ABSTRACT
A single chemical factor — the dissolved oxygen tension — plays a cardinal role in controlling the spatial distribution of many protists living in sediments and stratified water columns. It has been shown that microaerobic species grow best at low oxygen tensions, and those anaerobes require the almost complete absence of oxygen. It is clear that many of the methods commonly employed to study the behavior and various bioenergetic parameters of protists, especially those methods which provide homogeneously high oxygen tensions, are unsuitable. A population of microaerobic protists will, if left undisturbed and free to swim in an oxygen gradient, develop a pronounced aggregation close to the oxic-anoxic boundary. Anaerobic protists also have a chemosensory response to oxygen and their motility is minimal in the complete absence of oxygen. Most crude cultures of microaerobic and anaerobic protists also contain high numbers of bacteria which are microaerobic or facultatively so.
