The effects of algal density on growth of heterotrophic microflagellates

Abstract

The major role of heterotrophic nanoilagellates in the ocean is generally thought to be as grazers of bacteria, but they may also play an important role as grazers of photoautotrphs. The goal of the present study was to understand the basic growth kinetics of nanoilagellates feeding herbivorously. This was done using batch cultures in quasi steady-state growth. Growth (increase in biomass) can involve changes in both cell numbers and cell size. Because fixed samples were examined, it was necessary to quantiiy the effects of fixation on the cell volume of heterotrophic protozoa before proceeding with the growth studies. Fixation resulted in cell shrinkage, and the degree of shrinkage varied with heterotrophic protozoan species and with algal prey species. It was hypothesized that egestion of food particles upon fixation was a major cause of shrinkage. The growth rates of two heterotrophic nanoilagellates were determined to be hyperbolic xunctions of algal prey densities over a range of prey sizes. However, the specixic response of the two species varied. Paraphysomonas imperiorata appeared to respond primarily to prey cell numbers, and Strain HM-2 (unidentified species) responded most to available prey biomass (expressed as carbon or nitrogen). Minimum prey biomass for growth of both species ieeding herbivorously was within the ranges reported for similar species feeding bactivorously. The growth kinetics suggest that heterotrophic nanoilagellates are adapted to heterogeneous distribution of prey within their environment. The result of this study strongly suggests that previous studies of heterotrophic nanoilagellates based on the examination of fixed samples may have severely underestimated the role of theses taxa as herbivores. Herbivory by heterotrophic nanoilagellates may be much more important than previously thought.