Coral-Associated Prosthecochloris (CAP): The Reef's Most Specialized Green Sulfur Bacteria?

For decades, the green layer buried deep within the skeletons of stony corals was believed to consist almost entirely of the filamentous alga Ostreobium. While Ostreobium remains a dominant inhabitant of this hidden ecosystem, advances in metagenomics and comparative genomics have revealed another major player: Members of the green sulfur bacterial genus Prosthecochloris. These bacteria occupy one of the most unusual microbial habitats in nature—the microscopic pores and channels inside living coral skeletons.

Recent genomic work has shown that not all Prosthecochloris marina are alike. Instead, the species appears to contain two distinct ecological lifestyles. One consists of classical free-living marine strains inhabiting anoxic sediments. The other comprises a highly specialized evolutionary lineage now known as Coral-Associated Prosthecochloris (CAP), which has acquired numerous genetic adaptations enabling life within the dynamic environment of coral skeletons.

This distinction has important implications not only for understanding coral ecology but also for identifying bacterial strains that may participate in nutrient cycling within reef ecosystems. Recent metagenomic sequencing performed on samples of Hydrospace LLC's PNS Deep Cycle™ consortium recovered a nearly complete genome identified as Prosthecochloris marina. Comparison of that genome with published CAP lineages demonstrates that the Hydrospace isolate belongs within the coral-associated evolutionary clade rather than representing a conventional free-living sediment strain.

A Remarkably Specialized Green Sulfur Bacterium

Prosthecochloris marina belongs to the family Prosthecochloridaceae within the phylum Chlorobi (Green Sulfur Bacteria, GSB). Like other GSB, it performs anoxygenic photosynthesis, using reduced sulfur compounds—including hydrogen sulfide—instead of water as an electron donor.

Unlike cyanobacteria, green sulfur bacteria do not produce oxygen. Instead, they harvest extremely low levels of light using enormous chlorosomes, among the most efficient light-harvesting structures known in biology. This adaptation allows P. marina to thrive in habitats where light barely penetrates but sulfide remains available.

Carbon fixation occurs through the reverse tricarboxylic acid (rTCA) cycle, one of Earth's oldest carbon fixation pathways, while sulfur oxidation produces elemental sulfur that may later be further oxidized to sulfate. Many strains also possess nitrogen fixation genes, allowing atmospheric nitrogen to be converted into biologically useful compounds. These traits make P. marina exceptionally well suited for life in oxygen-poor marine sediments.

Discovery of Coral-Associated Prosthecochloris

Beginning around 2020, researchers studying coral microbiomes noticed that healthy coral skeletons frequently contained abundant green sulfur bacterial DNA. Rather than representing random sediment contaminants, these organisms formed a distinct phylogenetic cluster separate from environmental Prosthecochloris isolates. This lineage became known as Coral-Associated Prosthecochloris (CAP).

High-quality genomes recovered from coral skeleton enrichments—including strains SCSIO W1101, W1102 and W1103—demonstrated that CAP forms a monophyletic evolutionary lineage distinct from classical marine sediment strains. Importantly, these organisms were isolated from the skeletons of living corals such as Galaxea fascicularis, where they inhabit microscopic endolithic cavities beneath coral tissue. The discovery fundamentally changed scientists' understanding of coral microbiology. Coral skeletons are now recognized as biologically active microbial habitats rather than inert limestone.

Why Coral Skeletons Are Such Challenging Habitats

Living coral skeletons experience enormous environmental fluctuations every day. During daylight, photosynthesis by Ostreobium and the coral's symbiotic dinoflagellates produces large quantities of oxygen. At night, respiration dominates and oxygen rapidly declines, often producing strongly reducing conditions.

The result is a habitat characterized by:

• extremely low light availability

• rapidly changing redox chemistry

• localized sulfide production

• fluctuating pH

• steep oxygen gradients

• intense oxidative stress during daylight hours

For strict anaerobes, surviving these oscillations requires specialized protective mechanisms not typically found in free-living green sulfur bacteria. Comparative genomics has shown that CAP genomes possess precisely these adaptations.

