The functional responses of two benthic algivorous ciliated protozoa with differing feeding strategies

Surface-associated algivorous ciliated protozoa are common in the benthos of streams, but little is known about the feeding ecology of these organisms. We compared the functional responses of two algivorous ciliated protozoa, Oxytricha fallax (a filter feeder) and Trithigmostoma cucullulus (an encounter feeder). The ciliates were fed ¹⁴C-labeled Navicula cryptocephala in laboratory feeding experiments to determine their potential to consume significant amounts of algal prey. Logistic regression, and plots of the proportion of N. cryptocephala ingested vs. the total number offered, indicated functional responses of a typical rectangular hyperbolic (type II) form for both ciliates. Ingestion rates were estimated from regressions of the number of ¹⁴C-labeled N. cryptocephala cells ingested per ciliate vs. time. Maximum feeding rates and half-saturation concentrations were estimated by fitting the observed ingestion rates and experimental algal densities to a function of the Michaelis-Menten enzyme kinetics form using nonlinear regression. For O. fallax, the maximum feeding rate was estimated to be 1.07 N. cryptocephala cells per minute, and the half-saturation concentration was 3.9 × 102 N. cryptocephala per square centimeter. For T. cucullulus the maximum feeding rate was estimated to be 0.2 N. cryptocephala per minute, and the half-saturation concentration was 5.4 × 10³ N. cryptocephala per square centimeter. The data were also fitted using only the number of cells ingested at 60 and 120 min, by converting the endpoint consumption to rates. For O. fallax, the estimated maximum feeding rates were 1.3 and 1.0 N. cryptocephala per minute for 60 and 120 min, respectively, and estimated half-saturation concentrations were 5.1 × 10² and 3.5 × 10² N. cryptocephala per square centimeter. For T. cucullulus, estimated maximum feeding rates were 0.6 and 0.4 N. cryptocephala per minute for 60 and 120 min, respectively, and estimated half-saturation concentrations were 1.5 × 10⁴ and 1.1 × 104 N. cryptocephala per square centimeter. These results suggest that kinetic methods for estimating ingestion rates are more accurate than endpoint determinations. Based on field observations of periphyton densities, these ciliates potentially are consuming 4.8% of the total available standing crop of diatom biomass per day and this could represent up to 16% of total available daily primary production.