The evolutionary success of diatoms is often attributed to their unique life cycle, consisting of a long vegetative phase during which cell size gradually diminishes and a short sexual phase which is vital to restore cells to their original size. The life cycle is often described as a ‘sex clock’, and for a long time was hypothesized to be a self-sustained periodic process. Recent models, however, suggest that environmental conditions triggering sexual reproduction are necessary to sustain the periodicity of the clock. Nonetheless, knowledge on the relationship between environmental conditions, cell size distribution and sexual reproduction is scarce. In this PhD project I will use six raphid benthic diatoms to (1) conduct physiological experiments to assess the effect of environmental conditions on sexual reproduction, (2) generate sex transcriptomes and conduct comparative sequence analysis to elucidate genes and genetic pathways underlying raphid diatom sexual reproduction, (3) develop a methodology to detect natural sexual events and (4) link those to observations of temporal variation in population density, cell size distribution and seasonal changes in environmental conditions. Together, this will gain new insights about the genes, pathways and environmental cues controlling sexual reproduction in benthic diatoms, both in the lab and in the wild.