Culture characteristics and orthophosphate excretion of a marine oligotrich ciliate, Strombidium sulcatum, fed heat-killed bacteria

Ciliate microzooplankton have long been hypothesized as probably being important in nutrient regeneration. However, few estimates of excretion rates exist, especially with regard to phosphorus, identified as the limiting nutrient in many marine and estuarine systems. The utility of using heat-killed prey, which avoids prey uptake of excretory products, to measure excretion in feeding ciliates was examined. The growth characteristics of Strombidium sulcaturn fed heat-killed vs live bacteria were investigated and orthophosphate excretion rates were estimated. Organismal and orthophosphate concentrations were monitored over an 8 d incubation period in triplicate solutions of live bacteria, live bacteria plus ciliates, heat-killed bactena, and heat-killed bacteria plus ciliates. Ciliate cultures fed heat-lulled or live bacteria were indistinguishable in terms of the duration of different growth phases, maximum growth rates, maximum concentrations and average cell dry weights based on volume estimates. Calculated orthophosphate excretion rates of S. sulcatum, based on changes in concentrations of orthophosphate in cultures fed heat-killed bacteria, were highest for lag-phase cells, 13 X I O -~ pg-at. ciliate-' h-' (24.6 l g P mg-' dry wt h-') and decreased to zero for late stationary-phase cells.