Effect of temperature on growth, respiration, and nutrient regeneration by an omnivorous microflagellate.

The effect of temperature on the rates and extent of carbon and nitrogen cycling by the heterotrophic microflagellate Paraphysomonas imperforata (diameter, 7 to 12 mum) fed with the diatom Phaeodactylum tricornutum was investigated over an ecologically pertinent temperature range (14 to 26 degrees C). All physiological rates investigated increased with increasing temperature. Q(10) values were similar for all rate changes and were comparable to those which have been reported for other protozoa. In contrast to all rates, microflagellate gross growth efficiency and cell volume were unaffected by temperature. Decreases in the concentrations of particulate carbon and particulate nitrogen from grazed diatom cultures also were similar when summed over the entire growth phase of the microflagellate population. Therefore, the proportions of ingested carbon and nitrogen which were incorporated or remineralized by the microflagellate were independent of temperature between 14 and 26 degrees C. At temperatures above 18 degrees C, growth rates of P. imperforata were greater than the maximum growth rates reported for most phytoplankton. We conclude that the impact of P. imperforata on natural phytoplankton communities is not controlled by temperature above 18 degrees C but may be affected by the rate at which zooplankton or microzooplankton prey on the microflagellate, as well as the inability of the microflagellate to graze efficiently when phytoplankton are present at low cell densities.