Sediment bacteria attached to sand grains - indications to a protection from feeding by ciliates

Microbiological investigations of a marine, brackish sandy sediment area in a water depth of 10 to 12 m showed relatively high numbers of bacteria (4 x 108 - 2,4 x 109 bacteria g-1 dry weight of sediment). Benthic ciliates were estimated too. Their numbers varied between 1 and 87, with a mean of 21 individuals per gram dry weight of sediment. In general, the overwhelming part (80%), were small forms of about 30 µm length. 54% of the ciliates were predominantly bacterivorous. In feeding experiments using a fluorescent dye (DT AF), stained bacteria could be seen directly within the food vacuoles of the small ciliates, but this was the case only when free suspended bacteria were present. No uptake was visible if sand grains were added, which were colonized with stained bacteria. From these results, indications can be seen that small benthic ciliates cannot remove attached sediment bacteria. Taking into account that ciliates choose distinct size classes and certain strains of bacteria, less than a fraction of the total mass of bacteria is probably available as a potential food. This fact, together with a permanent reduction of ciliates by metazoa, ciliates themselves and hydrodynamic perturbations can explain the relatively low, but more or less constant number of the ciliate fauna at such locations