Survival and life strategy of the foraminiferan Globobulimina turgida through nitrate storage and denitrification

In a laboratory experiment, we examined the prolonged survival and behaviour of the benthic foraminiferan Globobulimina turgida under 3 simulated natural conditions: oxygenated with added nitrate, anoxic with added nitrate, and anoxic. The survival rates, adenosine triphosphate (ATP) reserve and intracellular nitrate pool of G. turgida were measured periodically under these conditions. Furthermore, to evaluate the efficiency and energy yield of the respiration system, denitrification rates of individual specimens were quantified using the acetylene inhibition and N2O microsensor technique at the start of the experiment. Our results demonstrate that the long-term (56 d) survival rate (64%) and ATP concentrations of G. turgida were not significantly different in oxygenated and anoxic, nitrate-containing conditions (Mann-Whitney test, p > 0.05). Thus, G. turgida can survive prolonged anoxia (3 mo) as long as nitrate is available to sustain its respiration. However, it remains unsure whether growth or reproduction can take place under anoxia. Short-term (21 to 35 d) survival rates were lower in nitrate-free, anoxic conditions (22% recovered alive compared to 62 to 82% in nitrate-oxic or nitrate-anoxic conditions), but foraminifera were observed to survive up to 56 d if respiring from their intra-cellular nitrate pool only. The foraminiferal nitrate pool appears very dynamic, as wide ranges of concentrations were measured in living specimens (0 to 463 mM ind.(-1)). We postulate that the scatter in the nitrate pool measurements highlights the ability of the foraminifera to actively collect and respire on nitrate, depending on individuals' history of exposure to oxygen and nitrate.