Characterization of a thaumarchaeal symbiont that drives incomplete nitrification in the tropical sponge Ianthella basta
Marine sponges represent one of the few eukaryotic groups that frequently harbor symbiotic members of the Thaumarchaeota, which are important chemoautotrophic ammonia-oxidizers in many environments. However, in most studies, direct demonstration of ammonia-oxidation by these archaea within sponges is lacking, and little is known about sponge-specific adaptations of ammonia-oxidizing archaea (AOA). Here, we characterized the thaumarchaeal symbiont of the marine sponge Ianthella basta using metaproteogenomics, fluorescence in situ hybridization, qPCR and isotope-based functional assays. "Candidatus Nitrosospongia bastadiensis" is only distantly related to cultured AOA. It is an abundant symbiont that is solely responsible for nitrite formation from ammonia in I. basta that surprisingly does not harbor nitrite-oxidizing microbes. Furthermore, this AOA is equipped with an expanded set of extracellular subtilisin-like proteases, a metalloprotease unique among archaea, as well as a putative branched-chain amino acid ABC transporter. This repertoire is strongly indicative of a mixotrophic lifestyle and is (with slight variations) also found in other sponge-associated, but not in free-living AOA. We predict that this feature as well as an expanded and unique set of secreted serpins (protease inhibitors), a unique array of eukaryotic-like proteins, and a DNA-phosporothioation system, represent important adaptations of AOA to life within these ancient filter-feeding animals. Originality-Significance StatementMany marine sponges harbor symbiotic members of the Thaumarchaeota, but there is generally only indirect evidence available about their functional role within these filter-feeding animals. Furthermore, the specific adaptations of thaumarchaeal symbionts to their sponge hosts are incompletely understood. In this study, we thoroughly characterized a thaumarchaeal symbiont residing in the reef sponge Ianthella basta and demonstrate by using a combination of molecular tools and isotope techniques, that it is the only ammonia-oxidizer in its host. In contrast to other sponges, I. basta does not contain nitrite-oxidizing microbes and thus excretes considerable amounts of nitrite. Furthermore, using metagenomics and metaproteomics we reveal important adaptations of this symbiont, that represents a new genus within the Thaumarchaeota, and conclude that it most likely lives as a mixotroph in its sponge host.