Abstract
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.
Highlights
Phytoplankton is exposed to darkness on timescales varying from daily, light–dark rhythm, to years if cells sink out of the water column and need to survive in the sediment
Light availability can be extremely low during winter convection in the North Atlantic, when phytoplankton cells can be exposed to dark periods ranging from days to weeks (McMinn and Martin 2013)
Growth rate of T. weissflogii was constantly low during the entire experiment
Summary
Phytoplankton is exposed to darkness on timescales varying from daily, light–dark rhythm, to years if cells sink out of the water column and need to survive in the sediment. Jochem (1999) investigated survival of several algae in darkness and noted two different survival strategies he categorized as: type I cells—which can suppress their metabolism and withstand long periods of darkness, but can react quickly to changing environmental conditions. This strategy is typified by diatoms, which dominate early spring blooms in temperate and cold regions (Barlow et al 1993; Wassmann et al 1999; Wiltshire et al 2008). This type of response is typical of small flagellates, which dominate the environment after the spring bloom (Colebrook 1979; McQuattersGollop et al 2007)
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