Thalassiosira oceanica and T. pseudonana: two different photoadaptational responses

Abstract

The oceanic centric diatom Thalassiosira oceanica (13-1) and the neritic 7. pseudonana (3H, both formerly T. pseudonana) dlffer in their photoadaptational responses. According to flow cytometnc studies with nutrient-saturated cultures in the laboratory at 15 "C, in vivo fluorescence and chlorophyll per cell of T. oceanica varied negligibly between cultures adapted to 70 and 300 pm01 m-' S-' The cultures decreased in cell density when exposed to outdoor light (2800 pm01 m-2 S-' at noon), and the still viable subpopulation exhibited a 70 to 75 % lower fluorescence and chlorophyll per cell than the laboratory cultures. In T pseudonana the same photoadaptive parameters were markedly dependent on photon flux. It grew actively albeit at a reduced rate in strong outdoor light, at which in vivo fluorescence was reduced by 30 to 60 %. Suppression of fluorescence was reversible and independent of cellular chlorophyll content (which increased somewhat) and was completed within 30 to 40 min. This 'sunglass' effect is apparently a manifestation of a protective mechanism against strong light and should be advantageous for a neritic opportunistic species. Rates of adaptation were within known limits. Neither species grew actively at 17 pm01 m-' S-', at which viable subpopulations retained the photoadaptational characteristics of the inoculum. Thus sudden transfer of algae to marginally low light arrests the photoadaptational response. Decreasing average fluorescence and volume per cell in slowly dying cultures of T pseudonana was solely due to increasing prominence of a subpopulation of cells with abnormally low fluorescence and small volume.