Microbial borings in benthic foraminifera from an heavy metal polluted coastal area (Portovesme, southwestern Sardinia)

A monitoring survey from coastal environments facing the industrial area of Portoscuso-Portovesme (South-Western Sardinia, Italy) has revealed intense bioerosional processes in tests of benthic foraminifera collected in water less than 2m deep. Electron Microscopic investigation show that 1) microbial communities extensively infest foraminifera, 2) heavy metals fluxes (Zn, Pb, Cd) and carbonate dissolution are correlated and this correspond to decrease in foraminifera richness, low population density, biodiversity and to marked increase in abnormality and pyritisation of the tests. In addition, carbonate dissolution induced by euendoliths is selective depending on Mg content and morpho-structural types of foraminiferal taxa. The bioerosional process on foraminiferal tests was analyzed via SEM (ESEM QUANTA 200, FEI, Hillsboro, Oregon, and partly with EVOLS15, ZEISS) at Cagliari University (Italy). The epoxy resin-casts of foraminifera were made at Erlangen University (Germany) in collaboration with M. Wisshak. To estimate quantitatively the impact of the microbial borings on the foraminifera and the selective bioerosion on high-Mg and low-Mg foraminiferal tests, 300 individuals were picked randomly from each sample on the fraction >63 μm and were observed with scanning electron microscope. A total of 1200 foraminifera was examined. As well as the bioerosional features on several morpho-structural taxa have been quantitatively calculated. The percentages of calcareous dissolution in foraminifera, distinguish between high-Mg and low-Mg tests, have been compared with heavy metals values, ph-pore waters and richness of specimens. Scanning Electron Microscope images of the test surfaces and of epoxy resin-casts of foraminifera provide the recognition of microboring traces and cavities produced by phototrophic (cyanobacteria, chlorophyta) and heterotrophic (fungi) organisms. Comparing foraminiferal hosts belonging to diversified morpho-structural types with the bioerosional features, our data point out that the increase in heavy metal fluxes is clearly marked by an increase of microbial infestation, reaching a peak at the more polluted sites. Consistent carbonate dissolution differences occur between porcelanaceous imperforate and hyaline perforate groups. When comparing hyaline with porcelanaceous specimens for each sampling site, it was observed that the percentage of bioerosion in hyaline specimens increases from 19.9% to 59.7%, whereas in porcelanaceous tests bioerosion increases from 35.1% to 80.3%. An opposite behavior show the benthic foraminifera assemblages, which abruptly decrease increasing the heavy metal fluxes, while the percentage of the teratological increases (until 11%) in accordance with the increase of the pollutants. Our multiproxy analyses show that the affects of the biochemical dissolution are controlled, as well, by the chemical composition and ultramicrostructure of the foraminifera substrates and by the bioavailability of organic matter in their skeleton texture