A cells view of ammonium assimilation

It is well known that dinoflagellates have a symbiotic relationship with photic zone corals and aid in the uptake and production of nutrients. Despite this being common knowledge the roles of the host and symbiont in this mechanism remains unclear but using the advance technique of NanoSIMS this paper sheds some light on the mechanism and the organisms’ roles.

The coral in question Acropora aspera and its associated dinoflagellate from the Symbiodinium genus were collect from a reef flat just of Heron Island Research Station and transferred into six independent acclimatisation tanks for a week. The labelling experiment was then carried out where the two experimental treatments were incubated in either normal seawater (control) or enriched ammonium seawater (pulse-chase). Enriched ammonium (¹⁵NH₄Cl) was added to seawater in powder form to a final concentration of 20mM which is 10 to 20 times above average levels but may represent elevated environmental levels. Samples of the coral branches were removed from the tanks at time periods of 0, 1, 3, 6 and 12h after the initial 1h-pulse period. One part of the sample was frozen for amino acid and symbiont density analysis and the other part was fixed for TEM (transmission electron microscopy) and NanoSIMS analysis.

The TEM was used to identify regions of interest (ROI) and subsequently these ROI were mounted onto a NanoSIMS holder for analysis. NanoSIMS uses a high-powered laser to remove secondary ions of ¹²C¹⁴N^- and ¹²C¹⁵N^- this then creates and image for each ion. These images are then overlaid onto each other to create a ratio of ¹⁵N/¹⁴N for the selected ROI. Cell densities of Symbiodinium were determined using a haemocytometer and the nitrogen and uptake rate was analysed using the standard combustion (950°C) method.

Observations from NanoSIMS indicated a rapid uptake of enriched ammonium with ¹⁵N clearly labelled in dinoflagellates within the 1h-pulse period and the concentration of ¹⁵N remained stable throughout the 11h-chase following the labelling event. In contrast the ¹⁵N-labelling of the coral host tissues remained substantially weaker during the entire experiment. The results emphasize to important points, with the first being that ammonium from the seawater is rapidly transported from the coelenteron to the symbionts. The second point is that the levels of ¹⁵N didn’t decrease over the 11h indicating no further incorporation of unlabelled nitrogen and that the symbionts retained most of the ¹⁵N-ammonium derived compounds.

Although the dinoflagellates are the main site of nitrogen fixation and are by the fastest and largest contributor they are not the only site with both the gastrodermal and epidermal host cells contributing a little to the N fixation. The fate of the assimilated ammonium is being metabolised into Glutamine (purine pathway to purine for storage as uric acid) and Glutamate (Krebs-cycle).

This paper further emphasises the importance to symbiotic algae in corals and that coral bleaching affects many different pathways including ammonium assimilation and fixation and gives an interesting insight into the interactions of nutrient metabolic pathways in corals and their symbionts.

Pernice, Mathieu, et al. "A single-cell view of ammonium assimilation in coral–dinoflagellate symbiosis." The ISME journal 6.7 (2012): 1314-1324.