The research, led by Stephanie G. Gardner of the University of Sydney together with Matthew R. Nitschke of the Australian Institute of Marine Science, Raphael F. Burkart-Radtke, the late Emma L. Johnston, and Graeme F. Clark, was conducted at One Tree Island, a research station perched on the southern Great Barrier Reef. The team worked under permit from the Great Barrier Reef Marine Park Authority and acknowledged the Bailai, Gurang, Gooreng Gooreng and Taribelang Bunda Peoples as Traditional Custodians of the Sea Country where the fieldwork took place. Their study, published open access on 9 October 2026, is titled Scale-Dependent Structuring of Sediment Bacterial Communities on Coral Reefs.

The methodological core of the study was 16S rRNA gene metabarcoding, a technique that reads short, diagnostic regions of the bacterial genome to inventory which taxa are present in an environmental sample. Rather than sampling a single reef in isolation, the researchers designed a deliberately multi-scale survey. They characterised sediment bacterial communities across 21 sites spanning the lagoonal habitats surrounding coral patch reefs, capturing variation at the scale of the whole lagoon. At 13 of those sites, they then zoomed in, comparing sediments from different microhabitats within each site to test whether local conditions, such as the immediate neighbourhood of a patch reef, left a detectable signature on the bacterial assemblages.

The environmental variables the team measured were chosen to represent the plausible drivers of microbial distribution. Sediment granulometry, the size distribution of sand and silt particles, shapes pore space, water flow and oxygen penetration, all of which matter to bacteria. Nutrient composition reflects the food available to microbial communities and often varies with proximity to bird colonies, algal patches or water circulation patterns. Distance from the island itself served as a proxy for spatial structure, capturing gradients in water residence time, terrigenous influence and lagoonal circulation that operate at scales far larger than any single patch reef.

The headline finding is stark: sediment bacterial communities were strongly structured by spatial context, and site-level variation exceeded microhabitat effects. In other words, two samples taken hundreds of metres apart in different parts of the lagoon differed more from each other than samples taken centimetres apart in different microhabitats within the same site. The researchers found that environmental gradients, particularly nutrient availability and distance-related spatial structure, contributed meaningfully to the composition of sediment bacterial communities. Microhabitat differences were present, but they were comparatively weak and inconsistent, appearing at some sites and not others rather than forming a reliable, repeatable pattern.

One of the more intriguing results concerns diversity. Overall alpha diversity, the number and evenness of bacterial taxa within individual samples, remained stable across reef zones. The lagoon did not contain microbial hotspots of exceptional local richness. Instead, the compositional differences between sites were driven by shifts in the relative abundance of specific taxa. The same broad cast of bacterial characters was present throughout the lagoon, but their proportions changed from place to place, like a symphony in which the same instruments play different melodies depending on the movement. This pattern suggests that environmental filtering, rather than the presence or absence of species, is the dominant force sculpting these communities.

The study also delivered a clear verdict on the contrast between benthic and pelagic microbial life. Sediment communities were more diverse than the adjacent seawater and contained a substantially higher proportion of habitat-exclusive taxa, species found in the sediment and nowhere else in the sampled system. This points to strong environmental filtering between the sea floor and the water column. The sediment is not simply a passive sink for whatever drifts down from above; it hosts a distinct, self-organised microbial ecosystem with its own specialists, shaped by the physical and chemical realities of life between sand grains.

Why does this matter beyond the lagoon at One Tree Island? Sediment-associated microbiomes play key roles in coral reef biogeochemistry, including the cycling of nitrogen and carbon that ultimately feeds or starves the reef’s larger inhabitants. Yet, as the authors note, the drivers of spatial variability in these tropical sediment communities have remained poorly understood. Many reef microbiome studies rely on limited spatial replication, sampling one or a few sites and extrapolating to the whole reef. The new findings demonstrate that coral reef sediment microbiomes are highly structured across lagoonal scales, which means that under-sampled studies risk mistaking local noise for the true signal, or missing the broader gradients that actually organise the system.

The practical implication for future research is a call for spatially explicit designs. If nutrient availability and distance-related spatial structure are the dominant drivers, then monitoring programmes and experiments need to replicate across the seascape, not just within a single site. This becomes especially urgent as reefs face warming waters, changing nutrient loads and increasing sedimentation from coastal development. The authors frame their results as a valuable baseline for understanding how bacterial community composition may respond to future environmental change. Because alpha diversity is stable while composition shifts, the most sensitive early-warning indicators of environmental stress may be changes in the relative abundance of particular taxa rather than any loss of local richness.

The study also carries a human story. The authors dedicated the research to the late Professor Emma L. Johnston AO, who died in December 2025 and was a co-author on the work, a distinguished Australian ecologist whose career spanned estuarine and marine contamination ecology. The fieldwork was supported logistically by the One Tree Island Research Station and funded through the Australian Research Council’s Securing Antarctica’s Environmental Future programme, with open access funding organised by the Council of Australian University Librarians and its member institutions. Published under a Creative Commons Attribution 4.0 licence, the paper invites other researchers to build on its dataset. For a field racing to understand how reef ecosystems will fare under climate change, knowing where the microbial structure lives, in the map of the lagoon rather than in the shadow of each reef, is a compass correction that could redirect years of future sampling.

