THE EFFECT OF pH IN INTENSIVE MICROALGAL CULTURES. II. SPECIES COMPETITION

The results of a series of competition experiments between the chlorr hyte Dunaliella tertiolecta (Dun) Butcher and the diatom Phaeodactylum tricomutum (TFX-l) Bc,nlin demonstrate conclusively that Phaeodactylum tricornutum dominates in intensive marine cultures when the pH rises above ""10. This dominance results because of the diatom's unique ability among marine species to tolerate alkaline conditions. When the pH is regulated both freshwater and marin<" algae, once firmly established in culture at their respective pH optima, can resist invasion from competing species. Hence, pHcontrol may be a method for maintaining species other than P. tricornutum in mass culture. When Dunaliella tertiolecta, however, is grown under even slight pH stress it becomes sus9eptible to invasion by Phaeodactylum tricornutum. This susceptibility to takeover by P. tricornutum increases with increasing pH. In contrast, the freshwater chlorophyte Chiarella vulgaris Beij., which also is sensitive to increasing pH, is capable of remaining dominant at any pH within its tolerance range when invaded by the pHinsensitive chlorophyte, Scenedesmus obliquus (Turp.) Kutz. Although allelopathic interactions may be responsible for the success of Chiarella vulgaris under seemingly stressful conditions, the success of Phaeodactylum tricornutum at increasingly higher pH seems to be related primarily to the alga's pH tolerance characteristics and not to any chemical interactions with competing species.