Tolerance of Infaunal Benthic Foraminifera for Low and High Oxygen Concentrations.

Ammonia beccarii is irregularly distributed in the subtidal sediment of the southern North Sea, with substantial numbers occurring as deep as 35 cm below the water-sediment interface. Deep infaunal specimens are insensitive to high oxygen concentrations (+/-225 {mu}M), and all specimens isolated from different depth intervals continued their normal activities (feeding and growth) when exposed to dysaerobic oxygen content (<12.5 {mu}M). Specimens of E. excavatum, Q. seminulum, and E. scabra, when subjected to the same conditions, behave similarly to A. beccarii. These benthic foraminifera have very low oxygen requirements. The chambers of A. beccarii that are formed in situ at different depth intervals in the sediment have a wide range in the porosity (i.e., % of area occupied by pores) which is adequate for gas exchange under both high and low oxygen conditions. However, chambers formed in the laboratory under dysaerobic conditions have a significantly higher porosity (mainly due to larger pores) than do chambers constructed in well oxygenated water. Foraminifera live at the oxic-anoxic boundary throughout the sediment and therefore must occasionally be subjected to completely anoxic conditions. A. beccarii, E. excavatum, and Q. seminulum actively survived at least 24 h without oxygen, indicating that they are capable of facultative anaerobic metabolism.