Deciphering the expression and regulation of nitrate reductase by Symbiodiniaceae from different coral species

Coral holobionts are very efficient at assimilating nitrogen through heterotrophic feeding or the uptake of dissolved inorganic nitrogen. Although NO3- is the most abundant source of nitrogen in the ocean, corals preferably uptake NH4+ due to its reduced state and energetically favorable assimilation. However, in conditions of low availability of environmental NH4+, coral holobionts are capable of depleting environmental NO3-. Symbiodiniaceae are vital partners of the symbiosis for nutrient assimilation. In addition to providing translocated photosynthates, they account for most of the uptake of dissolved inorganic nitrogen. While the uptake of NH4+ by the coral host and its symbiotic partners is a well-known process, NO3- assimilation is poorly studied. Coral hosts are unable to reduce nitrate as they lack the necessary enzymes, whereas Symbiodiniaceae have been shown to express the enzyme nitrate reductase (NR). However, the evidence supporting the active reduction of nitrate by the symbiotic algae during symbiosis is scarce and equivocal. Rigorous studies on nitrate assimilation by Symbiodiniaceae are lacking yet essential for the understanding of coral holobiont functioning. We have previously shown that the expression and regulation of NR in free-living Symbiodiniaceae of two different strains is a dynamic and reversible process impacted by NO3- and NH4+ concentrations. In particular, we noted the active degradation of NR in the presence of NH4+. NR has never been identified in in hospite coral symbionts. We investigated the expression of NR in Symbiodiniaceae during symbiosis. For this purpose, different coral species were depleted in nitrogen before they were exposed to an enrichment in NO3-. Expression of NR in the symbionts was investigated using western blotting and qRT-PCR. In addition, freshly isolated symbionts (FIS) from corals were placed in incubation with NO3- following their extraction from coral tissues and the expression of NR was investigated over time. This was performed in order to compare the ability of Symbiodiniaceae to express this enzyme in hospite versus ex hospite. This study aimed at deciphering the functioning of an essential enzyme involved in coral nutrition, shedding a new light and raising new questions on the process of inorganic nitrogen assimilation by the holobiont.