Green activity in the dark – ecophysiological studies of microalgae during the Polar night

• Main Authors: Leu, Eva, Hoppe, Clara, Smoła, Z, Bernhardsson, I
• Format: Conference Object
• Language: unknown
• Published: 2015
• Subjects: Psi; Svalbard; polar night
• Online Access: https://epic.awi.de/id/eprint/41611/; https://hdl.handle.net/10013/epic.48486

Photoautotroph organisms are facing severe challenges at high latitudes where the sun stays below the horizon for an extended period of time during winter. Abundances of pelagic microalgae during this time of the year are extremely low, and many species are thought to overwinter as resting stages in the sediment. Here, we investigated for the first time the physiological state of these cells in the middle of the Polar night in Svalbard at almost 79°N. The study was divided into two parts: (I) a characterization of the photophysiological state of pelagic microalgae sampled from surface waters, and (II) a study on growth initialization and bloom development from sediment surface material. In (I), we found that algal cells from surface water contained large amounts of chlorophyll and were capable of photosynthetic carbon fixation without previous acclimation to light. Fluorometric measurements of photosynthetic yield (FRRF) pointed at an initially very slow energy transfer from photosystem (PS) II towards PSI, which disappeared after short exposure to even dim light intensities. Although ambient light levels at the water surface were too low to allow measurable rates of primary production, carbon fixation (measured over 24 hours at low light) did not require any pre-acclimation to light exposure. The rates of primary production exhibited a strong positive correlation with the applied light levels. In (II), we incubated surface sediment material with nutrient-rich seawater at three different light intensities (+ dark control) under two different daylengths for three weeks, and followed the development of photosynthetic yield, chl a, primary production, and community species composition. We found a strong influence of both daylength and light level on photophysiology and primary production, and observed a very strong increase in the latter towards the end of the experimental period. These results prove that pelagic microalgae in surface waters during the polar night maintain an (almost) fully functional photosynthetic apparatus, although ambient light is too weak to allow measurable photosynthetic activity. They are capable to perform photosynthetic carbon fixation without previous acclimation to light. Algal growth from resting stages from the sediment can be induced by very low light intensities in the middle of wintertime.