Structure and composition of intertidal biofilms which enhance the settlement of the central-Mediterranean reef builder Dendropoma cristatum (Biondi 1859).

Introduction Marine biofilms are complex matrix of microorganisms and mucopolysaccharide substances which develop on all submerged surfaces, providing a variety of physical-bio-chemical cues which affect the settlement of a wide range of benthic organisms. Biofilm community composition, 3-D structure and metabolic activity vary over time. Few studies report the biological and physical changes in marine biofilm development in relationship to their effects on benthic colonisation. Biofilm maturity has been reported to enhance the settlement rate of the central-Mediterranean Dendropoma cristatum (Biondi, 1859), an intertidal vermetid gastropod which builds up ecologically relevant reefs, subjected to the increasing human pressure on the coast. Studies on the settlement of reef-forming species are relevant to implement effective strategies for the conservation of the key-habitats they construct. In order to understand if quali-quantitative features of the microbial assemblage (e.g. biological diversity and physical structure) may explain the positive relationship among the biofilm maturity and the settlement of this Mediterranean reef-builders, a combination of molecular techniques and microscopic observations has been employed. Materials and Methods Biofilms at increasing maturity were obtained on limestone blocks fixed on vermetid reef for 13, 23 and 32 days (NW Sicily). Each treatment was replicated twice. After block collection, the Automated Ribosomal Intergenic Spacer Analysis (ARISA) was carried out to describe the prokaryotic and eukaryotic diversity of the 3 microbial assemblages. Scanning electron microscope observations were performed for further eukaryotic identification and comparison of the substratum colonisation among treatments. Results Overall 43 prokaryotic and 41 eukaryotic OTUs were revealed by the ARISA. The number of bacterial OTUs for each assemblage varied between 7 and 14 and between 4 and 17 for the Eukaryota. Cluster analysis of the ARISA profiles within the 3 assemblages showed high similarity among replicates from the same treatment, both for the prokaryotic and the eukaryotic biofilm component. SEM observations showed a progressive increase of the microbial coverage on the substratum with days of development, as DNA concentration measures confirmed. The assemblage proceeds from single-living or colonial diatoms and fungal hyphae toward the formation of a polymeric matrix embedding other biofilm components. Furthermore, the number of diatom cells drastically decreases from 55 to 7 cells/9 x 10-4 cm2 over time. Conclusions The microbial assemblage composition and structure changes with time of exposure. These variations have a local effect on the settlement rate of the Mediterranean reef-builders D. cristatum. Whether the effect of the biofilm maturity in the settlement rate increase depends on higher microbial biomass accumulation with age or to specie-specific interactions with one or more biofilm components is to be elucidated.