Light response of karyostrophy in the benthic pennate diatom Pleurosigma strigosum (Bacillariophyceae): A complimentary photoprotective process?

Pennate diatoms are an ecologically and evolutionarily successful group of algae, dominating in sedimentary habitats where they form biofilms with high productivity and diversity. Their success has been attributed to directed motility, which is used to explore the microscale environmental gradients present in sediments, particularly regarding light, optimizing photosynthesis while avoiding photodamage. Some pennate diatoms can exhibit a process termed karyostrophy, the contraction of the chloroplasts toward the cell center when exposed to high light. Karyostrophy has long been hypothesized to play a photoprotective role; however, its light dependency and physiological effects remain poorly characterized. This study investigated the light-dependent kinetics and photophysiological effects of karyostrophy in the diatom Pleurosigma strigosum. Chloroplast contraction was found to be light-dependent, being induced under irradiances above 60 μmol photons · m^-2 · s^-1, with the rate and extent of contraction increasing with light intensity. The process was reversible, with chloroplasts returning to their original conformation under low light, although at a slower rate. Cell-level photophysiological measurements indicated that karyostrophy enhanced self-shading in proximal cell regions, improving the capacity of the cells to recover from light stress. Non-photochemical quenching (NPQ) was also affected by chloroplast contraction, with distal regions of the cell exhibiting significantly higher NPQ activation. These findings suggest that karyostrophy might serve as a complementary photoprotective mechanism, acting alongside whole-cell motility and NPQ. This study provides the first quantitative characterization of the light response of karyostrophy, highlighting its possible role in optimizing light utilization and protecting against photodamage.