Growth of interstitial ciliates in association with ciliate bacterivory in a sandy hyporheic zone

In situ growth rates of interstitial ciliates of a sandy hyporheic zone were determined by a modification of the dilution technique. Preconditioned sediment was used as a dilution substrate. Field incubation was conducted in the upper 3 cm sediment layer; samples were taken on 3 days (Days 0, 21 and 35) in October to November 2002. Mean growth rates varied between 0.070 d–1 (Pleuronema spp.) and 0.089 d–1 (other small scuticociliates) for the 21 d incubation period and between 0.093 d–1 (Hymenostomatia) and 0.145 d–1 (Placus spp.) for the 35 d incubation period. Mean generation times ranged between 4.8 and 9.9 d. Ciliate carbon production, determined on the basis of significant growth and mortality rates and summed up for the respective incubation period, ranged between 7 ng C g–1 sediment dry weight (DW) (Cinetochilum margaritaceum) and 226 ng C g–1 sediment DW (Hymenostomatia) for the 21 d incubation period, and was 6 ng C g–1 sediment DW for the 35 d incubation period (Placus spp. ). Losses due to predation ranged from 13.3 to 17.9% d–1 for ciliate carbon biomass and from 114.9 to 250.1% d–1 for ciliate carbon production. Fluorescence in situ hybridisation (FISH) was used to determine taxon-specific bacteria ingestion. Ingestion rates for C. margaritaceum, other small scuticociliates and Pleuronema spp. were 26, 50 and 86 bacteria ind.–1 h–1, respectively. Based on the estimated gross growth rates and on ciliate biomass and bacteria ingestion, we calculated the carbon requirement and the bacteria carbon ingestion required to enable this growth. The mean contribution of bacterivory to the carbon requirement for C. margaritaceum, other small scuticociliates and Pleuronema spp. amounted to only 7.0, 11.8 and 2.2%, respectively.