Gruber-Vodicka, Harald R.

Abstract Microbes perform critical functions in corals, yet most knowledge is derived from the photic zone. Here, we discover two mollicutes that dominate the microbiome of the deep-sea octocoral, Callogorgia delta, and likely reside in the mesoglea. These symbionts are abundant across the host’s range, absent in the water, and appear to be rare in sediments. Unlike other mollicutes, they lack all known fermentative capabilities, including glycolysis, and can only generate energy from arginine provided by the coral host. Their genomes feature several mechanisms to interact with foreign DNA, including extensive CRISPR arrays and restriction-modification systems, which may indicate their role in symbiosis. We propose the novel family Oceanoplasmataceae which includes these symbionts and others associated with five marine invertebrate phyla. Its exceptionally broad host range suggests that the diversity of this enigmatic family remains largely undiscovered. Oceanoplasmataceae genomes are the most highly reduced among mollicutes, providing new insight into their reductive evolution and the roles of coral symbionts.

AbstractIn addition to abundant animal communities, corals from all ocean depths support diverse microbial associates that are important to coral health. While some of these microbes have been classified taxonomically, understanding the metabolic potential of coral-associated bacteria and how they interact with their coral hosts is limited by a lack of genomic data. One example is Mycoplasma and other members of the class Mollicutes which are widespread coral associates. Here we investigated the association between two novel members of the class Mollicutes and the deep-sea octocoral Callogorgia delta. We screened C. delta, a closely related species C. americana, sediment, and water for mollicutes using 16S metabarcoding. One ASV was found in most colonies screened (99/108) and often dominated the microbiome (up to 99%). Another ASV was detected at lower abundance and prevalence in these corals. Both were absent in all water and were absent or rare in the sediment. We sequenced metagenomes and metatranscriptomes to assemble and annotate genomes and propose the names Ca. Oceanoplasma callogorgiae and Ca. Thalassoplasma callogorgiae. The genomes were small, revealed a reliance on the arginine dihydrolase pathway for ATP production, and contained CRISPR-Cas systems with extensive arrays. CARD-FISH microscopy unveiled an abundant bacterium in the mesoglea which is likely to be Ca. O. callogorgiae. These novel mollicutes cluster with others from diverse invertebrate hosts. Altogether, this work describes the association of these novel mollicutes in C. delta, provides insight into widespread coral associates, and identifies a novel clade of marine mollicutes whose diversity remains largely undiscovered.

AbstractPlacozoa is an enigmatic phylum of simple, microscopic, marine metazoans1,2. Although intracellular bacteria have been found in all members of this phylum, almost nothing is known about their identity, location and interactions with their host3–6. We used metagenomic and metatranscriptomic sequencing of single host individuals, plus metaproteomic and imaging analyses, to show that the placozoan Trichoplax sp. H2 lives in symbiosis with two intracellular bacteria. One symbiont forms an undescribed genus in the Midichloriaceae (Rickettsiales)7,8 and has a genomic repertoire similar to that of rickettsial parasites9,10, but does not seem to express key genes for energy parasitism. Correlative image analyses and three-dimensional electron tomography revealed that this symbiont resides in the rough endoplasmic reticulum of its host’s internal fibre cells. The second symbiont belongs to the Margulisbacteria, a phylum without cultured representatives and not known to form intracellular associations11–13. This symbiont lives in the ventral epithelial cells of Trichoplax, probably metabolizes algal lipids digested by its host and has the capacity to supplement the placozoan’s nutrition. Our study shows that one of the simplest animals has evolved highly specific and intimate associations with symbiotic, intracellular bacteria and highlights that symbioses can provide access to otherwise elusive microbial dark matter.