Rhodoliths can act as daily resolution paleotemperature archives in the Red Sea

communications earth & environment Article A Nature Portfolio journal https://doi.org/10.1038/s43247-026-03603-y Rhodoliths can act as daily resolution paleotemperature archives in the Red Sea Check for updates 1234567890():,; 1234567890():,; Lena Y. Li 1,2 , Juan P. Bernal-Tamayo 3, Steffen Hetzinger 4, Jochen Halfar5, Walter A. Rich Michael D. Fox 6, Maggie D. Johnson6, Hubert Vonhof 2, Ralf Schiebel2 & Bernd R. Schöne1 6 , Long-term, high-frequency temperature records are critical for evaluating the vulnerability of marine ecosystems to ongoing ocean warming. Commonly used paleoceanographic archives, such as corals and bivalves, face regional and ecological constraints. Rhodoliths, free-living coralline red algal nodules, are established and globally accessible alternatives, yet their complex three-dimensional growth structures hinder the construction of continuous age models. Here we present a workflow combining staining-based growth calibration, semi-automated increment detection using microcomputed tomography, and dynamic time warping to assemble a daily chronology spanning 133 days (March–July) from seven fruticose thalli of a single rhodolith from the central Red Sea. The resulting multi-thallus chronology, combined with a multi-element temperature proxy (Magnesium-toStrontium), yields closer agreement with in-situ logger temperatures (R² = 0.91, RMSE = 0.63 °C) than individual thalli or single-element proxies, establishing a proof-of-concept framework for dailyresolved paleotemperature reconstructions from tropical rhodoliths. Foundation species such as scleractinian corals and coralline algae construct the physical framework of tropical coral reef ecosystems. These reef structures support approximately 25% of marine biodiversity and generate tens of billions of dollars in annual economic value1,2. However, coral reefs are increasingly threatened by rising ocean temperatures3, which drive widespread habitat loss and functional decline4. Assessing reef vulnerability and informing conservation strategies requires an understanding of baseline historical temperature variability at ecologically relevant spatiotemporal scales. Furthermore, while satellite-derived sea surface temperatures (SSTs) provide valuable monthly to daily records, they do not capture the fine-scale temperature fluctuations experienced within coral reefs5. This is a critical limitation, as high-frequency temperature variability is a key predictor of organismal stress tolerances and coral bleaching risk6. Past environmental variability in reef environments is often reconstructed using the skeletons of corals7,8, bivalves9,10, and foraminifers11,12. These biogenic carbonates archive physical, structural, and geochemical information that reflects ambient conditions during growth13. However, most of the established archives and proxies have limitations. The biogeographic distributions of many species used as archives for the reconstruction of the environment and climate, such as corals and bivalves, are shaped by their environmental tolerances. As ocean conditions change, these suitable habitats are projected to contract, potentially restricting populations to climate refugia and reducing the spatial coverage of available environmental archives14,15. Ontogenetic changes, such as shifts in growth rate or the amount of elements incorporated into their skeletons with age, can further complicate chronological reconstruction and geochemical interpretation16. In contrast, coralline algae are globally distributed across photic and mesophotic zones17–20 with minimal ontogenetic effects21. Certain species can live for centuries and form seasonal growth bands, offering the potential for high-resolution, long-term environmental reconstructions22. While coralline algae have been gaining traction as environmental and climatic archives since the early 2000s23–26, most proxy work with coralline algae has focused on polar and temperate oceans27–29, with relatively few studies addressing their utility in tropical and subtropical systems30,31. Though coralline algae are also vulnerable to local and global environmental change32, they are likely to become even more ecologically important as coral reefs change33, due to their ability to withstand light variability, suppress competing fleshy macroalgae and build and bind reef-frameworks, potentially buffering some of the loss in key ecosystem functions associated with coral declines34,35. However, most encrusting coralline algal species grow laterally rather than vertically, making it difficult to distinguish boundaries between 1 Institute of Geosciences, Johannes Gutenberg University Mainz, Mainz, Germany. 2Climate Geochemistry, Max Planck Institute for Chemistry, Mainz, Germany. Applied Mathematics and Computational Science, King Abdullah University of Science and Technology, Thuwal, Kingdom of Saudi Arabia. 4Institute of Geosciences, Christians-Albrechts-University Kiel, Kiel, Germany. 5Department of Chemical & Physical Sciences, University of Toronto Mississauga, Mississauga, ON, Canada. 6Biological and Environmental Sciences and Engineering Division, King Abdullah University of Science and Technology, Thuwal, Kingdom of Saudi Arabia. e-mail: 3 Communications Earth & Environment | (2026)7:439 1 https://doi.org/10.1038/s43247-026-03603-y Article Fig. 1 | Rhodolith sample and study site. a Rhodolith, with the left half showing overview microCT scan (voxel size = 80 μm), and the right half showing the whole, air-dried sample. b Map showing location of the Red Sea in a global context (Esri, Maxar, Earthstar Geographics and the GIS User Community). c Inset showing the summer sea surface temperature (°C) of the Red Sea (NOAA 1/4° Daily Optimum Interpolation SST). Yellow star indicates sampling locality; logo indicates King Abdullah University of Science and Technology (KAUST). individuals, which reduces the interpretability of their growth histories. The encrusting forms that do produce the continuous, layered growth that tends to yield higher proxy correlations36 are largely limited to high-latitude regions37. Free-living coralline algae can grow as discrete nodules, termed rhodoliths (or maerl), and are globally distributed across wide depth, temperature, and salinity gradients32. Despite this, rhodoliths have been relatively overlooked and underutilized as high-resolution archives because of their complex three-dimensional morphology, variable banding periodicity, and fragmented growth histories38. The Red Sea is a valuable system for studying thermal resilience in coral reefs39. This semi-enclosed basin is oligotrophic and hypersaline, with high evaporation and intense solar irradiation40. Shallow reef flats in the Red Sea experience seasonal temperature ranges of 18 to ~38 °C, and rare events have produced fluctuations greater than 10 °C in a single day41. Central and southern Red Sea reef flats frequently reach summer temperatures that match or excee (...truncated)