Effects of Algal Density on Growth of Heterotrophic Microflagellates

摘要

The major role of heterotrophic nanoflagellates in the ocean is generally thought to be as grazers of bacteria, but they may also play an important role as grazers of photoautotrophs. The goal of the present study was to understand the basic growth kinetics of nanoflagellates feeding herbivorously, using batch cultures in quasi steady-state growth. Growth can involve changes in both cell numbers and cell size. Because fixed samples were examined, it was necessary to quantify 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 is hypothesized that egestion of food particles upon fixation was a major cause of shrinkage. The growth rates of two heterotrophic nanoflagellates were determined to be hyperbolic functions of algal prey densities over a range of prey sizes. Minimum prey biomass for growth of both species feeding herbivorously was within the ranges reported for similar species feeding bactivorously. The growth kinetics suggest that heterotrophic nanoflagellates are adapted to heterogeneous distribution of prey within their environment. The results of the study strongly suggest that previous studies of heterotrophic nanoflagellates based on the examination of fixed samples may have severely underestimated the role of these taxa as herbivores. (Copyright (c) Joon Won Choi, 1988.)