Filamentous calcareous alga provides substrate for coral-competitive macroalgae in the degraded lagoon of Dongsha Atoll, Taiwan

How the Ecology of Calcified Red Macroalgae is Investigated under a Chemical Approach? A Systematic Review and Bibliometric Study

Characteristics such as calcareous morphology and life cycle are used to understand the ecology of calcified rhodophytes. However, there is limited information regarding their chemical profiles and biological activities. Therefore, a systematic review (PRISMA) was conducted to assess the influence of the chemistry of calcareous rhodophytes on ecological interactions in the marine environment. The keywords used were: ["Chemical AND [Ecology OR Interaction OR Response OR Defense OR Effect OR Cue OR Mediated OR Induce]"] AND ["Red Seaweed" OR "Red Macroalgae" OR Rhodophy?] AND [Calcified OR Calcareous] in Science Direct, Scielo, PUBMED, Springer, Web of Science, and Scopus. Only English articles within the proposed theme were considered. Due to the low number of articles, another search was conducted with three classes and 16 genera. Finally, 67 articles were considered valid. Their titles, abstracts, and keywords were analyzed using IRaMuTeQ through factorial, hierarchical and similarity classification. Most of the studies used macroalgae thallus to evaluate chemical mediation while few tested crude extracts. Some substances were noted as sesquiterpene (6-hydroxy-isololiolide), fatty acid (heptadeca5,8,11-triene) and dibromomethane. The articles were divided into four classes: Herbivory, Competition, Settlement/Metamorphosis, and Epiphytism. Crustose calcareous algae were associated with studies of Settlement/Metamorphosis, while calcified algae were linked to herbivory. Thus, the importance of chemistry in the ecology of these algae is evident,and additional studies are needed to identify the substances responsible for ecological interactions. This study collected essential information on calcified red algae, whose diversity appears to be highly vulnerable to the harmful impacts of ongoing climate change.

Knowledge mapping analysis of the global seaweed research using CiteSpace

Seaweed research has gained substantial momentum in recent years, attracting the attention of researchers, academic institutions, industries, policymakers, and philanthropists to explore its potential applications and benefits. Despite the growing body of literature, there is a paucity of comprehensive scientometric analyses, highlighting the need for an in-depth investigation. In this study, we utilized CiteSpace to examine the global seaweed research landscape through the Web of Science Core Collection database, assessing publication trends, collaboration patterns, network structures, and co-citation analyses across 48,278 original works published since 1975. Our results demonstrate a diverse and active research community, with a multitude of authors and journals contributing to the advancement of seaweed science. Thematic co-citation cluster analysis identified three primary research areas: "Coral reef," "Solar radiation," and "Mycosporine-like amino acid," emphasizing the multidisciplinary nature of seaweed research. The increasing prominence of "Chemical composition" and "Antioxidant" keywords indicates a burgeoning interest in characterizing the nutritional value and health-promoting properties of seaweed. Timeline co-citation analysis unveils that recent research priorities have emerged around the themes of coral reefs, ocean acidification, and antioxidants, underlining the evolving focus and interdisciplinary approach of the field. Moreover, our analysis highlights the potential of seaweed as a functional food product, poised to contribute significantly to addressing global food security and sustainability challenges. This study underscores the importance of bibliometric analysis in elucidating the global seaweed research landscape and emphasizes the need for sustained knowledge exchange and collaboration to drive the field forward. By revealing key findings and emerging trends, our research offers valuable insights for academics and stakeholders, fostering a more profound understanding of seaweed's potential and informing future research endeavors in this promising domain.

