NITROFORAM · The New Players in the Marine Nitrogen Cycle: Benthic Foraminifera

The New Players in the Marine Nitrogen Cycle: Benthic Foraminifera

Project context and objectives NITROFORAM aimed to investigate the microbial ecology of benthic foraminifera in relation to the marine nitrogen (N) cycle. Specifically, the main questions were: 1. How many species of foraminifera can accumulate intracellular NO3 stores in anoxic sediments? How many of them have the potential to denitrify, and how substantial is their contribution to the global N-cycle? 2. How are these species of foraminifera related and how did the anaerobic respiration in eukaryotes evolve? 3. How are the survival and NO3- uptake mechanisms different in the various foraminiferal species in anoxic marine habitats? Work performed The research for objectives 1 and 2 focused on the investigation of the occurrence of nitrate accumulation and denitrification capacity within foraminifera, as well as the phylogeny, ecology and environmental affiliation of nitrate-accumulating foraminiferal species. Benthic foraminifera, single-celled marine eukaryotes, inhabit all marine environments from brackish intertidal flats to deep-sea trenches, and from tropics to poles. Denitrification capacity and nitrate content were investigated for 67 foraminiferal species sampled from different marginal marine environments in Europe and in Chile. The investigated species represented all the major foraminiferal groups: the miliolids, rotaliids and textulariids, allogromiids and lagenids. In addition, 55 specimens belonging to the genus Gromia were tested for nitrate content. This aquatic ameboid protist genus bears an organic test and is thought to be the sister-group of foraminifers within Rhizaria. The conclusions that could be drawn from this research were striking: 1. The ability to accumulate and respire NO3 was found in almost all the major foraminiferal groups and also within Gromia. 2. Nitrate storage was common in foraminifers from very diverse benthic marine environments, including sediments below oxygen minimum zones, continental slopes, shelves and coastal sediments. 3. The phylogenetic analyses indicated that denitrification was incorporated with the protomitochondrion in the very first eukaryotes and that more eukaryotic phyla could have retained the trait. 4. The foraminiferal nitrate respiration also has implications for geochemical cycling of major nutrients. The contribution of foraminifers in ocean sediments to the removal of fixed nitrogen by respiration may in some areas equal the importance of bacterial denitrification. To achieve the third objective, laboratory experiments addressing the prolonged survival and behaviour of benthic foraminifera in the absence or presence of oxygen and nitrate were conducted. For this first experimental round, the benthic foraminifera Globobulimina turgida was chosen as a model organism, since it has a well-documented nitrate storing and denitrification capability, and is known to thrive in oxygen-free sediment environments. The results showed that G. turgida are capable of long-term survival (more than 86 days) under simulated oxygen-free conditions if nitrate is readily available to sustain cellular respiration. In the absence of desired electron acceptors (oxygen and nitrate), the survival rates of G. turgida are reduced (average of 25 days) but foraminifera can still survive up to two months if utilising their internal nitrate pool only. Therefore, the facultative anaerobe capacity of G. turgida and its relatively long-term survival through the intracellular nitrate pool allows foraminifera to reside in a wide range of sediment depths and inhabit various sediment redox zones. The results also show similar adenosine triphosphate (ATP) concentrations in incubated specimens under different oxygen/nitrate conditions, indicating that the metabolic activity of G. turgida remain equal in both environments. Parallel laboratory experiments to study migration and nitrate uptake mechanisms of Globobulimina turgida under anoxic conditions were also performed. Experimental cores were designed and built to check if the foraminifers must migrate to the strata containing NO3 or if they can use their pseudopods like a straw. The results showed that G. turgida cannot extend their pseudopods for nitrate uptake through several millimetres of sediment, but must physically migrate upwards, closer to nitrate-containing strata. However, foraminiferal migration patterns in control cores with no nylon net were erratic, suggesting that individuals move in random orientations until they find their preferred conditions (i.e. free nitrate or oxygen). A second experiment showed foraminifera actively collect nitrate in both the presence and absence of oxygen, although the uptake was initiated faster if oxygen was absent from the environment .Our results showed that not only oxygen but also other electron acceptors (namely nitrate) may play an important role in the foraminiferal distribution in sediment. Microhabitat distribution of deep infauna may be related to respiration pathway and migration activity in respect to refuelling and subsequent depletion of their nitrate storage. Main results Our discovery of the widespread occurrence of nitrate accumulation and denitrification within the unicellular eukaryotic Phyla Foraminifera and Gromiida (Piña-Ochoa et al. 2010, PNAS) upend the classical view on mechanisms responsible for nitrogen loss from the world's oceans. It has been thought previously that these processes were performed only by prokaryotes. We have shown that the contribution of eukaryotes to the removal of fixed nitrogen by respiration may equal the importance of bacterial denitrification. Furthermore, our work had also broadened the perspective of the foraminifer's ecology, and metabolic and ecological versatility of the eukaryotic cell.

Data: CORDIS, © European Union