Subject of Research: Spatial and environmental drivers of sediment bacterial community structure in coral reef lagoons

Article Title: Scale-Dependent Structuring of Sediment Bacterial Communities on Coral Reefs

Article References: Gardner, S. G., Nitschke, M. R., Burkart-Radtke, R. F., Johnston, E. L., & Clark, G. F. (2026). Scale-Dependent Structuring of Sediment Bacterial Communities on Coral Reefs. Microbial Ecology. https://doi.org/10.1007/s00248-026-02908-x

Image Credits: AI Generated

DOI: 10.1007/s00248-026-02908-x

Keywords: coral reefs, sediment microbiome, 16S rRNA metabarcoding, bacterial communities, One Tree Island, Great Barrier Reef, spatial structure, nutrient availability, sediment granulometry, environmental filtering, microbial ecology, patch reefs

The study, published in the journal Ecology and Evolution, reports the results of a survey tracking coral feeding scars and surviving coral tissue after predation by individual crown-of-thorns starfish. The fieldwork was carried out at the University of Sydney’s One Tree Island Research Station on the southern Great Barrier Reef, in protected areas where healthy populations of fish and other predators keep the starfish at naturally low densities. In these balanced conditions, the researchers found something surprising: living coral tissue remained after almost 75 percent of the starfish feeding events they examined.

Dr Shawna Foo, a University of Sydney Horizon Fellow in the School of Life and Environmental Sciences who led the research, explained that the starfish at One Tree Reef behave very differently from the destructive swarms that have plagued other parts of the Great Barrier Reef. Rather than demolishing an entire coral colony in a single assault, starfish at low density appear to consume a relatively consistent amount of coral during each meal before moving on to another colony. This grazing pattern frequently leaves live coral tissue remnants behind, giving the corals a chance to recover.

That distinction matters enormously for how scientists and reef managers understand the species. Crown-of-thorns starfish are native to the Great Barrier Reef, where they have long been part of the ecological fabric. Since the 1960s, however, the reef has experienced four major outbreaks, erupting at intervals of approximately 15 years, each capable of causing mass coral destruction. The new findings do not diminish the significance of these devastating events, which coral populations struggle to recover from. Instead, they highlight an overlooked upside of low-density starfish populations functioning within a healthy marine ecosystem.

The mechanism behind the starfish’s constructive role lies in the competitive hierarchy of corals themselves. On many reefs, fast-growing species in the family Acroporidae, which includes the familiar branching and table corals of the genus Acropora, can rapidly colonize available space and shade out slower competitors. The study found that remnants of these fast-growing Acroporidae corals did not regenerate over the feeding scars left by starfish. Regeneration occurred mostly among much slower-growing Montipora and massive Porites corals, which gradually reclaimed the damaged areas. By preferentially trimming back the dominant, fast-growing species, low-density starfish populations open space for slower-growing corals to persist.

In effect, the starfish act as reluctant gardeners, pruning the most aggressive competitors and preventing any single coral species from monopolizing the reef. This increases the structural and taxonomic diversity of the coral community, a quality widely associated with healthier, more resilient reef ecosystems. Foo noted that by feeding on fast-growing corals such as Acropora, low-density populations create space for slower-growing species, helping to increase overall coral diversity across the reef.

The results align closely with a foundational concept in ecology known as the intermediate disturbance hypothesis. Professor Maria Byrne, a co-author of the study, said the findings are consistent with this important framework in ecology and conservation. The hypothesis proposes that occasional, moderate disturbance within an ecosystem can actually help maintain biodiversity, because it prevents dominant species from excluding others while stopping short of the catastrophic damage that eliminates species altogether. Crown-of-thorns starfish feeding at low densities appear to fit this pattern precisely, applying a measured pressure that keeps coral competition in balance without pushing the system toward collapse.

Understanding when starfish shift from ecological regulators to reef destroyers is a central question for conservation. The balance appears to hinge on predation. Diminishing populations of crown-of-thorns predators, including the giant triton snail and multiple species of coral reef fishes, heighten the vulnerability of the Great Barrier Reef and marine environments worldwide to outbreaks. Where those predators thrive, as in the protected zones surveyed at One Tree Reef, starfish numbers remain low and their feeding becomes a modest, even beneficial, component of reef dynamics. Where predator populations have collapsed, starfish can multiply into swarms that consume coral faster than it can regrow.

The research also carries practical implications for how managers respond to outbreaks. Current control efforts rely heavily on culling programs in which divers inject starfish to remove them from affected reefs. Foo emphasized that the new findings can help determine when and why culling should occur, and what the ecological endpoint of a cull should be. Once an outbreak has been suppressed, she argued, the goal should not necessarily be to eliminate crown-of-thorns starfish entirely. Instead, culling needs to be complemented by strong marine management that protects the starfish’s natural predators and keeps populations in check, allowing the species to resume its natural role rather than tipping back toward destructive abundance.

The study, an observational survey of coral feeding scars conducted on the Great Barrier Reef, reframes one of the ocean’s most feared invertebrates as a species whose impact depends entirely on context. In a functioning ecosystem with its predators intact, the crown-of-thorns starfish is a native coral predator performing a role that has shaped reef communities for millennia. Only when that balance is disrupted does it become the reef-wrecker of popular imagination. For a Great Barrier Reef facing warming waters, bleaching and repeated outbreaks, the message from One Tree Island is a nuanced one: the enemy of corals can, in the right numbers, be part of their salvation, and conservation strategies that restore ecological balance may prove more powerful than eradication alone.

Subject of Research: The ecological role of crown-of-thorns starfish predation in maintaining coral reef biodiversity

Article Title: Crown-of-thorns starfish can help reefs thrive

Article References: Crown-of-thorns starfish can help reefs thrive. (n.d.). Original publication

Image Credits: AI Generated

DOI: Not provided

Keywords: crown-of-thorns starfish, coral reefs, Great Barrier Reef, biodiversity, intermediate disturbance hypothesis, marine ecology, coral predation, One Tree Island, reef management, culling, Acropora, University of Sydney