Functional diversity of microboring Ostreobium algae isolated from corals

Bacterial Microbiota of Ostreobium, the Coral-Isolated Chlorophyte Ectosymbiont, at Contrasted Salinities

Microscopic filaments of the siphonous green algae Ostreobium (Ulvophyceae, Bryopsidales) colonize and dissolve the calcium carbonate skeletons of coral colonies in reefs of contrasted salinities. Here, we analyzed their bacterial community's composition and plasticity in response to salinity. Multiple cultures of Pocillopora coral-isolated Ostreobium strains from two distinct rbcL lineages representative of IndoPacific environmental phylotypes were pre-acclimatized (>9 months) to three ecologically relevant reef salinities: 32.9, 35.1, and 40.2 psu. Bacterial phylotypes were visualized for the first time at filament scale by CARD-FISH in algal tissue sections, within siphons, at their surface or in their mucilage. Ostreobium-associated microbiota, characterized by bacterial 16S rDNA metabarcoding of cultured thalli and their corresponding supernatants, were structured by host genotype (Ostreobium strain lineage), with dominant Kiloniellaceae or Rhodospirillaceae (Alphaproteobacteria, Rhodospirillales) depending on Ostreobium lineage, and shifted Rhizobiales' abundances in response to the salinity increase. A small core microbiota composed of seven ASVs (~1.5% of thalli ASVs, 19-36% cumulated proportions) was persistent across three salinities in both genotypes, with putative intracellular Amoebophilaceae and Rickettsiales_AB1, as well as Hyphomonadaceae and Rhodospirillaceae also detected within environmental (Ostreobium-colonized) Pocillopora coral skeletons. This novel knowledge on the taxonomic diversity of Ostreobium bacteria paves the way to functional interaction studies within the coral holobiont.

The trace metal economy of the coral holobiont: supplies, demands and exchanges

The juxtaposition of highly productive coral reef ecosystems in oligotrophic waters has spurred substantial interest and progress in our understanding of macronutrient uptake, exchange, and recycling among coral holobiont partners (host coral, dinoflagellate endosymbiont, endolithic algae, fungi, viruses, bacterial communities). By contrast, the contribution of trace metals to the physiological performance of the coral holobiont and, in turn, the functional ecology of reef-building corals remains unclear. The coral holobiont's trace metal economy is a network of supply, demand, and exchanges upheld by cross-kingdom symbiotic partnerships. Each partner has unique trace metal requirements that are central to their biochemical functions and the metabolic stability of the holobiont. Organismal homeostasis and the exchanges among partners determine the ability of the coral holobiont to adjust to fluctuating trace metal supplies in heterogeneous reef environments. This review details the requirements for trace metals in core biological processes and describes how metal exchanges among holobiont partners are key to sustaining complex nutritional symbioses in oligotrophic environments. Specifically, we discuss how trace metals contribute to partner compatibility, ability to cope with stress, and thereby to organismal fitness and distribution. Beyond holobiont trace metal cycling, we outline how the dynamic nature of the availability of environmental trace metal supplies can be influenced by a variability of abiotic factors (e.g. temperature, light, pH, etc.). Climate change will have profound consequences on the availability of trace metals and further intensify the myriad stressors that influence coral survival. Lastly, we suggest future research directions necessary for understanding the impacts of trace metals on the coral holobiont symbioses spanning subcellular to organismal levels, which will inform nutrient cycling in coral ecosystems more broadly. Collectively, this cross-scale elucidation of the role of trace metals for the coral holobiont will allow us to improve forecasts of future coral reef function.

Every refuge has its price: Ostreobium as a model for understanding how algae can live in rock and stay in business

Ostreobium is a siphonous green alga in the Bryopsidales (Chlorophyta) that burrows into calcium carbonate (CaCO₃) substrates. In this habitat, it lives under environmental conditions unusual for an alga (i.e., low light and low oxygen) and it is a major agent of carbonate reef bioerosion. In coral skeletons, Ostreobium can form conspicuous green bands recognizable by the naked eye and it is thought to contribute to the coral's nutritional needs. With coral reefs in global decline, there is a renewed focus on understanding Ostreobium biology and its roles in the coral holobiont. This review summarizes knowledge on Ostreobium's morphological structure, biodiversity and evolution, photosynthesis, mechanism of bioerosion and its role as a member of the coral holobiont. We discuss the resources available to study Ostreobium biology, lay out some of the uncharted territories in Ostreobium biology and offer perspectives for future research.

The role of the endolithic alga Ostreobium spp. during coral bleaching recovery

In this study, we explore how the Caribbean coral Orbicella faveolata recovers after bleaching, using fragments from 13 coral colonies exposed to heat stress (32 °C) for ten days. Biological parameters and coral optical properties were monitored during and after the stress. Increases in both, the excitation pressure over photosystem II (Qm) and pigment specific absorption (a*_Chla) were observed in the stressed corals, associated with reductions in light absorption at the chlorophyll a red peak (D_e675) and symbiont population density. All coral fragments exposed to heat stress bleached but a fraction of the stressed corals recovered after removing the stress, as indicated by the reductions in Q_m and increases in D_e675 and the symbiont population observed. This subsample of the experimentally bleached corals also showed blooms of the endolithic algae Ostreobium spp. underneath the tissue. Using a numerical model, we quantified the amount of incident light reflected by the coral, and absorbed by the different pigmented components: symbionts, host-tissue and Ostreobium spp. Our study supports the key contribution of Ostreobium spp. blooms near the skeletal surface, to coral recovery after bleaching by reducing skeleton reflectance. Endolithic blooms can thus significantly alleviate the high light stress that affects the remaining symbionts during the stress or when the coral has achieved the bleached phenotype.

Genomic adaptations to an endolithic lifestyle in the coral-associated alga Ostreobium

The green alga Ostreobium is an important coral holobiont member, playing key roles in skeletal decalcification and providing photosynthate to bleached corals that have lost their dinoflagellate endosymbionts. Ostreobium lives in the coral's skeleton, a low-light environment with variable pH and O₂ availability. We present the Ostreobium nuclear genome and a metatranscriptomic analysis of healthy and bleached corals to improve our understanding of Ostreobium's adaptations to its extreme environment and its roles as a coral holobiont member. The Ostreobium genome has 10,663 predicted protein-coding genes and shows adaptations for life in low and variable light conditions and other stressors in the endolithic environment. This alga presents a rich repertoire of light-harvesting complex proteins but lacks many genes for photoprotection and photoreceptors. It also has a large arsenal of genes for oxidative stress response. An expansion of extracellular peptidases suggests that Ostreobium may supplement its energy needs by feeding on the organic skeletal matrix, and a diverse set of fermentation pathways allows it to live in the anoxic skeleton at night. Ostreobium depends on other holobiont members for vitamin B12, and our metatranscriptomes identify potential bacterial sources. Metatranscriptomes showed Ostreobium becoming a dominant agent of photosynthesis in bleached corals and provided evidence for variable responses among coral samples and different Ostreobium genotypes. Our work provides a comprehensive understanding of the adaptations of Ostreobium to its extreme environment and an important genomic resource to improve our comprehension of coral holobiont resilience, bleaching, and recovery.