Growth of Two Facultatively Heterotrophic Marine Centric Diatoms1,2

SUMMARY The facultative, heterotrophs Cyclotella cryptica (sclone WT‐1‐8) and Coscinodiscus sp. were selectively isolated from coastal waters by dark incubation of organically enriched solid medium. C. cryptica grows in the dark with glucose and galactose, and Coscinodiscus sp. with glucose. Clone WT‐1‐8 of C. cryptica grows about twice as fast with glucose as a previously studied clone (0‐3A). In the dark with, 5 × 10−6 M glucose C. cryptica divides every 3 days, while with 5 × 10−5 M glucose Coscinodiscus sp. divides every 10 days. Heterotrophic growth of either diatom for 1 year does not cause a major reduction in carbon, nitrogen, chlorophyll a, and chlorophyll c contents, or in photosynthetic ability, compared to light‐grown cells. It is possible that facultative heterotrophy is of ecological benefit to these diatoms, probably for slow growth and survival during extended periods of dim light or darkness.