Excretion of nitrogen and phosphorus by the soil ciliate Colpoda steinii when fed the soil bacterium Arthrobacter sp

Nitrogenous excretion of Colpoda steinii Maupas was investigated when this ciliate was fed the soil bacterium, Arthrobacter sp. NCIMB strain 10683, in batch cultures. Pure bacterial cultures were first grown to steady state in a chemostat before the cells were harvested and added to the batch cultures with C. steinii. Ammonium was the major form of N excreted. Ciliate excretion was influenced by the temperature of the culture medium. Ammonium excretion by C. steinii when fed N-replete Arthrobacter sp. (8.5% N, C:N = 5.0) was more rapid at 15 degrees C than at either 10 degrees or 5 degrees C. At 5 degrees C, few ciliate trophozoites were present after 10 days and most of the NH+4-N released into the medium at this temperature was probably due to lysis of the ciliates. The N content of the bacterial prey also influenced ciliate excretion. When the ciliates were fed N-limited Arthrobacter sp. (4.1% N, C:N = 8.6) at 10 degrees C, there was an initial lag of 2 days when no N was excreted and the final quantity of N released into the culture medium after 14 days was only 60% of that released when the ciliates were fed the same bacterial strain with a larger N content (8.5% N, C:N = 5.0). In a comparison made between feeding C. steinii on P-limited (0.46% P, C:P = 101) and P-replete (1.72% P, C:P = 26) Arthrobacter sp., P was only mineralized with the latter. Ciliate populations supplied with P-limited Arthrobacter sp., however, declined from the onset of the experiment and few trophozoites or cysts were present after 14 days. An experiment with C. steinii grown actively in the second vessel of a two-stage chemostat, successively using P-replete (1.72% P) and P-limited (0.46% P) Arthrobacter sp., confirmed that the P content of the prey influenced the quantity of P released by C. steinii.