CAP Versus Non-CAP: Two Ecological Strategies Within One Species

One of the most fascinating discoveries of recent comparative genomic studies is that Prosthecochloris marina includes both free-living and coral-associated strains. The original type strain, P. marina V1, was isolated from coastal marine sediments and represents the classical ecological lifestyle expected for green sulfur bacteria. It is an obligate anaerobic photoautotroph specialized for permanently anoxic marine muds.

Tang et al. 2020

By contrast, CAP strains—including SCSIO W1101 and related genomes—retain the core machinery of green sulfur bacteria while possessing additional genetic systems apparently acquired during adaptation to coral skeletons. Comparative genome analyses have identified several recurring differences.

Enhanced Oxidative Stress Protection

Perhaps the most significant distinction involves coping with oxygen.

Although CAP organisms remain fundamentally anaerobic phototrophs, their genomes contain expanded systems involved in reactive oxygen species (ROS) detoxification and oxidative stress response. These include enzymes capable of detoxifying superoxide radicals and peroxides that periodically accumulate inside illuminated coral skeletons. These systems likely allow CAP to tolerate transient oxygen exposure rather than grow aerobically.

Expanded Respiratory Capacity

Several CAP genomes also encode high-affinity respiratory components—including cbb3-type cytochrome c oxidases—that may enable survival under microoxic conditions where oxygen concentrations are extremely low but not entirely absent. Researchers speculate that these systems permit CAP cells to persist through daily oxygen fluctuations rather than functioning as fully aerobic organisms.

Horizontal Gene Transfer

Comparative analyses reveal numerous genomic islands within CAP genomes that appear to have been acquired through horizontal gene transfer. Many of these genes involve environmental sensing, oxidative stress management, membrane transport, extracellular interactions, and metabolic flexibility. These acquisitions likely reflect long-term adaptation to the complex microbial community inside coral skeletons rather than simple evolution within isolated sediment environments.

Extracellular Polymeric Substance (EPS) Production

CAP genomes frequently contain expanded machinery for extracellular polymeric substance synthesis. EPS production likely helps stabilize microbial biofilms inside the porous coral skeleton while buffering cells against rapid environmental fluctuations. These biofilms may also create localized microenvironments that maintain favorable redox conditions for sulfur cycling.

Why Does P. marina Include Both CAP and Non-CAP Strains?

The existence of two very different ecological lifestyles within one species initially appears surprising. However, bacterial species often encompass multiple ecotypes that remain genetically similar while occupying distinct ecological niches. Comparative genomic studies suggest that CAP strains diverged from free-living ancestors relatively recently in evolutionary time. Rather than representing a separate species, CAP appears to constitute a specialized lineage that acquired adaptive genes through horizontal gene transfer while retaining the core metabolic identity of P. marina.

Nie et al. 2023

Average Nucleotide Identity (ANI) values remain sufficiently high for these organisms to be classified within the same species, despite substantial ecological divergence. In essence, free-living P. marina occupies marine sediments, whereas CAP has become highly specialized for the endolithic habitat inside living coral skeletons.

What CAP May Be Doing Inside Coral Skeletons

Although many details remain under investigation, several ecological roles have gained substantial support.

Sulfur Cycling

Green sulfur bacteria oxidize hydrogen sulfide while producing elemental sulfur and sulfate.

Meanwhile, neighboring sulfate-reducing bacteria regenerate sulfide from sulfate.

Together these organisms form a tightly coupled sulfur cycle that continually recycles sulfur compounds within the skeleton.

Carbon Fixation

Using the reverse TCA cycle, CAP fixes inorganic carbon into organic biomass. Some of this fixed carbon may ultimately become available to neighboring microorganisms through excretion, cell turnover or grazing.

Nitrogen Fixation

Several CAP genomes encode nitrogenase genes. Whether nitrogen fixed by CAP significantly contributes to coral nutrition remains uncertain, but diazotrophy could enrich nutrient-poor skeletal microhabitats.

