Photoinhibition risk in marimo (<i>Aegagropila brownii</i>) during ice transition periods based on field observations and laboratory assessments

Abstract The green algae Aegagropila brownii (marimo) form large spherical aggregates in Lake Akan, Japan, where they are exposed to extreme seasonal environmental fluctuations. In particular, the transitional periods before ice formation and after ice thawing present a rare combination of low water temperature and high light (LT‐HL), which may cause photoinhibition (light‐induced reduction in photosystem II [PSII] efficiency). To examine the seasonal photosynthetic responses of marimo, we conducted assessments under both field and controlled laboratory conditions, focusing on transitional periods. Using continuous monitoring of water temperature and underwater light intensity photosynthetic photon flux density (PPFD), in combination with chlorophyll fluorescence measurements, we evaluated photosynthetic activity and photoprotective capacity in marimo collected across five seasonal stages: (i) summer, pre‐freezing (late December; before surface ice formed above the marimo colony), (ii) ice‐covered (March; under continuous ice and snow cover), (iii) immediately post‐thawing (early April); (iv) after ice had melted above the marimo colony, and (v) post‐thawing (early May) periods. Our results showed that marimo maintained high PSII efficiency ( F v / F m &gt; 0.6) in both summer and ice‐covered periods, whereas marked photoinhibition occurred in the immediate post‐thawing period ( F v / F m &lt; 0.3 on sun‐exposed surfaces). Although PSII repair under low light was rapid, it plateaued after 30 min of exposure. Full recovery of F v / F m and nonphotochemical quenching occurred within 20 to 30 days under natural conditions. These findings suggest the physiological vulnerability of marimo during seasonal transitions and emphasize the need for conservation strategies that address habitat degradation and light‐induced stress events driven by climate variability.