Photosynthetic light requirement near the theoretical minimum detected in Arctic microalgae

Abstract

Photosynthesis is one of the most important biological processes on Earth, providing the main source of bioavailable energy, carbon, and oxygen via the use of sunlight. Despite this importance, the minimum light level sustaining photosynthesis and net growth of primary producers in the global ocean is still unknown. Here, we present measurements from the MOSAiC field campaign in the central Arctic Ocean that reveal the resumption of photosynthetic growth and algal biomass buildup under the ice pack at a daily average irradiance of not more than 0.04 ± 0.02 µmol photons m^-2 s^-1 in late March. This is at least one order of magnitude lower than previous estimates (0.3-5 µmol photons m^-2 s^-1) and near the theoretical minimum light requirement of photosynthesis (0.01 µmol photons m^-2 s^-1). Our findings are based on measurements of the temporal development of the under-ice light field and concurrent measurements of both chlorophyll a concentrations and potential net primary production underneath the sea ice at 86 °N. Such low light requirements suggest that euphotic zones where photosynthesis can occur in the world's oceans may extend further in depth and time, with major implications for global productivity estimates.

Fig. 1. Schematic illustration of sampling sites and depths.

MOSAiC sampling sites and depths around RV Polarstern during spring 2020 relevant to this study are illustrated in a schematic way. Biological parameters from the upper mixed layer (circular arrows) were collected from 20 m depth via rosettes deployed through holes in the ice at Ocean City and next to RV Polarstern (see refs. ^,), as well as the ship’s underway system at 11 m depth. Sea ice cores for sea ice Chlorophyll a and Net Primary Production P were collected at the first-year ice coring site. Light measurements were collected with OptiCALs (Optical Chain And Logger) gg, hh, and ee down to 50 m water depth as well as the lightharp, the latter inside the ice column only. Exact locations of the different sampling sites are illustrated in Fig. S2. Please note that the ice was drifting about six times faster than the underlying water column so that specific locations on the pack ice do not represent different sampling locations in the ocean.

Fig. 2. Temporal development in phytoplankton activity and biomass.

Development of (A) potential net ¹⁴C productivity (as a proxy for Net Primary Production) under reference conditions, and (B) 11 m water column Chlorophyll a (Chl-a) concentrations (light green) and 50 m depth-integrated Chl-a standing stocks (dark green) as a function of time. Chl-a-specific ¹⁴C productivity production is displayed in Fig. S4.

Fig. 3. Temporal development in light availability.

Photosynthetically active radiation (PAR) at the sea ice bottom (blue) and in the water column (3–50 m depth-integrated values; green) for the different OptiCALs (Optical Chain And Logger sensors ee, gg, hh) and light harp as a function of time. Shaded areas indicate error bands of 20% and 12.2% uncertainty for OptiCALs and lightharp, respectively. A lead that opened on April 17th near sensor gg strongly increased light availability in the water column at this location for 3–4 days.

Fig. 4. Relationship between photosynthetic biomass and light availability.

Underway Chlorophyll a (Chl-a) concentrations (11 m depth; log scale) as a function of 3–50 m depth-integrated daily average photosynthetically active radiation (PAR) values with exponential fits of the period before (r² = 0.03 indicating no relationship between Chl-a and PAR) and after (r² = 0.66 indicating an exponential increase in Chl-a with increasing PAR) the change point determined after Beaulieu & Killick. A lead that opened on April 17th at sensor gg strongly increased light availability at this location for 3–4 days (see Fig. 3). The respective data points were excluded from this calculation (see Fig. S9 for full data).