Biofilm Stabilization

EPS production probably helps organize the complex microbial architecture inside coral pores. These biofilms may stabilize environmental gradients that support numerous interacting microorganisms.

CAP and SRB: A Potential Syntrophic Partnership

Perhaps the strongest ecological hypothesis emerging from recent genomic work involves syntrophy between CAP and sulfate-reducing bacteria (SRB). Researchers recovered CAP genomes together with previously undescribed sulfate-reducing bacterial genomes from the same coral skeleton enrichments. Metabolic reconstruction suggests an elegant reciprocal relationship.

Chen et al. 2019

Sulfate reducers generate sulfide. CAP consumes sulfide during photosynthesis. CAP subsequently oxidizes sulfur compounds back toward sulfate. The sulfate reducers then regenerate sulfide, completing a tightly integrated sulfur cycle. Although this metabolic partnership remains incompletely verified experimentally, genomic evidence strongly supports the existence of such syntrophic interactions.

What Scientists Still Do Not Know

Despite rapid advances, important questions remain unanswered. Researchers have yet to demonstrate experimentally that CAP directly and significantly promote coral health (i.e., are not merely exploiting a favorable habitat created by the coral). Similarly, the extent to which CAP supplies fixed nitrogen or organic carbon to coral hosts remains uncertain. It also remains unknown whether CAP actively protects corals against disease, contributes to skeletal stability, or simply participates in microbial nutrient cycling without significantly affecting the host. These questions remain active areas of investigation.

Genomic Analysis of the P. marina Strain in PNS Deep Cycle™

To better characterize the microbial consortium present in PNS Deep Cycle™, Hydrospace LLC submitted samples to Microbial Marine for high-depth metagenomic sequencing followed by genome assembly, binning, and phylogenomic classification.

The resulting P. marina genome exhibited exceptionally high quality:

• Genome completeness: 99.99%

• Estimated contamination: 1.02%

• Genome size: approximately 2.87 Mb

• Predicted genes: 2,727

• GTDB-Tk classification: Prosthecochloris marina

• Average genomic identity: 98.97% relative to reference genomes

These quality metrics provide high confidence that the recovered genome accurately represents a single bacterial population rather than an assembly artifact. More importantly, functional annotation revealed numerous characteristics consistent with published CAP genomes.

The genome retained the expected core features of green sulfur bacteria, including anoxygenic photosynthesis, reverse TCA carbon fixation, sulfur oxidation pathways, and nitrogen fixation genes. However, it also encoded several features associated with coral-associated lineages, including robust oxidative stress response systems, pathways involved in oxidative pentose phosphate metabolism, extracellular polymeric substance biosynthesis, and expanded protein degradation capabilities.

The genome also contained genes involved in biofilm formation, chitin degradation, cellulose degradation, and other complex carbohydrate processing. Collectively, these pathways suggest an organism capable of participating in both primary production and organic matter recycling within structured microbial communities. Notably, the largest annotated functional category involved protein degradation, comprising twenty-one predicted genes. Such an expanded proteolytic repertoire is uncommon among classical obligately autotrophic green sulfur bacteria and is consistent with a bacterium adapted to nutrient recycling within dense microbial biofilms.

To further belabor these points, the taxonomy tool matched PNS Deep Cycle to Prosthecochloris marina at 98.97% Average Nucleotide Identity (ANI) or 16S similarity--but did not match with any previously described strain. In comparative genomics of endolithic ecosystems, P. marina is the exact species that splits into two radically different lifeways:

1. The Free-Living Type Strain (P. marina V1): Isolated from coastal marine mud. It is a strict, obligate anaerobe that lacks the capacity to process or tolerate oxygen.

2. The CAP Clade Strains (e.g., P. marina SCSIO W1101, W1102): Enriched directly from stony coral skeletons.

Because this strain's functional profile explicitly contains both the strict anaerobic tools (sulfide/sulfur oxidation) and the specialized survival adaptations (oxidative pentose phosphate pathway and robust ROS scavenging tools), it is a textbook CAP variant. A free-living mud-dwelling P. marina simply cannot harbor those aerobic pathways without destroying its own metabolic paradigm.

