Forget Host-Specificity: Desulfoluna Just Floats to Any Sponge Party
- Ray Sullivan
- Jun 29
- 3 min read
Marine sponges are more than just simple filter-feeders; they are ancient and ecologically important residents of coral reefs found globally. These remarkable animals, along with their diverse microbial communities, form a "holobiont" that plays a crucial role in numerous biogeochemical cycles, including the often-overlooked natural organohalogen cycle. A recent study from Max Häggblom’s Lab, Rutgers Dept. of Biochemistry & Microbiology has unveiled fascinating insights into these complex relationships, particularly regarding bacteria that can break down halogenated compounds.
Sponges are uniquely rich sources of biogenic organohalides, especially brominated compounds. These can include a vast array of bioactive molecules like phenolics, pyrroles, and fatty acids, produced as secondary metabolites by the sponges or their symbionts. Intriguingly, these natural compounds bear structural similarities to troubling human-made pollutants, such as brominated flame retardants. The presence of these brominated metabolites within the sponge environment appears to select for a population of dehalogenating bacteria – microbes that can break down these compounds – which actively reside within the sponge. Desulfoluna spongiiphila, for example, is a known predominant debrominating species found in marine sponges worldwide.
A key question in microbiology is how microbial communities are assembled and maintained within their hosts. For sponges, it's generally understood that their microbiomes can be host-specific, meaning particular bacterial groups are consistently associated with certain sponge species. However, whether this host-specificity extends to the dehalogenating bacteria remained unknown.
To delve into this, Hall, et al. employed nanopore long-read sequencing of nearly full-length ribosomal RNA operons. This advanced technique, developed in Lee Kerkhof’s lab at Rutgers, allows for strain-level resolution, offering a much more detailed view of microbial community composition and identity than previous methods. The study collected eight different sponge species from two sites on Australia's Great Barrier Reef (GBR). They also established anaerobic enrichment cultures from these sponges using brominated compounds as electron acceptors to specifically enrich for dehalogenating bacteria.
The study revealed several key findings:
The research corroborated previous studies, showing that the overall sponge microbiomes in the GBR were indeed host-specific, from the phylum down to the strain level. Bacterial communities within individuals of the same sponge species were more similar to each other than to those in different sponge species.
Reductive debromination activity was consistently observed in anaerobic enrichment cultures established from all GBR sponges (all eight species tested). This highlights that the capability for dehalogenation is broadly distributed among the microbes associated with these sponges.
Despite their importance, Desulfoluna spp. showed no evidence of host-specificity at the strain level. While they were enriched from every sponge analyzed, and their relative abundance often increased in subcultures, phylogenetic analysis revealed a diverse set of Desulfoluna strains that did not cluster based on their host sponge species. This striking finding suggests that these dehalogenating bacteria are likely horizontally transferred or acquired from the surrounding seawater rather than being vertically transmitted from parent sponges.
In stark contrast to Desulfoluna, other significant bacterial symbionts, such as Nitrospira spp. (critical for nitrogen cycling) and Candidatus Synechococcus spongiarum SH4 (phototropic symbionts), did demonstrate strong host-specificity. Strains of these bacteria detected in the same sponge host were more closely related to each other than to strains found in other hosts. This implies that these specific symbionts are more likely to be vertically transmitted or selectively retained by the host.
These findings significantly advance our understanding of how sponge microbiomes are assembled, illustrating that different members of the microbial community can have distinct modes of transmission and retention within the same host. This nuanced view supports the idea that some bacterial symbionts are selectively maintained (possibly due to their critical contribution to the host's health), while others are acquired from the environment.
Furthermore, the study highlights the potential of sponge-associated dehalogenating bacteria as an untapped resource for marine pollution remediation. Given that organohalide contaminants are ubiquitous, persistent, and pose severe health risks, identifying and characterizing microbes capable of dehalogenation is of great interest for bioremediation applications. The discovery that Desulfoluna species are cosmopolitan and active dehalogenators in sponges, regardless of specific host, opens new avenues for exploring their use in cleaning up contaminated marine environments.
Hall LA, Scott KD, Webster N, Kerkhof LJ, Häggblom MM. Dehalogenating Desulfoluna spp. are ubiquitous in host-specific sponge microbiomes of the Great Barrier Reef. ISME J. 2025 Jan 2;19(1):wraf113. doi: 10.1093/ismejo/wraf113. PMID: 40448582.
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