Marine epiphytic diatoms: a review of environmental controls and ecosystem roles

Epiphytic diatom assemblages constitute a crucial component of coastal microbenthic ecosystems, yet they remain underexplored compared to other microbenthic organisms. This review provides a comprehensive synthesis of current knowledge on the ecological patterns, drivers, and functional significance of epiphytic diatoms colonizing marine macrophytes. Special attention is given to the relationships between diatoms and their hosts, including seagrasses (Posidonia oceanica, Zostera marina, Z. noltii, Thalassia testudinum, Cymodocea nodosa) and macroalgae (Caulerpa taxifolia, Padina sp., Eucheuma denticulatum), which provide substrates that influence diatom colonization through morphological traits, leaf turnover rates, and biochemical interactions. The distribution and assemblage structure of epiphytic diatoms are modulated by both biotic interactions (e.g., grazing, competition, host characteristics) and abiotic factors (e.g., salinity, nutrient and light availability, and hydrodynamics). Spatial variability in epiphytic assemblages emerges across latitudinal, regional, and microhabitat gradients, whereas temporal variability is driven by seasonal cycles, environmental instability, and host phenology. Dominant genera such as Cocconeis exhibit specific ecological strategies linked to substrate type and environmental conditions. Growth forms (adnate, erect, motile) represent functional adaptations to the structural and physicochemical properties of different host surfaces. The review highlights the functional relevance of epiphytic diatoms not only as primary producers and key trophic resources but also as sensitive bioindicators of environmental status. Changes in assemblage composition can signal variations in nutrient loading, hydrological regimes, and habitat degradation, making epiphytic diatoms a useful tool for biomonitoring and assessment of coastal ecosystems. Despite growing recognition of their ecological importance, studies on epiphytic diatom assemblages remain scarce, particularly concerning functional traits, host specificity, and large-scale biogeographic patterns.