Photophysiology of the cryptophytes Proteomonas sulcata and Hemiselmis aquamarina with varying light intensity

Cryptophytes are often found in deep, dimly-lit layers of water bodies which has led to the assumption that they are a low-light adapted taxon. Recent studies have suggested this assumption to be false, but the limits of cryptophyte photoacclimation have yet to be thoroughly tested across the taxon. To address this lack of experimentation, we investigated the photophysiology of two differently-pigmented cryptophytes, Proteomonas sulcata and Hemiselmis aquamarina. Both were grown at 75 μmol photons m−2 s−1, then moved to six different light intensities between 45 and 1500 μmol photons m−2 s−1. Growth rates increased to a maximum of ~ 1 d−1 at 421 (P. sulcata) and 363 (H. aquamarina) μmol photons m−2 s−1, and declined only when intensities were >700 μmol photons m−2 s−1, but growth was maintained at 1500 μmol photons m−2 s−1 for both species. Fv/Fm values for both species were high (>0.6) up to 310 μmol photons m−2 s−1, indicating high photosystem II quantum efficiency. Changes in chlorophyll-a (chl-a) and phycobiliproteins, alloxanthin:chl-a ratios and non-photochemical quenching were also consistent with effective high-light acclimation. Both species showed adequate ability to acclimate to even the highest intensity tested. These findings suggest that cryptophytes may possess a greater capacity to adjust to high irradiance conditions than previously recognized.