Anthropogenic CO2 emissions have altered seawater chemistry causing ocean acidification and affecting the ability of marine calcifiers to build and maintain calcium carbonate structures. Calcifiers such as corals and rhodoliths play key roles in benthic carbon cycling through photosynthesis, respiration, calcification, and dissolution. Despite growing attention to their influence on seawater carbonate chemistry, organism-scale studies in tropical ecosystems, such as the Colombian Caribbean, remain limited. We investigated short-term carbon fluxes by measuring photosynthesis–respiration and calcification–dissolution rates in the coral Siderastrea radians and rhodoliths under natural light at different times of day. In situ incubations at Isla Grande (Corales del Rosario and San Bernardo National Natural Park) showed that both organisms actively modulate seawater carbonate chemistry, but via distinct diel patterns and metabolic strategies. Rhodoliths exhibit highly variable metabolism, with high daytime photosynthesis and pronounced day-night contrasts in calcification, reflecting strong coupling between light, photosynthesis, and CaCO3 precipitation, along with a tendency for nocturnal dissolution. In contrast, S. radians displays lower absolute productivity and calcification rates but maintains metabolic stability through a more regulated coupling of photosynthesis and respiration and a moderated light-to-dark calcification ratio. The higher photosynthesis-to-calcification ratio in S. radians indicates a strategy prioritizing carbon fixation over skeletal accretion, whereas rhodoliths allocate more of their photosynthetic output to CaCO3 deposition than corals. These results highlight the distinct organism-level metabolic strategies and indicate that species-specific patterns of photosynthesis and calcification determine their contributions to short-term carbon fluxes and the surrounding water carbonate dynamics.

Coral species abundance and biodiversity estimates are typically based on colony macromorphology. However, such measurements often underestimate the true diversity within coral communities because morphology does not necessarily reflect behavioral or genetic divergence. We previously reported on the unusual spawning behavior of the southern Caribbean population of the brain coral Diploria labyrinthiformis (Linnaeus, 1758), which spawns in both spring and autumn. Here, using data collected from 2013 to 2021, we show that the D. labyrinthiformis population in Curaçao is comprised of two behaviorally- and genetically distinct lineages, with 93% of colonies spawning exclusively in one season or the other. The two lineages could not be distinguished based on macromorphological differences or depth but represented clearly distinct genetic clusters (FST = 0.098) based on genome-wide sequencing. We tested for prezygotic and postzygotic gametic barriers between these two lineages by fertilizing eggs released in spring 2019 with sperm collected and cryopreserved in autumn 2018. Fertilization was successful and the resulting larvae developed normally, thus eliminating gametic incompatibility or early life postzygotic barriers as explanations for the divergence between these groups. Using observations from 19 other Caribbean localities, we confirmed the co-occurrence of discrete spring- and autumn-spawning populations in several locations across a range of latitudes. Thus, we show that seasonal, temporal reproductive isolation (allochrony), but not gametic reproductive isolation, is a strong barrier to gene flow in sympatric lineages of this critically endangered reef-building coral. More broadly, our findings underscore the role of allochrony as a mechanism capable of driving genetic divergence among cryptic, sympatric coral species.

Ostreobium comprise endolithic algae commonly seen in conjunction with scleractinian corals. In the past, it was solely recognized as a coral skeleton bioeroder. Yet, their relationship with corals is critical because they give photosynthetic byproducts and help the coral when it loses its primary symbionts due to stress. The variety of these algae among coral species of the genus Porites in the tropical easter Pacific and western Atlantic was investigated here. We extracted Ostreobium samples from seven Porites species including two from the Tropical Easter Pacific-TEP ( P. panamensis, P. lobata) and five from the Caribbean (P . furcata, P. porites, P. colonensis, P. branneri, and P. astreoides). We also compared the new rbcL sequences from algae found within various coral species from other parts of the world. A biogeographic analysis and two methodologies, PTP (Poisson tree process) and GMYC (General Mixed Yule-Coalescent), were used to delineate the different species. The findings revealed a significant degree of genetic diversity within Ostreobium, with more than 15 groups of no more than three individuals and 40 individual lineages. Its origins date back to the Ordovician, 500 Ma, and it does not appear to preserve host-specificity. The sampled locations include a wide variety of Ostreobium still, biogeographically, varied patterns were confirmed, making it impossible to pinpoint the precise origins of most clades. The ancestry analyses revealed convergent events for the emergence of Ostreobium in a few genera of local corals, but the phenomenon also occurred in genera from far-off places.

Vastly understudied, mesophotic coral ecosystems lie below shallow reefs (> 30 m depth) and comprise ecologically distinct communities. Brooding reproductive modes appear to predominate among mesophotic-specialist species and may limit genetic connectivity among populations. Using reduced representation genomic sequencing, we assessed spatial population genetic structure (at 50 m depth) in an ecologically important mesophotic-specialist species, Agaricia grahamae among locations in the Southern Caribbean. We also tested for hybridisation with the closely related (but depth-generalist) species Agaricia lamarcki, within their sympatric depth zone (50 m). In contrast to our predictions, no spatial genetic structure was detected between the reefs of Curaçao and Bonaire (~ 40 km apart) within A. grahamae. However, cryptic taxa were discovered within both nominal species, with those in A. lamarcki (incompletely) partitioned by depth and those in A. grahamae occurring sympatrically (at the same depth). Hybrid analyses and demographic modelling identified contemporary and historical gene flow among cryptic taxa, both within and between A. grahamae and A. lamarcki. These results (1) indicate that spatial genetic connectivity in these ecologically important mesophotic species may be maintained over large geographic distances and (2) highlight that gene flow links taxa within this relativity diverse Caribbean genus.