Because published comparative genomic studies have shown that coral-associated P. marina lineages possess expanded accessory genomes supporting life within coral skeletons, the close phylogenomic affinity of the Hydrospace isolate to these lineages, together with its metabolic profile, very strongly supports classification of this strain within the CAP evolutionary clade. Although definitive ecological behavior can only be demonstrated experimentally, the genomic evidence places this isolate squarely among the highly specialized coral-associated variants described in recent literature.

Conclusion

The discovery of Coral-Associated Prosthecochloris has transformed our understanding of reef microbiology. What was once believed to be a simple sediment bacterium has emerged as a highly specialized inhabitant of one of nature's most dynamic microbial ecosystems. Comparative genomics demonstrates that Prosthecochloris marina encompasses at least two ecological strategies: Classical free-living sediment strains and coral-associated lineages that possess expanded genetic toolkits for surviving fluctuating oxygen conditions, participating in sulfur cycling, and interacting with complex endolithic microbial communities.

The Prosthecochloris marina strain maintained by Hydrospace LLC was originally isolated from anaerobic sandy sediments collected immediately adjacent to a healthy Porites sp. bommie. While the isolate was not recovered directly from coral skeleton, its origin is entirely consistent with its genomic classification as a Coral-Associated Prosthecochloris (CAP). Coral skeletons continuously exchange microorganisms with the surrounding benthic environment through bioerosion, skeletal erosion, mucus shedding, groundwater flow, and sediment resuspension. Consequently, sediments immediately surrounding healthy coral colonies are expected to serve as reservoirs for members of the coral microbiome, including endolithic microorganisms.

Interestingly, this isolation history also illustrates why Prosthecochloris marina contains both CAP and non-CAP lineages. Although CAP strains are genomically specialized for life within coral skeletons, the close ecological connectivity between coral skeletons and adjacent reef sediments likely provides opportunities for these organisms to disperse between habitats. Whether CAP populations actively cycle between sediments and coral skeletons or simply persist transiently in nearby sediments remains an open question. Nevertheless, recovering a genomically verified CAP strain from sediments immediately adjacent to a healthy Porites colony fits well with current models of microbial exchange within the coral holobiont and the surrounding reef ecosystem.

Can we—or should we—assume that this 'origin story' proves our strain is a true Coral-Associated Prosthecochloris (CAP)? No. While the proximity of the collection site to a healthy Porites colony is certainly intriguing, isolation source alone cannot establish ecological identity. Rather, it is the combination of high-quality phylogenomic classification, comparative genomic analysis, and the presence of hallmark CAP-associated adaptations that supports placement of our isolate within the CAP lineage. Its collection from anaerobic sediments immediately adjacent to a healthy coral colony is therefore best viewed not as proof, but as an ecological observation that is entirely consistent with what is currently understood about microbial exchange between coral skeletons and the surrounding reef environment.

It is within this context that Hydrospace LLC partnered with Microbial Marine to perform high-resolution metagenomic sequencing and comparative phylogenomic analysis of our strain. Rather than serving as the sole basis for our conclusions, these genomic data have substantially strengthened earlier ecological observations and informed hypotheses by providing multiple independent lines of evidence that our strain belongs within the Coral-Associated Prosthecochloris (CAP) lineage. In turn, this placement suggests that the organism very likely possesses the specialized adaptations characteristic of CAP strains, making it a promising candidate for further investigation into its potential roles within the coral holobiont and its possible application in supporting the health and resilience of both wild and captive stony corals.

The high-quality P. marina genome recovered from PNS Deep Cycle™ closely matches this emerging picture, exhibiting both the conserved physiology expected of green sulfur bacteria and the accessory genomic features characteristic of coral-associated lineages. While many aspects of CAP ecology remain under investigation, growing genomic evidence indicates that these bacteria represent integral members of the coral holobiont. As research continues, CAP organisms almost certainly will become increasingly important models for understanding microbial adaptation, coral resilience, and the hidden biogeochemical processes occurring beneath the surface of living reefs.

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