Thirty-one species of marine unicellular algae in pure culture were tested for survival in darkness at 20 C in seawater buffered at pH 7.6–7.8 and enriched with nitrate, orthophosphate, silicate, vitamins (B12, thiamin, biotin), and trace-metal ions (chelated with ethylenediamine tetraacetate). None of the species showed any significant growth in darkness but six species [including two cyanophytes, two chlorophytes (Nannochloris oculata, Tetraselmis maculata), one rhodophyte (Porphyridium cruentum), and one diatom (Phaeodactylum tricornutum)] resumed growth in light after 24 weeks in darkness (the maximum period tested). Another chlorophyte survived 18 weeks, two chrysomonads and one cryptomonad 12–14 weeks, and most other algae (including ten diatoms) 6–8 weeks of darkness. Skeletonema costatum, Hemiselmis virescens, Coccolithus huxleyi, and Amphidinium carteri showed the lowest survival potential ranging from one to three weeks. The observed darkness-survival may have ecological relevance to th...