Testing sea urchin and green sea turtle consumption of the allelopathic macroalga Galaxaura divaricata

Galaxaura divaricata is a partially calcified macroalga that hampers coral recruitment, growth, and recovery via the excretion of allelopathic secondary metabolites. Herbivorous fishes are not major consumers of Galaxaura spp. and there is a need to understand feeding preferences for Galaxaura divaricata in other macroherbivores, like sea urchins and green sea turtles that could act as potential controlling agents. Under certain environmental conditions, G. divaricata can proliferate and overgrow degraded reefs for several years, as documented for several coral patch reefs in the lagoon of Dongsha Atoll, South China Sea. This study aimed to experimentally test the feeding preferences of five species of sea urchin and two individual green sea turtles, Chelonia mydas, for G. divaricata. Specifically, we quantified and compared the consumption rates of the allelopathic G. divaricata with Gracilaria edulis, a nonallelopathic, fleshy red alga, known to be highly favored by herbivores. Results showed that the five urchin species fed on both G. edulis and G. divaricata. However, urchins consumed 2-8 times less wet weight of G. divaricata (range 0.3-3.1 g urchin-1 24 h-1) compared to G. edulis (range 0.6-18 g urchin-1 24 h-1), suggesting that urchin grazing may exert some control on G. divaricata abundance but is likely ineffective for a large-scale removal of the alga. Further, both green sea turtles avoided G. divaricata and selectively fed on G. edulis. More experiments are needed to test the potential role of herbivores in controlling the overgrowth of coral competitive and allelopathic macroalgae, like Galaxaura on coral reefs.

Zonal macroalgae blooms influenced by different aquaculture discharges in the Xuwen fringing reef, southern China

Reef-building corals are experiencing major impacts from climate change and increasing anthropogenic activities worldwide. Coral reef degradation is associated with the loss of coral species abundance and diversity, followed by an ecological imbalance that commonly occurs with a shift from coral- to macroalgae-dominated community states. The species composition of the bloom-forming macroalgae in the Xuwen National Coral Reef Reserve, southern China, and the associated drivers affecting the composition remain unclear. We investigated the relationship between zonal macroalgae blooms and different aquaculture discharges in this coral reef in March 2016. The structure of macroalgae communities varied greatly among the three study sites, with green and brown algae dominating the high-level pond aquaculture discharge zone; red, green and brown algae dominating the tidal flat aquaculture discharge zone; and green and brown algae dominating in the enclosure aquaculture discharge zone. A significant amount of variation in the structure and composition of the macroalgae community was explained by temperature, total suspended solids (TSS), chlorophyll a (Chla) and dissolved inorganic nutrients (DIN), while temperature and TSS had a significant relationship with these variables. Our findings highlight the negative effects of aquaculture sewage discharges on the health of coral reefs, and we recommend effective and integrated management of anthropogenic sewage discharge into coral reefs.

Calcified macroalgae and their bacterial community in relation to larval settlement and metamorphosis of reef-building coral Pocillopora damicornis

Calcified macroalgae play an important role in the settlement and metamorphosis of invertebrate larvae in coral reef ecosystems. However, little is known about the algal-associated bacterial communities and their effects on larval settlement. In this study, the responses of larvae of the coral Pocillopora damicornis to calcified algae (Porolithon onkodes, Halimeda cylindracea, Halimeda opuntia and Amphiroa fragilissima) were evaluated. The results revealed that Por. onkodes and H. cylindracea significantly enhanced the rates of settlement and metamorphosis, whereas fewer larvae settled on Am. fragilissima and H. opuntia. Amplicon pyrosequencing of the V3-V4 region of 16S rDNA was applied to investigate the relationship between algal bacterial community and larval settlement. Principal coordinates analysis demonstrated that the bacterial community composition of H. opuntia was more similar to that of Am. fragilissima, but clearly distinct from those of H. cylindracea and Por. onkodes. Furthermore, the relative abundances of bacteria were highly diverse among different algae. H. opuntia had higher percentages of Thalassobius, Pelagibius and SM1A02, whereas the abundances of Mycoplasma and Suttonella were significantly higher in H. cylindracea than other algae. Our results showed that larval settlement/metamorphosis was strongly correlated with the bacterial community composition and with the relative abundance of a few operational taxonomic units.