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Objective: This study is aimed to provide a first insight into the microphytobenthos (MPB) species composition, biomass and exopolymeric substances (EPS) of the organic enriched Cabras lagoon (Western Mediterranean Sea, Italy), due to the lack of information on the presence and distribution of microphytobenthic assemblages in this system, although their importance has been shown in other organic-enriched coastal lagoons.
Methods: Surface sediment samples were collected at three sites of the lagoon (C1, C2, C3), differing in sediment granulometry, on 4 occasions, over one year. MPB biomass, expressed as chlorophyll a and organic matter, was evaluated as well as the quantity of two operationally distinct EPS fractions. Sampled communities were also characterized in light and confocal microscopy to assess the spatial distribution and composition of microphytobenthic species in relation with sediment characteristics. Alcian Blue cytochemical staining of EPS samples was also used to ascertain, during light microscopy observations, the presence of acidic groups in EPS residues.
Results and Conclusions: MPB assemblages developed in biofilms comprising both eukaryotic and prokaryotic members. Communities were dominated by diatom species, mainly of the genera Navicula and Nitzschia, associated with cyanobacteria and green algae in most samples. Microorganisms were distributed in patches of different thickness and embedded in a polymeric matrix, characterized by both neutral and acidic polysaccharidic residues. Spatial and temporal patterns in species composition and biomass were also observed, likely due to the habitat heterogeneity within each site. Sediment properties seemed to influence the diatom assemblages. The muddy nature of sediments at site C2 enabled the development of epipelic forms, such as Surirellales diatoms, known for their high motility through the sediment while sandy bottoms at site C1 and C3 supported the growth of episamnic species (i.e. among the genera Amphora, Cocconeis and Cymbella), that are able to adhere to the grains with stalks, tubes and/or apical pads. Cyanobacteria dominated the extremely variable (salinity, water temperature, light and water availability) site C2 over the other MPB members, likely due to their physiological versatility and capacity of producing protective exopolymers.