Photosynthetic adaptations of polar phytoplankton to extreme low light

Abstract

Polar phytoplankton are vital to global aquatic ecosystems, driving primary production, biogeochemical cycling, carbon sequestration, biodiversity, and climate regulation. Polar phytoplankton’s slow but significant productivity at exceptionally low light suggests possible adaptations for low-light photosynthesis. We hypothesized that maintaining photosynthesis under extremely low light involves suppressing energetically wasteful charge recombinations in Photosystem II. These recombinations desynchronize the four-step cycle of Photosystem II oxygen evolution. We used single turnover variable chlorophyll fluorescence to detect changes in recombination in polar diatoms and green algae in response to temperature and photon delivery spacing. Prolonged synchronous cycling indicates fewer wasteful recombination reactions and, thus, more efficient photosynthetic energy conversion under low light. We observed that higher photon delivery rates and colder temperatures result in less recombination within taxa. Further, polar taxa synchronized cycles for longer durations than temperate taxa under comparable conditions. Our findings support our hypothesis that diverse polar phytoplankton have evolved capacities to suppress energetically wasteful charge recombinations and sustain photosynthesis under extremely low light. This research challenges the conventional understanding of the limits on photosynthesis under light limitation, helping unravel polar ecosystem dynamics and predict their ecosystem responses to climate change.