Original paper

Algal biocenosis in the benthic mucilaginous aggregates of the Tyrrhenian sea, with emphasis on the exopolysaccharide-producing microalgal community

de Philippis, Roberto; Faraloni, Cecilia; Sili, Claudio; Vincenzini, Massimo

Abstract

With the aim to investigate on the algal biocenosis of the benthic mucilages in the Tyrrhenian sea and on the possible role of the exopolysaccharide-producing cyanobacteria in the formation of these aggregates, a research in various sites of the Tuscan Archipelago was carried out during the period April 1999-August 2001. The two Chrysophyceae Nematochrysopsis marina and Chrysonephos lewisii and the Phaeophycea Acinetospora crinita alternatively dominated the algal biocenosis of the samples; with regard to the microalgal community, fourteen genera of diatoms were detected into the mucilaginous aggregates, Synedra, Licmophora and Navicula being the genera most frequently found. As concerns the cyanobacterial community, Leptolyngbya and Lyngbya were found in almost all samplings, while other forms, belonging to the genera Oscillatoria, Rivularia, Gloeocapsa and Synechocystis, were observed at a significantly lower frequency. Polysaccharidic investments were observed in diatoms belonging to the genera Amphora, Navicula and Pseudo-nitzschia and in all the cyanobacteria found, with the sole exception of the filaments of Oscillatoria. The comparison of the microalgal and cyanobacterial biocenosis of benthic and pelagic mucilages sampled in the same area showed that the most frequently found diatoms were substantially the same, whereas a larger number of motile cyanobacterial genera, typically devoid of polysaccharidic envelopes, was observed in pelagic mucilages, probably in correlation with their capability to move into the water column to reach the depth with the optimal light intensity. The microscopic observation of the natural samples and the results obtained with the cultures of the microalgae and cyanobacteria isolated from the benthic aggregates suggest that only diatoms give a significant contribution to the formation of the mucilages, while the role of cyanobacteria in the benthic aggregates seems to be related to their capability to create a thin layer of polysaccharidic material, constituting the first step in the adhesion and colonization of the benthic surfaces by the microbial community.

Keywords

algal biocenosiscyanobacteriadiatomsbenthic mucilaginous aggregatespelagic mucilaginous aggregatesoutermost polysaccharidic investmentsexopolysaccharidestyrrhenian sea