic communities in the northern Adriatic Sea

“…Consequently, dissolved organic matter leaches out of the marine snow, attracting not only chemotactic prokaryotes (Stocker et al 2008) but also their predators such as highly motile nanoflagellates (Fenchel & Blackburn 1999, Turk et al 2010. Bacterial grazing mortality can therefore exceed bacterial growth and colonization (Kiørboe et al 2004, Mari et al 2004. As marine snow ages, however, it becomes more refractory due to the intense utilization of easily degradable compounds (Müller-N iklas et al 1994, Turk et al 2010) and the production of refractory capsular material by marine-snow-attached bacteria (Heissenberger & Herndl 1994, Heissenberger et al 1996.…”

Marine heterotrophic bacteria, protozoan and metazoan zooplankton may experience protein N or mineral P limitation in coastal waters

“…Moreover, studies have indeed suggested that the reproduction rates of the dominant copepods can be limited by the docosahexaenoic acid (22:6 n-3, DHA) content of food algae during the summer season (see Evjemo et al 2008). Ciliates may consume minor fractions of heterotrophic bacteria (Lawrence & Snyder 1998, Mari et al 2004). This could mitigate P limitation, but it is an open question whether large herbi vorous ciliates (20 to 50 µm) are able to consume significant amounts of bacteria (linkage not represented in Fig.…”