Evidence for algal heterotrophy in Lake Tahoe, California‐Nevada1

Abstract

Significant differences in the uptake of 14C organic compounds between light and dark bottle incubations of water were recorded from Lake Tahoe. The response to light did not occur in situ below the maximum depth of inorganic carbon photoassimilation and was completely inhibited by a photosynthetic inhibitor. Dark acetate uptake in the deep euphotic zone (ca. 90–150 m) was strongly inhibited by a eucaryotic inhibitor. Microautoradiographic analysis showed that two species of green algae in Tahoe were capable of acetate transport at labeled substrate additions within previously determined ambient limits; in axenic culture these two species grew heterotrophically on acetate. Activities of key enzymes of the major inducible pathway for acetate assimilation were high per unit ATP in the region of the water column where acetate uptake was light stimulated. These data strongly support the hypothesis of heterotrophically active phytoplankton populations at the bottom of the euphotic zone.