Guillermo Diaz-Pulido

Geniculate coralline algae (Corallinales, Rhodophyta) are a diverse group of calcifying macroalgae, loosely united by flexible upright axes formed by alternating calcified (intergenicula) and uncalcified segments (genicula), a trait that has independently arisen several times. To standardize terminology, the concept of growth forms (i.e., the external gross morphology of a specimen) was developed previously for non‐geniculate (without joints) corallines, but no such efforts have been undertaken for geniculate corallines. Here, we adapted the growth‐‐form concept to geniculate corallines. We propose that, based on their branching pattern and two‐ or three‐dimensional branch arrangement, geniculate corallines can be grouped into seven growth forms: unbranched (unbranched uprights), feather‐like (complanate, pinnate branching), fan‐like (complanate, dichotomous branching), irregular (complanate or multiplanar without a clear branching pattern), whorled (multiplanar, verticillate branching), arbuscular (multiplanar, dichotomous branching), and plumose (multiplanar, pinnate branching). We have provided a dichotomous key to assign specimens to different growth forms and discussed examples of each of these growth forms as well as the concept of intergrades between them. Additionally, we have provided a glossary of terms used in the growthform definitions and for intergenicular morphology. These tools will be a helpful resource for ecologists, taxonomists, beachcombers, and anyone interested in these comparatively little‐studied but diverse macroalgae.

The current challenge of defining growth forms in the non‐geniculate coralline red algae was addressed. Since the 19th century, those who have worked on this large and globally distributed group of algae have tried to summarize and systematically describe their growth forms. This effort resulted in a plethora of terms, which could sometimes be confusing, overlapping, and difficult to apply in the field. We reviewed the past literature to provide a coherent key to the non‐geniculate coralline growth forms, incorporating the latest observations. Our revision expands the 10 currently recognized growth forms ( unconsolidated , encrusting , warty , lumpy , fruticose , discoid , layered , foliose , ribbon‐like , and arborescent ) by redefining some of them, by reintroducing the columnar growth form, and by adding two new growth forms— convoluted and imbricate —to include recently documented specimens. This updated and refined tool better encompasses the diverse external morphologies of non‐geniculate corallines. The goal was to facilitate taxonomic descriptions and to support ecological studies of non‐geniculate corallines given the extreme difficulty of identifying species using only external characters.

Crustose coralline algae (CCA) comprise hundreds of different species and are critical to coral reef growth, structural stability and coral recruitment. Despite their integral role in reef functioning, little is known about the diversity and structure of bacterial communities associated with CCA. We address this knowledge gap by characterising the surface microbial communities of 15 Indo‐Pacific CCA species across eight different families from the Great Barrier Reef, using 16S rRNA amplicon sequencing. CCA microbial community composition was distinct and found to primarily differentiate by algal host species. When looking at the core bacterial communities, divergence across CCA microbiomes was additionally correlated to host phylogeny. CCA from similar light environments and depths also had more similar microbial communities, suggesting the potential role of environmental parameters in influencing microbial community organisation. The fundamental descriptions of CCA bacterial communities for a wide range of Indo‐Pacific species presented here provide essential baseline information to further inform CCA microbial symbiosis research.

To address a significant knowledge gap in chemical ecology underpinning larval settlement processes on coral reefs, we examined the tissue-associated metabolomes using nuclear magnetic resonance (NMR) spectroscopy across 14 species of crustose coralline red algae (CCA) and one non-coralline calcareous red alga collected from the Great Barrier Reef, Australia. We further explored the relationship between algal metabolites and the settlement success of fifteen reef-building coral species across five families, as well as a key coral predator, the crown-of-thorns starfish (CoTS; Acanthaster cf. solaris). We found that algal metabolomes are highly variable and differ among species, phylogenetic lineages, and reef habitats, highlighting the combined influence of evolutionary history and environmental context on algal metabolomes. We also identified strong, positive correlations between specific algal metabolites, particularly disaccharides and trisaccharides (e.g., raffinose, maltose), and glycine betaine with high settlement success in both corals and CoTS. This study provides the most comprehensive analysis to date of coralline algal metabolomes and their ecological significance to coral reef ecosystems. These results provide novel chemical, biological and ecological insights that may be used to inform the optimisation of coral aquaculture techniques for reef restoration, as well as potential strategies for controlling CoTS outbreaks to mitigate ongoing reef decline.

