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Ocean acidification (OA) effects on photophysiology and calcification were examined in Halimeda discoidea, a calcifying macroalga that produces tropical reef sediments. Photosynthetic parameters, including maximum photosynthetic rate (P sub(max)), photosynthetic efficiency ([alpha]) and compensating irradiance (I sub(c)) were determined in short-term assays on live thalli after a 10 d exposure to 4 levels of CO sub(2) partial pressures (p CO sub(2); 491, 653, 982 and 1201 [mu]atm) under saturating (300 [mu]mol photons m super(-2) s super(-1)) and sub-saturating (90 [mu]mol photons m super(-2) s super(-1)) irradiance in an aquaria study. Morphology of aragonite crystals produced in segments formed on adult thalli was characterized using scanning electron microscopy (SEM). Further, we examined crystal morphology and changes in inorganic content of non-living segments exposed to elevated (1201 [mu]atm) and ambient p CO sub(2) for 27 d to assess OA effects on carbonate sediments generated from H. discoidea. Even though P sub(max) was higher under elevated p CO sub(2), this photophysiological response did not result in higher calcification rates. Based on crystal measurements and SEM imagery, aragonite crystals within new segments were indistinguishable across p CO sub(2) and irradiance treatments. Under high irradiance, new segments showed a greater investment in organic versus inorganic production. Non-living segments contained narrower crystals after 27 d exposure to elevated p CO sub(2) relative to controls, but differences were small (0.03 [mu]m) and did not contribute significantly to changes in normalized biomass or inorganic content. Based on these results, H. discoidea will likely produce new calcified segments with intact aragonite crystals under year 2100 p CO sub(2) levels at high and low irradiance, while aragonite crystals of the sediment may produce thinner needle carbonate muds.

Calcareous tropical green macroalgae of the genus Halimeda are key reef-builders, yet the drivers of their diversification and population dynamics remain poorly understood. This study analyzed the species diversity of Halimeda in the Xisha (Paracel) Islands based on tufA gene sequences, focusing on evaluating the genetic diversity, population structure, and historical dynamics of two widespread species—Halimeda discoidea and Halimeda macroloba. The results indicate new records of Halimeda cylindracea and Halimeda cf. stuposa in the Xisha (Paracel) Islands. More importantly, H. discoidea and H. macroloba exhibited significantly different evolutionary histories. Specifically, H. discoidea showed a highly fragmented population structure, restricted gene flow, and a multimodal mismatch distribution, suggesting a complex historical process or long-term stability. In contrast, H. macroloba exhibited lower population differentiation, extensive gene flow, and non-significant neutrality test results, indicating long-term demographic stability without recent, drastic population events. Further validation based on gene flow analysis and divergence time estimation revealed that the lineage divergence of H. discoidea is older, while H. macroloba represents a lineage with a relatively younger evolutionary origin restricted to the Indo-Pacific region. This striking dichotomy clearly illustrates the interplay between intrinsic species-specific traits (e.g., dispersal capacity) and extrinsic historical factors (e.g., paleo-oceanographic events), leading to contrasting evolutionary outcomes among widespread marine taxa. By elucidating how differing evolutionary histories influence patterns of genetic diversity, this study provides a predictive framework for evaluating the resilience and guiding conservation priorities for critical marine calcifiers in the context of rapid environmental change.

The human population growth has been followed by an infection rate increase of pathogenic bacteria. The continual use of synthetic antibacterial chemicals has negative effects, pathogenic bacterial resistance of high doses. Therefore, the discovery of a new antibacterial agent is an urgent need. The search for new drug agents as antibacteria has expanded to marine organisms, especially seaweeds. This research aimed to measure the clear zone of seaweeds against bacterial tests and to evaluate the ability of seaweed as an antibacterial agent. The seaweed samples were collected from Pantai Panjang, Bengkulu, Indonesia, using a purposive sampling method. Maceration method used to extract the antibacterial agent from seaweeds. Agar diffusion method was used as an antibacterial activity assay against Escherichia coli, Salmonella typhii, and Staphylococcus aureus as the tested bacteria. There were three species of seaweeds found in the location e.g., Halimeda discoidea, Halimeda micronesica, and Caulerpa taxifolia. The antibacterial assay indicated that H. discoidea and C. taxifolia have the antibacterial activity against all tested bacteria with a clear zone that categorized into moderate and high. Meanwhile, H. micronesica had the potential as an antibacterial agent against E. coli and S. typhii with a clear zone categorized into moderate and high. In the future, it could be a potential new drug agent, especially for an antibacterial agent.

Bacteria associated with seaweed were involved in the production of metabolites associated with their host, so they tend to have nearly the same metabolites to defend themselves or as antibiotics. The aim of this study was to isolate and to identify bacteria with antimicrobial activities from the marine green algae (Halimeda discoidea). Laboratory analysis was for the isolation of bacteria, selection of bacteria with antimicrobial activities, antibacterial activity tests and Minimum Inhibitory Concentration (MIC). Results showed that six bacterial isolates were isolated from outside and inside of the algae, which have antimicrobial activities. One was selected for further work which was have more antimicrobial activities. The bacteria showed antimicrobial activity against Gram-positive bacteria (Staphylococcus aureus) but not against Gram-negative bacteria (Escherichia coli). The bacterial growth curve was at an optimum phase at the 10th to 16th hour. Microscopic analysis and biochemical tests showed that isolated bacteria were a rod-shaped Gram-negative, acid-free, non-endosporous, reacted positively to gelatin, citrate and carbohydrate, reacting negatively on motile, and urease. We conclude that the bacteria isolated from green algae which had more antimicrobial properties belongs to the genus Pseudomonas guezennei.