Marine carbonate cements, biofilms, biomineralization, and skeletogenesis:Some bivalves do it all

Recent years have seen a growing interest in the direct and indirect roles of organisms, and more explicitly of organic matter, in the crystallization of minerals in sediments. Two bivalve genera form extensive biofilms that are apparently responsible for the growth of a variety of isolated crystals together with a marine cement. Granicorium indutum and Samarangia quadrang ularis are infaunal species that cover their shells with a cemented coating of sand, sculpted to mimic the surface ornament typical of many bivalves. Granicorium has an exceptionally mobile mantle margin that produces large v olumes of mucus, harboring a diverse microbial community. The sand grains s urrounding both species are initially bound by a biofilm that provides stru ctural integrity but also acts, like others, as a template for the crystall ization of a variety of carbonate polymorphs, These include varied prismati c crystals, rice-grain and wheatsheaf forms and, more importantly, large vo lumes of acicular crystals indistinguishable from typical marine cements. T he distribution of these crystals is related to positions accessible to the mantle of the animal and in Samarangia appears to result from the active e mplacement of grains and mucus several times a year. Summing the evidence, it seems that these species are responsible not only for the biomineralizat ion involved in the formation of their shells but also for crystallization mediated through biofilms and for the generation of cements that are morpho logically indistinguishable from typical marine cements. The boundaries bet ween these three strategies may be closer than we think.