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 red seaweed Asparagopsis taxiformis has the potential to reduce methane emissions from livestock, but key challenges remain in developing scalable, ocean-based cultivation protocols using a closed-lifecycle approach. A critical milestone in this effort is developing seeding methodologies and identifying effective substrates for the attachment of gametophyte germlings. This study evaluates the suitability of synthetic and natural twine substrates and a sodium alginate binder for A. taxiformis germling cultivation under laboratory conditions. Polypropylene, nylon, cotton and sisal twines and a smooth polypropylene control surface were seeded with a germling solution, either directly or within a sodium alginate binder. Germling cover (%) was assessed after six weeks of tank-based cultivation and again following unravelling and exposure to moderate-velocity flow to simulate ocean deployment conditions. Polypropylene twine retained the highest germling cover (104% cover post-exposure), while cotton twine (70% cover) showed promise as a biodegradable alternative. Reduced germling cover was observed on the sodium alginate binder treatments across the majority of substrates, with germling loss particularly high (81% reduction in cover) on the smooth surface of the polypropylene control. By demonstrating a scalable seeding methodology and identifying two effective, widely available substrates for cultivation; polypropylene and cotton twine, this study provides a foundation for developing seeded twine for A. taxiformis mariculture. Field trials will be essential to validate this nursery-stage performance at-sea. Ultimately, optimising seeding techniques and cultivation substrates will be key to scaling A. taxiformis mariculture and unlocking its potential for sustainable bioproduct applications.

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.

The global problem of ocean acidification and localised decline in water quality are major threats to coral reefs worldwide. This study examined the individual and interactive impacts of global and local stressors by investigating the effects of increased seawater pCO₂, elevated nutrient concentrations and reduced light levels on linear growth and metabolic rates of the common branching crustose coralline alga Lithophyllum cf. pygmaeum. We found complex interactions between factors on algal growth and photosynthetic rates, but overall, growth was significantly enhanced by pCO₂ enrichment under all light and nutrient combinations. This is the first study to report a positive growth response in coralline algae to elevated pCO₂ using linear extension methods. In contrast, the combination of reduced light levels and high nutrient concentrations simulating poor water quality conditions reduced algal growth rates by up to 67% (compared to individuals exposed to high light, low nutrients and elevated pCO₂). Decreased light levels reduced linear growth, P_gross and P_net rates by 33%, 18% and 24%, respectively, highlighting the critical role of light in coralline algal physiology. We suggest that poor water quality may counteract any CO₂ fertilisation effect under ocean acidification conditions on the growth of coralline algae, and this has implications for coral reef conservation as it emphasises the importance of improving water quality to maintaining coral reef functions. These results further highlight the need for multifactorial experiments to better understand the interplay between global and local processes on coralline algae growth.

The capacity of many macroalgae to rapidly absorb soluble inorganic nutrients and convert them into primary biomass provides opportunities for their use in the bioremediation of nutrient-enriched waters. Marine-based macroalgal cultivation has the potential to target diffuse source nutrient discharges and produce valuable bioproducts such as fertiliser, soil conditioning agents and agricultural feed additives and phycocolloids. Whilst macroalgal diversity offers benefits for improving and maintaining healthy marine ecosystems, it also presents a challenge for determining the best candidate species for cultivation as a nutrient biofilter and, ultimately, a source of bioproducts. We developed and applied a multi-criteria selection model to identify likely candidate macroalgal species for cultivation in Queensland’s coastal waters for biofiltration and bioproduct development, focusing on native species and product development for the agronomical and horticultural sectors. From a species database of 1380 macroalgal species, candidates were initially selected based on their regional abundance, distribution and morphological characteristics considered amenable to in-situ cultivation and harvesting. From a resulting shortlist of 17 species, five systematic literature searches were applied to identify biofilter potential, bioproduct potential and current cultivation status. Of the shortlisted species, Ulva lactuca, Gracilaria edulis and Hypnea cervicornis ranked most highly in the multi-criteria analysis, with several other species showing good potential, but requiring further investment in understanding key biological processes and the development of cultivation protocols.