Anja Scherwass

Detritus as a potential food source for protozoans: utilization of fine particulate plant detritus by a heterotrophic flagellate, Chilomonas paramecium, and a ciliate, Tetrahymena pyriformis

We investigated the direct utilization of fine particulate detritus (dried and homogenized plant material in the size range of bacteria) as a food source for protozoans using axenic cultures of the cryptomonad, heterotrophic flagellate, Chilomonas paramecium, and the hymenostome ciliate, Tetrahymena pyriformis. When fed media containing only particulate detritus, these species revealed growth rates similar to those reported for field populations. The growth rates of Chilomonas fed exclusively particulate detritus were similar to those obtained on a bacterial diet. Considering the high percentage of detritus particles in the size range of bacteria in lakes, our results imply that direct utilization of detritus by protozoans may form an additional pathway of carbon in aquatic food webs that has generally been overlooked.

Structure, dynamics and control of the ciliate fauna in the potamoplankton of the River Rhine

Planktonic ciliate abundance and taxonomic composition were investigated from November 1998 to January 2000 in the River Rhine at Cologne, Germany. The composition of the ciliate fauna revealed the occurrence of both typical benthic (drifting peritrichs) as well as pelagic ciliates (oligotrichs) in the potamoplankton. Oligotrich ciliates prevailed in the potamoplankton community. Peritrichs, represented almost exclusively by sessile species, appeared to be washed out from the substrate at increased water discharge during flood events. Highest abundances of ciliates were registered during the phytoplankton spring bloom, indicating a bottom-up control of ciliates during this time, an observation which was supported by laboratory experiments. In summary, the ciliate potamoplankton community of the River Rhine is controlled by a seasonally varying set of factors: 1) Flood events in early spring and autumn cause a short-term succession from drifting peritrichs to pelagic oligotrichs, 2) the phytoplankton spring bloom supports oligotrich population growth, and 3) the low water level and subsequently high grazing pressure by benthic predators (mainly the mussel Corbicula) reduce the abundance of ciliates. The latter was probably the major reason for the extremely low ciliate abundances observed during summer.

Determination of the abundance of ciliates in highly turbid running waters – an improved method tested for the River Rhine

The enumeration of ciliates from riverine samples is often confounded by the problems of relatively low ciliate abundances combined with high turbidity. We successfully tested a method to determine abundance and taxonomic composition of planktonic ciliates in highly turbid running waters. The concentration of samples by sedimentation in Utermohl-chambers was combined with density gradient centrifugation (to exclude inorganic particles) and quantitative protargol staining (for taxonomic determination of species). The newly developed technique was compared with established methods (live counting, counting in Utermohl chambers and quantitative protargol staining without prior exclusion of inorganic particles). We could demonstrate that the newly developed technique gave, in addition to the quantification of ciliates, a detailed overview of taxonomic composition in the investigated samples.

Grazing Effects of Ciliates on Microcolony Formation in Bacterial Biofilms

The attachment to surfaces and the subsequent formation of biofilms are a life strategy of bacteria offering several advantages for microorganisms, for example, a protection against toxins and antibiotics and profits due to synergistic effects in biofilm environ‐ ment. Moreover, biofilm formation is thought to serve as grazing protection against predators. From pelagic systems it is known that feeding of bacterivorous protists may strongly influence the morphology, taxonomic composition and physiological status of bacterial communities and thus may be an important driving force for a change in bacterial growth and shift in morphology towards filaments and flocs. Bacteria in biofilms had to evolve several other defence strategies: production of extrapolymeric substances (EPS) or toxins, formation of specific growth forms with strong attach‐ ment, specific chemical surface properties and motility. In addition, bacteria can communicate via quorum sensing and react on grazing pressure. The results of the case study presented here showed that even microcolonies in bacterial biofilms are affected by the activity of grazers, though it may depend on the nutrient supply. Feedback effects due to remineralization of nutrients because of intensive grazing may stimulate biofilm growth and thereby enhancing grazing defence. Predator effects might be much more complex than they are currently believed to be.

Discrepancies Between Molecular and Morphological Databases of Soil Ciliates Studied for Temperate Grasslands of Central Europe

By measuring the change in soil protist communities, the effect of human land use on grasslands can be monitored to promote sustainable ecosystem functioning. Protists form the active link in the rhizosphere between the plant roots and higher trophic organisms; however, only few morphological species and their ecological values have yet been described in this context. To investigate the communicability between morphological and molecular databases used in the molecular barcoding of protists and in the biomonitoring of grassland soil, the present high-throughput sequencing (HTS) study (N=150) covered the area of central Europe (mesoscale) known to be well studied for ciliated protists. HTS delivered 2,404 unique reads identifying taxa in all major ciliophoran classes but exact reference matches were few. The study identified clear discrepancies between databases for well-studied taxa, where molecular databases contained multiple gene variants for single morphospecies of dominant taxa. Gene variants presented own biogeography - the eukaryotic microdiversity along gradients (e.g., land-use intensity, soil water). It is possible that many of the so called novel phylogenetic lineages and hidden diversity pointed out in environmental surveys could be evidence for the severe lack of molecular data for already known and morphologically described species, present in morphological databases.