First survey of epibenthic diatom communities from Arabian Gulf coasts: a floristic and ecological analysis

Benthic diatoms are a key component of coastal marine ecosystems, contributing significantly to primary production, biogeochemical cycles, and the functioning of food webs. Due to their high sensitivity to environmental changes, these microalgae also provide effective tools for monitoring the ecological status of marine environments. Despite their importance, communities associated with biological substrates, such as seagrasses and animal organisms, remain poorly studied, particularly regarding the mechanisms that govern their structure and distribution. This aspect takes on even greater importance in environments characterized by extreme conditions, such as the Arabian Gulf, where high temperatures, significant seasonal variations, and high salinity levels represent major stressors for marine organisms. In such contexts, the study of microalgal communities offers a unique opportunity to understand adaptation and resilience processes and to analyze the relative roles of environmental and biological factors in determining community structure. The main objective of this thesis was to investigate the factors that regulate the structure of benthic diatom communities associated with different biological substrates, with a particular focus on the role of environmental variability and substrate characteristics. To this aim, two different systems were analyzed: communities associated with marine seagrasses and communities associated with corals, which represent ideal models for distinguishing between environmental and biological control. The study of seagrass revealed that community structure is strongly influenced by seasonality. The variations between winter and summer are marked, both in terms of abundance and community composition, with significantly higher abundance values during the winter months and a reduction during the summer. This trend is associated with changes in community structure, characterized by strong dominance during periods of higher abundance and greater evenness and diversity during periods of lower biomass. The genus Cocconeis represents the dominant component, with a clear seasonal turnover of the main species. Although differences related to the type of phanerogam have been observed, these are secondary to the effect of environmental variability, indicating that in this system, ecological control is primarily environmental in nature. In contrast, the study of coral-associated communities revealed a very different picture. These communities are more stable, with a more balanced distribution of species, high levels of evenness, and low dominance. The composition is more diverse and not dominated by a single taxon, suggesting conditions favourable to the coexistence of multiple species. In this case, the determining factor is the host: the identity of the coral significantly influences community structure, highlighting an active role for the biological substrate. The morphological characteristics of the coral and the presence of mucus create heterogeneous microhabitats that promote community diversity and stability. In this system, therefore, ecological control is mainly biological in type. A comparison of the two systems highlights how benthic diatom communities can be regulated by different levels of ecological control. In seagrass, environmental factors predominate, while in corals, the host substrate plays a dominant role. This difference reflects the different roles of the substrate, which can act as a passive support or as an active element capable of directly modulating the community structure. The results of this thesis highlight how the structure of benthic diatom communities is the result of the interaction between environmental and biological factors, the relative importance of which differs depending on the system under consideration. These communities therefore serve as effective models for understanding the mechanisms that regulate microalgal ecosystems and represent particularly useful tools for monitoring marine environments, especially in extreme contexts subject to anthropogenic pressures such as the Arabian Gulf.