Morphology of a ferric iron-encrusted biofilm forming on the shell of a burrowing bivalve (Mollusca)

The shell of the bivalve Montacuta ferruginosa is usually covered with a rust-coloured and ferric iron-encrusted biofilm. The latter is a structured microbial mat with 3 separate layers. The outer layer is essentially microbial (filamentous bacteria and protozoa). The intermediary layer is both microbial and mineral (heavily ferric iron-encrusted filamentous bacteria and protozoa), The inner layer is essentially mineral (ferric iron deposits) and generally devoid of living microorganisms. The most abundant microorganisms in the biofilm are filamentous bacteria related to Beggiatoaceae. The genesis of this mineral-microbial mat could be partly due to a sequence of processes: (1) ferric iron deposition within bacterial sheaths in the outer layer; (2) the release and accumulation of heavily ferric iron-encrusted sheaths after lysis of the bacteria in the intermediary layer; (3) degradation of bacterial sheaths and accumulation of ferric iron minerals in the inner layer.