Anaerobic Sulfur Oxidation Underlies Adaptation of a Chemosynthetic Symbiont to Oxic-Anoxic Interfaces

A dynamic epibiont community associated with the bone-eating polychaete genus Osedax

Osedax, the deep-sea annelid found at sunken whalefalls, is known to host Oceanospirillales bacterial endosymbionts intracellularly in specialized roots, which help it feed exclusively on vertebrate bones. Past studies, however, have also made mention of external bacteria on their trunks. During a 14-yr study, we reveal a dynamic, yet persistent, shift of Campylobacterales integrated into the epidermis of Osedax, which change over time as the whale carcass degrades on the sea floor. The Campylobacterales associated with seven species of Osedax, which comprise 67% of the bacterial community on the trunk, appear initially dominated by the genus Arcobacter (at early time points <24 mo), the Sulfurospirillum at intermediate stages (~50 mo), and the Sulfurimonas at later stages (>140 mo) of whale carcass decomposition. Metagenome analysis of the epibiont metabolic capabilities suggests potential for a transition from heterotrophy to autotrophy and differences in their capacity to metabolize oxygen, carbon, nitrogen, and sulfur. Compared to free-living relatives, the Osedax epibiont genomes were enriched in transposable elements, implicating genetic exchange on the host surface, and contained numerous secretions systems with eukaryotic-like protein (ELP) domains, suggesting a long evolutionary history with these enigmatic, yet widely distributed deep-sea worms. IMPORTANCE Symbiotic associations are widespread in nature and we can expect to find them in every type of ecological niche. In the last twenty years, the myriad of functions, interactions and species comprising microbe-host associations has fueled a surge of interest and appreciation for symbiosis. During this 14-year study, we reveal a dynamic population of bacterial epibionts, integrated into the epidermis of 7 species of a deep-sea worm group that feeds exclusively on the remains of marine mammals. The bacterial genomes provide clues of a long evolutionary history with these enigmatic worms. On the host surface, they exchange genes and appear to undergo ecological succession, as the whale carcass habitat degrades over time, similar to what is observed for some free-living communities. These, and other annelid worms are important keystone species for diverse deep-sea environments, yet the role of attached external bacteria in supporting host health has received relatively little attention.

Differential regulation of degradation and immune pathways underlies adaptation of the ectosymbiotic nematode Laxus oneistus to oxic-anoxic interfaces

Eukaryotes may experience oxygen deprivation under both physiological and pathological conditions. Because oxygen shortage leads to a reduction in cellular energy production, all eukaryotes studied so far conserve energy by suppressing their metabolism. However, the molecular physiology of animals that naturally and repeatedly experience anoxia is underexplored. One such animal is the marine nematode Laxus oneistus. It thrives, invariably coated by its sulfur-oxidizing symbiont Candidatus Thiosymbion oneisti, in anoxic sulfidic or hypoxic sand. Here, transcriptomics and proteomics showed that, whether in anoxia or not, L. oneistus mostly expressed genes involved in ubiquitination, energy generation, oxidative stress response, immune response, development, and translation. Importantly, ubiquitination genes were also highly expressed when the nematode was subjected to anoxic sulfidic conditions, together with genes involved in autophagy, detoxification and ribosome biogenesis. We hypothesize that these degradation pathways were induced to recycle damaged cellular components (mitochondria) and misfolded proteins into nutrients. Remarkably, when L. oneistus was subjected to anoxic sulfidic conditions, lectin and mucin genes were also upregulated, potentially to promote the attachment of its thiotrophic symbiont. Furthermore, the nematode appeared to survive oxygen deprivation by using an alternative electron carrier (rhodoquinone) and acceptor (fumarate), to rewire the electron transfer chain. On the other hand, under hypoxia, genes involved in costly processes (e.g., amino acid biosynthesis, development, feeding, mating) were upregulated, together with the worm's Toll-like innate immunity pathway and several immune effectors (e.g., bactericidal/permeability-increasing proteins, fungicides). In conclusion, we hypothesize that, in anoxic sulfidic sand, L. oneistus upregulates degradation processes, rewires the oxidative phosphorylation and reinforces its coat of bacterial sulfur-oxidizers. In upper sand layers, instead, it appears to produce broad-range antimicrobials and to exploit oxygen for biosynthesis and development.

