Growth of Arctic diatoms at extreme low light and impact on populations dynamics – Dim

In the Arctic Ocean, microalgae experience extreme seasonal variations in available light, from the polar night under the ice pack to the midnight sun in ice-free waters. We have recently discovered that they could grow under the extreme low-light regime typical of the polar night exit. The objective of the Dim project is to quantify growth of Arctic microalgae under extreme low light, and identify the regulation mechanism of cellular metabolism that make it possible. The project includes i) a series of laboratory experiments to characterize the growth of an Arctic diatom at low light, ii) a field campaign in Baffin Bay during winter 2023-2024 to document in situ growth of the various taxonomic groups present between January and April, and iii) modelling experiments to determine the possible role of low-light winter growth on inoculating the spring bloom of micro-algae in the Arctic Ocean. In the first two laboratory experiments the polar diatom Fragilariopsis cylindrus will be grown under continuous light and sine light-dark cycles closely reproducing natural ones, at 7 light levels from ~0.01 to 10 mmol photons m-2 s-1. In a third experiment, various forms of organic carbon will be added to study the possible role of heterotrophy, and in a fourth experiment, we will compare F. cylindrus growth at very low light with that of 3 other Arctic diatoms and a typical prasinophyte, Micromonas polaris. In these experiments, we will try to determine the possible contribution of the following strategies that may allow growth at extremely low light: maximization of light capture and efficient photosynthesis, minimization of respiration, optimization of the use of reserves, notably lipids, and possible use of heterotrophy to complement energy provided by photosynthesis. A detailed characterization of photophysiology, together with proteomics, lipidomics, and transcriptomics analyses will allow deciphering the role of each of these possible strategies. During the field campaign, growth of microalgae in sea ice and in the water column under sea ice will be monitored from December 2023 to April 2024 in the vicinity of Qikiqtarjuaq (Nunavut, Canada) under landfast ice. Every four days, we will document: the light field at the bottom of sea ice and in the water column with ultra-sensitive radiometers, micro-algae pigments, light-absorption properties and photosynthetic parameters, abundance and taxonomy of phytoplankton and zooplankton, carbon and nitrogen assimilation by microalgae, zooplankton grazing rate, nutrients, hydrological properties and vertical fluxes of particles. Additionally, phytoplankton communities and species will be identified using high throughput amplicon sequencing targeting the V4 and V9 regions of the 18S rRNA and the 16S rRNA from chloroplasts. Finally, we will use a numerical framework allowing for the simulation of various plankton functional types to test our hypotheses regarding the ecological impacts of the physiological responses of the Arctic diatom community to the polar-night light conditions (trait-based model coupled to the MIT general circulation model). We plan the following numerical experiments: (i) a series of local sensitivity analyses in a 1-D water column implementation to quantify the range of physiological and biological parameter values required to simulate a positive net growth of numerical diatom types in extreme low light regimes, (ii) test of the role of the simulated predation pressure (top-down control) on the biomass of the phytoplankton community at the onset of the light conditions sought to initiate positive net growth, and (iii) use of a 3-D configuration of Baffin Bay in order to quantify the role of diatom adaptations to extreme low light conditions on the spatio-temporal patterns in the phenology and amplitude of the phytoplankton bloom.