Darkness modulates polyunsaturated aldehyde production in two coastal diatoms

Diatoms are eukaryotic phytoplankton responsible for about 20 % of Earth's total primary production. In temperate regions, they produce seasonal bloom events. After these blooms, diatoms settle in the water column to coastal seabeds, where they remain in darkness. In such conditions, diatoms can form resting stages that ensure their viability for future blooms. During bloom development, diatoms could produce polyunsaturated aldehydes, bioactive molecules with significant ecological functions. This study examined the impact of periods of darkness on polyunsaturated aldehydes production in two coastal diatoms, Cyclotella cryptica and Skeletonema pseudocostatum. We checked that after 75 days in darkness, both species remained viable for blooming and maintained their photosynthetic capabilities. Moreover, our findings indicated that darkness enhanced polyunsaturated aldehydes production in both species, resulting in significantly higher aldehyde concentrations than those observed under light:dark (control) conditions. For C. cryptica, polyunsaturated aldehydes levels rose from 0.112 ± 0.123 fmol cell^-1 to 0.573 ± 0.376 fmol cell^-1, and for S. pseudocostatum, from 0.012 ± 0.006 fmol cell^-1 to 0.083 ± 0.054 fmol cell^-1. The ecological implications that may arise from increased polyunsaturated aldehydes production in dark conditions are discussed.