Comparative photoheterotrophy, chemoheterotrophy, and photolithotrophy in a eutrophic reservoir and an oligotrophic lake1

Autoradiography was used to determine the role in the uptake of NaH14CO3 and [3H]glucose of specific microorganisms of two sizes (0.45–10 µm and 10–165 µm) in a eutrophic reservoir and an oligotrophic lake. Ultraplankton (0.45–10 µm) dominated plankton numbers and metabolic activity in both systems. An increase in photolithotrophy during the fall was due to an increase in the activity, numbers·ml−1, or both, of specific phytoplankters rather than an increase in the percentage of viable cells. Many of the most abundant species were not the most productive in terms of inorganic C fixation. Single species dominated total inorganic C fixation in both systems. Light‐mediated uptake of [3H]glucose was extensive in Flathead Lake; light bottle uptake averaged 365% of dark bottle uptake. Autoradiographs indicated that bacteria were responsible for the observed light‐mediated uptake of [3H]glucose in both lakes. Chemoheterotrophy dominated all Lake Texoma samples. Simultaneous in situ NaH14CO3 and [3H]glucose incubations in Lake Texoma revealed an ability of Oscillatoria agardhii (Gomont) to be mixotrophic as well as photoheterotrophic.