Response of the sea‐ice diatomFragilariopsis cylindrusto simulated polar night darkness and return to light

Abstract

Arctic photoautotrophic communities must survive through polar night darkness until light returns in spring. We tracked changes in the cellular resource allocations and functional capacities of a polar sea-ice diatom, Fragilariopsis cylindrus, to understand acclimation processes in both darkness and during the subsequent return to light. We measured parameters at specific time-points over 3 months of darkness, and then over 6 d after a return to light. Measured parameters included cell number and size, cellular carbon and nitrogen quotas, lipid and pigment contents, concentration of key proteins of the photosynthetic system, photosynthetic parameters based on both variable fluorescence and carbon assimilation, and the level of nonphotochemical quenching. A stable functional state was reached within a few days after the transition to dark and was then maintained throughout 3 months of darkness. The dark period resulted in a decrease of lipid droplet cell quota (−6%), chlorophyll a cell quota (−41%) and the maximum carbon fixation rate per cell (−98%). Return to light after 1.5 months of darkness resulted in a strong induction of nonphotochemical quenching of excitation and a fast recovery of the maximum carbon fixation rate within 1 d, followed by a rapid increase in the cell number. Return to light after 3 months of darkness showed an increase of mortality or a profound downregulation induced over longer periods of darkness. *Correspondence: philippe-israel.morin.1@ulaval.ca Additional Supporting Information may be found in the online version of this article.Authors Contribution Statement: Morin P. I. is the main author of this work with major contributions to designing and running the experiments, analyzing the data, and writing the article. All coauthors helped with running the experiments and/or revising the article. Campbell D. A., J. Lavaud, and M. Babin helped with the structure of the manuscript and the discussion of the data.

The possible fates of Fragilariopsis cylindrus (polar diatom) cells exposed to prolonged darkness

“…Nymark et al (2013, and references therein) have previously reported that Phaeodactylum tricornutum cells complete an ongoing cell cycle when placed in the dark. Morin et al (2019) and Lacour et al (2019) made the same observation with Fragilariopsis cylindrus and Chaetoceros neogracilis, respectively. The total cell concentration then remained stable until the end of DA but after 30 d in the dark, the abundance of P1 cells began slowly to decrease while P2 and P3 populations began to emerge (Fig.…”

Under-Ice Phytoplankton Blooms: Shedding Light on the “Invisible” Part of Arctic Primary Production

“…Among the small flagellated cells, two species that periodically dominate phytoplankton assemblages in Arctic waters were found throughout the Polar night: Micromonas polaris and Phaeocystis pouchetii (Vader et al, 2015). Natural microalgal assemblages, and in particular diatoms, are able to survive extended periods of darkness from months to years (Zhang et al, 1998;McMinn and Martin, 2013), while retaining their ability to resume physiological activity quickly once light returns (Kvernvik et al, 2018;Lacour et al, 2019;Morin et al, 2020). Diatoms also seem to retain their photophysiological characteristics during extended periods of darkness relatively unchanged as compared to flagellates, which possibly enables them to utilize the returning light in early spring very efficiently (van de Poll et al, 2020).…”

Always ready? Primary production of Arctic phytoplankton at the end of the polar night

“…Measurements and experiments during the polar night in Svalbard indicate that phytoplankton are capable to reinitiate photosynthesis almost immediately upon reillumination in the laboratory, even if in situ primary production is not measurable (Berge et al 2015;Kvernvik et al 2018). From experiments with single strains of polar phytoplankton, there is increasing evidence that species differ in the extent to which they manage to decrease metabolic energy needs in the dark as well as in their capacity to reestablish photosynthesis and growth upon re-illumination, yet overall capacities for recovery are remarkable (Kennedy et al 2019;Lacour et al 2019;Morin et al 2019;van de Poll et al 2020a). In view of this, it seems to be the return of sufficient sunlight (Cohen et al 2020) for net growth rather than germination cues or molecular clocks that determines the start of phototrophic production.…”