The effect of constant darkness and short light periods on the survival and physiological fitness of two phytoplankton species and their growth potential after re-illumination

We tested the survival potential and fitness of two different algae strains (the diatom Thalassiosira weissflogii and the cryptophyceae Rhodomonas sp.) under different growth conditions (complete darkness and short light intervals, simulating conditions in a deep mixed water column) at different temperatures, plus the effect of these conditions on the physiological fitness and growth after re-illumination was examined. Both species survived the experimental conditions without significant cell loss or physiological damage. Two different survival strategies were observed: (1) the diatom T. weissflogii immediately reduced its metabolic rate and stopped cell division. The effect on chlorophyll a (chl-a) content and photosynthetic capacity was negligible. At 10 °C, T. weissflogii used the short light windows to metabolize carbohydrates and growth. (2) The cryptophyte Rhodomonas sp. initially continued to grow after transfer into all trials. However, the cell number decreased after day 6. Carbohydrate and chl-a content went on to decrease dramatically (70 and 50%, respectively). After 3 days of re-illumination, T. weissflogii grew faster than of Rhodomonas sp.. The diatom seemed to benefit from better start conditions and would out-compete the cryptophyte during a spring bloom. Our results highlight that these algae groups have different strategies in dealing with darkness, which potentially endow diatoms with a competitive advantage in deep mixed waters and in the season of early spring.