Tessa M. Page

Scleractinian (stony) corals build reef frameworks through calcium carbonate deposition, yet all methods for assessing skeletal growth - staining, SEM, or ex vivo µCT - are limited to endpoint measurements on dead specimens. Here, we utilise in vivo X-ray computed microtomography (µCT) to non-destructively quantify skeletal growth in the reef-building coral Stylophora pistillata for the first time. Our in vivo µCT approach applied to one individual over 16 days revealed a volume increase from ∼630 to ∼700 mm³ that can be partitioned into external vertical extension and internal lateral thickening, generating high-resolution 4D reconstructions of the evolving skeletal architecture. These measurements were consistent with established approaches but provide additional unique insights into internal growth dynamics. We demonstrate that in vivo μCT enables micron-scale monitoring of calcification processes of living stony corals, thereby representing a powerful new tool to probe coral growth dynamics in the face of rapidly changing environmental conditions.

How organisms respond to increasing temperatures could be attributed to existing thermal tolerances or that certain populations are living well below their thermal limits. To address these ideas, we exposed geographically distinct (1144–2332 km apart) lineages of the dominant reef-building crustose coralline alga, Porolithon cf. onkodes, from the Australian Great Barrier Reef and Lord Howe Island to an increasing temperature (1 °C h⁻¹) experiment, where individual average oxygen production was measured continuously. Molecular phylogenetic analysis revealed the existence of hidden lineages within this alga, but individuals are morpho-anatomically identical. The tropical, low latitude lineage supported the climate variability hypothesis, in which some populations existing in already warmer and more stable thermal environments may be living at or near their thermal thresholds. On average, there was a ~ 92% decrease in O₂ produced after a 1 °C increase in the tropical, low latitude lineage. However, the high latitude lineage did not support this hypothesis, as individuals continuously decreased the amount of O₂ produced with increasing temperature. The central lineage responded uniquely, maintaining a stable level of O₂ for almost 5 °C above their acclimation temperature. Our results indicate that the climate variability hypothesis only partially explains the thermal tolerance in this alga, and we suggest local oceanographic processes, latitudinal effects, and importantly, cryptic speciation influences the responses of different lineages of the critically important reef-building alga P. cf. onkodes to rising temperatures.

The availability and stoichiometry of dissolved nutrients are known to have a significant effect on coral growth, biomineralisation, and stress tolerance. However, previous mechanistic studies have focused primarily on the photosynthetic symbionts. Here we studied the physiological and transcriptomic responses of the coral Stylophora pistillata exposed to four different concentration combinations of dissolved inorganic nitrogen (N) and phosphorus (P) over a period of eight weeks. Despite no significant effects on coral growth or calcification, corals from low P conditions bleached and had significantly fewer symbionts than those from high-phosphate treatments. Low P concentrations induced upregulation of ion transmembrane transporter activity, and downregulation of transcripts involved in phospholipid biosynthesis, protein processing, and protein maturation. Further, all enriched biological processes were related to phosphate metabolism. Our results suggest that S. pistillata controls a variety of molecular pathways to counteract the negative effects of insufficient nutrient supply.

Linguistic uncertainty is a prime source of uncertainty pervading ecology and conservation. Coralline algae are a widespread and diverse group of calcifying red macroalgae that underpin coastal ecosystem function and service provision. Recent increasing interest in coralline algae in the scientific literature has revealed a diverse but confusing terminology at organism to habitat scales. Coralline algal research and conservation are international and multidisciplinary, so there are geographic and disciplinary imbalances in research and conservation efforts. To reach consensus and reduce uncertainty, we propose a unified terminology. We review trends in cultural and scientific use of coralline algal terms and propose a system based on six morphologies: (1) attached, (2) free‐living geniculate, (3) encrusting and free‐living nongeniculate coralline algae, the latter either being (4) nucleated or (5) non‐nucleated thalli or (6) fragments. We take inspiration from other coastal systems that have achieved consensus through umbrella terms, such as ‘coral’ and ‘kelp’, to accelerate global progress in coralline algal research and conservation. We characterise 14 coralline algae–dominated habitat global types, falling within seven functional groups, four biomes and four realms: (1) freshwater coralline streams; (2) coralline tide pools; (3) intertidal coralline rims and (4) turf; (5) coralline sea caves; (6) coral–algal reefs; (7) algal ridges; (8) coralligenous reefs; subtidal (9) carbonate crusts, (10) coralline barrens and (11) turf; and (12) articulith, (13) maerl and (14) rhodolith beds, which fall into the coralline algal bed functional group. We hope this unified terminology promotes data comparison, enables cross‐boundary and cross‐sector sharing of best practices, develops capacity for meta‐analyses and improves conservation strategies.

