EXPERIMENTAL AND FIELD EVIDENCE OF ANTARCTIC FORAMINIFERAL TOLERANCE TO ANOXIA AND HYDROGEN-SULFIDE

The common occurrence of foraminifera in strata deposited under anoxic environmental conditions throughout the geologic record raises the question of whether foraminifera actually survive anoxic or reducing conditions. To test this, benthic foraminifera from McMurdo Sound, Antarctica were exposed to anoxic or reducing conditions for 30 days. These Antarctic forms are known to be exposed to anoxic events in their natural environment. An adenosine triphosphate (ATP) assay was used to determine foraminiferal survival, and their ultrastructure was examined using transmission electron microscopy (TEM) to evaluate any possible effect that might have resulted from exposure to anoxic or reducing conditions. The experimental treatments did not have any statistically significant effects upon the foraminifers with regards to survival or average ATP content per individual including the four most common living species. Some ultrastructural evidence for encystment (fibrillar membrane-bound bodies in the cytoplasm of many specimens) was observed. Field data include the observation that Globocassidulina cf. G. biora resides in sediments to depths of 7 cm, as evidenced by ultrastructural investigations. A specimen of Globocassidulina cf. G. biora collected from sediments containing anoxic pore waters showed numerous bacteria within the confines of the organic lining. The occurrence of cytoplasmic inclusions similar in appearance to carboxysomes within these bacteria suggest possible affinities to the chemolithotrophic Thiobacillus. Bacteria of this type were not observed in specimens collected from oxygenated pore waters. These field and laboratory findings suggest that certain foraminifera may be facultative anaerobes able to withstand reducing conditions.