Skeletal Mineralogy Patterns of Antarctic Bryozoa

Bryozoans represent an extremely diversified phylum in terms of biomineralogy. The mineral composition of the bryozoan skeleton is most likely controlled by the organism itself and to some degree by the ambient environmental conditions (e.g., pH, Mg/Ca ratio, temperature). The purpose of this study is to describe the mineral composition of nearly one-quarter of all Antarctic bryozoan species known to date and to discuss, locally and in a global context, the potential factors that control mineralization in bryozoan skeletons. Collections of bryozoans were gathered by divers and with van Veen grabs (0.1 m(2)) from depths of 6-300 m at King George Island, Antarctica, during the summer seasons of 1985, 2007, and 2011. An x-ray diffraction analysis of the skeletons indicated that all analyzed Antarctic bryozoans (256 individuals, 71 taxa) precipitated monomineral calcite skeletons. The magnesium content ranged from 0.2 to 10.1 mol% MgCO3. Most of the studied species (76%) consisted of intermediate-magnesium calcite. Yet extreme variations in mol% MgCO3 were found between individuals of the same species as well as between species and populations. The high variability found between specimens of the same species suggests that the mol% MgCO3 could be determined by the biology or physiology of an individual. The magnesium content was equally distributed in each of three growth forms (e.g., erect flexible, erect rigid, and encrusting). The mean mol% MgCO3 in Antarctic species was significantly lower than that in bryozoans from Mediterranean, Chile, and New Zealand but similar to that in Arctic bryozoans. This finding confirms the latitudinal-pattern hypothesis of a decreased Mg content toward polar regions.