Differential polysaccharide utilization is the basis for a nanohaloarchaeon : haloarchaeon symbiosis

Nanohaloarchaeota, a clade of diminutive archaea, with small genomes and limited metabolic capabilities, are ubiquitous in hypersaline habitats, which they share with the extremely halophilic and phylogenetically distant euryarchaea. Some of these nanohaloarchaeota and euryarchaea appear to interact with each other. In this study, we investigate the genetic and physiological nature of their relationship. We isolated the nanohaloarchaeon Candidatus Nanohalobium constans LC1Nh and the haloarchaeon Halomicrobium sp. LC1Hm from a solar saltern, reproducibly co-cultivated these species, sequenced their genomes, and characterized their metabolic/trophic interactions. The nanohaloarchaeon is a magnesium-dependent aerotolerant heterotrophic anaerobe of the DPANN superphylum; it lacks respiratory complexes and its energy production relies on fermentative metabolism of sugar derivatives, obtained by depolymerizing alpha-glucans or by acquiring the chitin monomer N-acetylglucosamine from the chitinolytic haloarchaeal host. Halomicrobium is a member of the class Halobacteria and a chitinotrophic aerobe. The nanohaloarchaeon lacks key biosynthetic pathways and is likely to be provided with amino acids, lipids, nucleotides and cofactors via physical contact with its host Halomicrobium . In turn, the ability of Ca . Nanohalobium to hydrolyse alpha-glucans boosts the host’s growth in the absence of a chitin substrate. These findings suggest that at least some members of the nanohaloarchaea, previously considered ecologically unimportant given their limited metabolic potential, in fact may play significant roles in the microbial carbon turnover, food chains, and ecosystem function. The behaviour of Halomicrobium , which accommodates the colonization by Ca . Nanohalobium, can be interpreted as a bet-hedging strategy, maximizing its long-term fitness in a habitat where the availability of carbon substrates can vary both spatially and temporarily.