Evidence for complete denitrification in a benthic foraminifer

Living without oxygen Several foraminiferal protozoa species grow in anoxic zones in marine sediment, but the type of anaerobic respiration that sustains them was not known. These organisms have now been found to accumulate nitrate intracellularly at concentrations more than 500 times the environmental values. The nitrate substitutes for oxygen in these anoxic habitats. The large amounts of nitrate that accumulate may even allow them to 'hold their breath' for more than a month. Denitrification, the biological conversion of nitrate to nitrogen, was believed to be restricted to bacteria and archaea. It has now been shown that highly abundant benthic foraminifers are also capable of denitrification, suggesting that much remains to be learned about the global nitrogen cycle. Benthic foraminifera are unicellular eukaryotes found abundantly in many types of marine sediments. Many species survive and possibly reproduce in anoxic habitats 1 , but sustainable anaerobic metabolism has not been previously described. Here we demonstrate that the foraminifer Globobulimina pseudospinescens accumulates intracellular nitrate stores and that these can be respired to dinitrogen gas. The amounts of nitrate detected are estimated to be sufficient to support respiration for over a month. In a Swedish fjord sediment where G. pseudospinescens is the dominant foraminifer, the intracellular nitrate pool in this species accounted for 20% of the large, cell-bound, nitrate pool present in an oxygen-free zone. Similarly high nitrate concentrations were also detected in foraminifera Nonionella cf. stella and a Stainforthia species, the two dominant benthic taxa occurring within the oxygen minimum zone of the continental shelf off Chile. Given the high abundance of foraminifera in anoxic marine environments 1 , 2 , 3 , these new findings suggest that foraminifera may play an important role in global nitrogen cycling and indicate that our understanding of the complexity of the marine nitrogen cycle is far from complete.