Crustose coralline algae (CCA) are one of the most important benthic substrate consolidators on coral reefs through their ability to deposit calcium carbonate on an organic matrix in their cell walls. Discrete polysaccharides have been recognized for their role in biomineralization, yet little is known about the carbohydrate composition of organic matrices across CCA taxa and whether they have the capacity to modulate their organic matrix constituents amidst environmental change, particularly the threats of ocean acidification (OA) and warming. We simulated elevated pCO₂ and temperature (IPCC RCP 8.5) and subjected four mid‐shelf Great Barrier Reef species of CCA to 2 months of experimentation. To assess the variability in surficial monosaccharide composition and biomineralization across species and treatments, we determined the monosaccharide composition of the polysaccharides present in the cell walls of surficial algal tissue and quantified calcification. Our results revealed dissimilarity among species' monosaccharide constituents, which suggests that organic matrices are composed of different polysaccharides across CCA taxa. We also observed that species differentially modulate composition in response to ocean acidification and warming. Our findings suggest that both variability in composition and ability to modulate monosaccharide abundance may play a crucial role in surficial biomineralization dynamics under the stress of OA and global warming.

Background: Crustose coralline algae (CCA) are calcifying red macroalgae that play important ecological roles including stabilisation of reef frameworks and provision of settlement cues for a range of marine invertebrates. Previous research into the responses of CCA to ocean warming (OW) and ocean acidification (OA) have found magnitude of effect to be species-specific. Response to OW and OA could be linked to divergent underlying molecular processes across species.Results: Here we showSporolithon durum, a species that exhibits low sensitivity to climate stressors, had little change in metabolic performance and did not significantly alter the expression of any genes when exposed to temperature and pH perturbations. In contrast,Porolithon onkodes, a major coral reef builder, reduced photosynthetic rates and had a labile transcriptomic response with over 400 significantly differentially expressed genes, with differential regulation of genes relating to physiological processes such as carbon acquisition and metabolism. The differential gene expression detected inP. onkodesimplicates possible key metabolic pathways, including the pentose phosphate pathway, in the stress response of this species.Conclusions: We suggestS. durumis more resistant to OW and OA thanP. onkodes, which demonstrated a high sensitivity to climate stressors and may have limited ability for acclimatisation. Understanding changes in gene expression in relation to physiological processes of CCA could help us understand and predict how different species will respond to, and persist in, future ocean conditions predicted for 2100.

Crustose coralline algae (CCA) are a group of calcifying red macroalgae crucial to tropical coral reefs because they form crusts that cement together the reef framework ¹ . Previous research into the responses of CCA to ocean warming (OW) and ocean acidification (OA) have found reductions in calcification rates and survival ^2,3 , with magnitude of effect being species-specific. Responses of CCA to OW and OA could be linked to evolutionary divergence time and/or their underlying molecular biology, the role of either being unknown in CCA. Here we show Sporolithon durum , a species from an earlier diverged lineage that exhibits low sensitivity to climate stressors, had little change in metabolic performance and did not significantly alter the expression of any genes when exposed to temperature and pH perturbations. In contrast, Porolithon onkodes , a species from a recently diverged lineage, reduced photosynthetic rates and had over 400 significantly differentially expressed genes in response to experimental treatments, with differential regulation of genes relating to physiological processes. We suggest earlier diverged CCA may be resistant to OW and OA conditions predicted for 2100, whereas taxa from more recently diverged lineages with demonstrated high sensitivity to climate stressors may have limited ability for acclimatisation.

Crustose coralline algae (CCA) are vital to coral reefs worldwide, providing structural integrity and inducing the settlement of important invertebrate larvae. CCA are known to be impacted by changes in their environment, both during early development and adulthood. However, long-term studies on either life history stage are lacking in the literature, therefore not allowing time to explore the acclimatory or potential adaptive responses of CCA to future global change scenarios. Here, we exposed a widely distributed, slow growing, species of CCA, Sporolithon cf. durum, to elevated temperature and pCO₂ for five months and their first set of offspring (F₁) for eleven weeks. Survival, reproductive output, and metabolic rate were measured in adult S. cf. durum, and survival and growth were measured in the F₁ generation. Adult S. cf. durum experienced 0% mortality across treatments and reduced their O₂ production after five months exposure to global stressors, indicating a possible expression of plasticity. In contrast, the combined stressors of elevated temperature and pCO₂ resulted in 50% higher mortality and 61% lower growth on germlings. On the other hand, under the independent elevated pCO₂ treatment, germling growth was higher than all other treatments. These results show the robustness and plasticity of S. cf. durum adults, indicating the potential for them to acclimate to increased temperature and pCO₂. However, the germlings of this species are highly sensitive to global stressors and this could negatively impact this species in future oceans, and ultimately the structure and stability of coral reefs.