The Longitudinal Dividing Bacterium Candidatus Thiosymbion Oneisti Has a Natural Temperature-Sensitive FtsZ Protein with Low GTPase Activity

FtsZ, the bacterial tubulin-homolog, plays a central role in cell division and polymerizes into a ring-like structure at midcell to coordinate other cell division proteins. The rod-shaped gamma-proteobacterium Candidatus Thiosymbion oneisti has a medial discontinuous ellipsoidal "Z-ring." Ca. T. oneisti FtsZ shows temperature-sensitive characteristics when it is expressed in Escherichia coli, where it localizes at midcell. The overexpression of Ca. T. oneisti FtsZ interferes with cell division and results in filamentous cells. In addition, it forms ring- and barrel-like structures independently of E. coli FtsZ, which suggests that the difference in shape and size of the Ca. T. oneisti FtsZ ring is likely the result of its interaction with Z-ring organizing proteins. Similar to some temperature-sensitive alleles of E. coli FtsZ, Ca. T. oneisti FtsZ has a weak GTPase and does not polymerize in vitro. The temperature sensitivity of Ca. Thiosymbion oneisti FtsZ is likely an adaptation to the preferred temperature of less than 30 °C of its host, the nematode Laxus oneistus.

Host-symbiont stress response to lack-of-sulfide in the giant ciliate mutualism

The mutualism between the thioautotrophic bacterial ectosymbiont Candidatus Thiobius zoothamnicola and the giant ciliate Zoothamnium niveum thrives in a variety of shallow-water marine environments with highly fluctuating sulfide emissions. To persist over time, both partners must reproduce and ensure the transmission of symbionts before the sulfide stops, which enables carbon fixation of the symbiont and nourishment of the host. We experimentally investigated the response of this mutualism to depletion of sulfide. We found that colonies released some initially present but also newly produced macrozooids until death, but in fewer numbers than when exposed to sulfide. The symbionts on the colonies proliferated less without sulfide, and became larger and more rod-shaped than symbionts from freshly collected colonies that were exposed to sulfide and oxygen. The symbiotic monolayer was severely disturbed by growth of other microbes and loss of symbionts. We conclude that the response of both partners to the termination of sulfide emission was remarkably quick. The development and the release of swarmers continued until host died and thus this behavior contributed to the continuation of the association.

FtsZ-mediated fission of a cuboid bacterial symbiont

Less than a handful of cuboid and squared cells have been described in nature, which makes them a rarity. Here, we show how Candidatus Thiosymbion cuboideus, a cube-like gammaproteobacterium, reproduces on the surface of marine free-living nematodes. Immunostaining of symbiont cells with an anti-fimbriae antibody revealed that they are host-polarized, as these appendages exclusively localized at the host-proximal (animal-attached) pole. Moreover, by applying a fluorescently labeled metabolic probe to track new cell wall insertion in vivo, we observed that the host-attached pole started septation before the distal one. Similarly, Ca. T. cuboideus cells immunostained with an anti-FtsZ antibody revealed a proximal-to-distal localization pattern of this tubulin homolog. Although FtsZ has been shown to arrange into squares in synthetically remodeled cuboid cells, here we show that FtsZ may also mediate the division of naturally occurring ones. This implies that, even in natural settings, membrane roundness is not required for FtsZ function.

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Ca. T. cuboideus cells are cuboid

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Septation is host oriented in Ca. T. cuboideus

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FtsZ localization pattern recapitulates that of new PG insertion

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FtsZ polymerizes into either straight or sharp-cornered filaments