Photoacclimation of cryptophytes and diatoms to light variability in the Western Antarctic Peninsula

Meltwater stratification during austral summer along the rapidly warming Western Antarctic Peninsula (WAP) exposes coastal phytoplankton to sudden light shifts. Such variability is thought to modulate phytoplankton dynamics, yet the photoacclimation capacity of Antarctic phytoplankton, especially cryptophytes, remains unclear. We grew a pennate diatom (Fragilariopsis cylindrus), a centric diatom (Porosira glacialis), and a cryptophyte (Geminigera cryophila) in microcosms subjected to low (30 μmol photons · m-2 · s-1), high (450 μmol photons · m-2 · s-1), and very low (6 μmol photons · m-2 · s-1) light over 24 days at 3°C, while monitoring cell growth, pigments, photo system II (PSII) quantum yield, and non-photochemical quenching (NPQ). All taxa maintained positive growth (0.12-0.25 · d-1) and rapidly adjusted pigment ratios after light shifts, demonstrating efficient photoacclimation. The small pennate diatom reached 10-fold higher cell densities than the centric diatom, although the latter accumulated more chlorophyll per cell and consumed twice as much inorganic nitrogen. Geminigera cryophila exhibited the most flexible short-term NPQ response, dissipating over 60% of excess energy during high light pulses, yet this flexibility did not translate into any apparent advantage in growth over both diatoms during the period of the experiment. Field data showed cryptophyte dominance in shallower, more stratified waters, whereas diatoms prevailed in more mixed waters. Our results show that all three species can acclimate to Antarctica's complex light regime over days to weeks, yet cryptophytes may have a competitive advantage under sustained high light at shorter timescales. As warming and glacial melting continue, potentially favoring smaller cells over large bloom-forming diatoms, it is crucial to understand how it will impact carbon export and trophic transfer.