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Pebble-like rhodoliths, which form a hidden seaweed ecosystem, collected from a depth of 38 m in the waters off Tanegashima Island, Kagoshima Prefecture, Japan.
Photo Credit: Aki Kato / Hiroshima University
Scientific Frontline: Extended "At a Glance" Summary: Rhodolith Diversity and Carbon Sequestration
The Core Concept: Rhodoliths are unattached, pebble-like marine nodules formed primarily by calcifying coralline algae that serve as vital habitats and contribute to long-term carbon storage in ocean sediments.
Key Distinction/Mechanism: Unlike many seaweed species that exhibit continuous distribution across depth gradients, coralline algae show distinct community compositions that change dramatically based on depth, with deeper mesophotic zones hosting unique, non-overlapping species compared to shallow-water counterparts.
Major Frameworks/Components:
- Marine Biodiversity: Rhodolith beds represent the largest areal extent of seaweed-based habitats, facilitating complex ecosystems.
- Blue Carbon: Calcified algal structures act as significant carbon sinks, sequestering atmospheric CO2 in marine sediments.
- Molecular Phylogenetics: Utilization of chloroplast (psbA, rbcL) and mitochondrial (COI-5P) genes to validate species divergence.
- Morpho-Anatomical Taxonomy: Critical evaluation of physical reproductive structures and anatomy to define biological units.
Branch of Science: Phycology, Marine Biology, Evolutionary Biology, Taxonomy.
Future Application: Ongoing efforts to reassess global coralline algal diversity using integrated molecular and morphological data to refine current taxonomy and improve predictive modeling for marine ecosystem resilience.
Why It Matters: Understanding the specific distribution and carbon-sequestering potential of these algae is essential for accurate climate change mitigation strategies and the conservation of "blue carbon" ecosystems.
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| Orientalilithon compactum, a new species described in this study (left). This study provided the first detailed account of both male (center, arrows) and female (right, arrows) reproductive structures in the genus Orientalilithon. Photo Credit: Adapted from Min-Khant-Kyaw, et al. Journal of Phycology. |
Rhodoliths may look like small rocks on the seafloor, but they are actually living algae that create habitats for marine life and contribute to long-term carbon storage. A new study found that the deeper "low-light" waters off Japan's Tanegashima Island harbor a surprisingly distinct and diverse community of these "living pink rocks," including four species completely new to science. Researchers identified at least twelve species in a small patch of seafloor 35 to 38 meters deep, but only three were also found in nearby shallow waters, suggesting the deeper habitat is not simply a continuation of the one near the surface.
The waters off Tanegashima Island—just south of Kyushu, the third-largest of Japan’s four main islands—in Kagoshima Prefecture are a hotspot for algal biodiversity. Due to the undersea geography and the presence of the warm Kuroshio ocean current, the biodiversity of the mesophotic zone (depths of 30 to 150 meters) in this area is especially rich and varied. However, the exact extent of algal diversity in the hotspot is not fully understood and is subject to intensive investigation.
A research team led by Hiroshima University has reported four new species of calcifying red algae, better known as coralline algae, which form rhodoliths in the waters off Tanegashima Island. In doing so, they have also revised the descriptions of two coralline algal genera, emphasizing the diversity of coralline algae in the area and showing that completely different biological communities can exist at different depths within the same marine area.
“Rhodoliths are unattached, pebble-like nodules, which are formed mainly by coralline algae,” says Aki Kato, associate professor at Hiroshima University’s Seto Inland Sea Carbon-neutral Research Center and a corresponding author of the paper. “In marine ecosystems, rhodolith beds represent the largest areal extent among seaweed habitats. These ecosystems occur across a wide depth range, from shallow waters to deep environments, and provide essential habitat that supports marine biodiversity. In addition, due to their calcified structures, rhodoliths contribute to long-term carbon storage in marine sediments, making them increasingly relevant in the context of blue carbon and climate change.”
Samples were collected by dredging the seafloor (30 to 40 meters deep) around two locations approximately 10 kilometers off the western coast of Tanegashima Island. The frozen samples were sorted, labeled, and stored at −20 °C until further study.
DNA was extracted from selected samples, and the chloroplast genes psbA and rbcL and the mitochondrial gene COI-5P were sequenced. These genes are the standard ones used in studying phylogenetic relationships among algae. In addition to phylogenetic analysis—which indicates likely evolutionary history—when DNA sequencing indicated undescribed species, the morphology and anatomy of the corresponding samples were observed by light microscopy and scanning electron microscopy.
The research team identified at least twelve coralline algae species from five genera. The four novel species they identified and named are Orientalilithon compactum, Roseolithon aggregatum, Roseolithon sparsituberculatum, and Sporolithon variotuberculatum. In describing O. compactum, the research team provided the first detailed account of both male and female reproductive structures in the genus Orientalilithon.
“Taxonomy provides an essential framework for making biodiversity visible and for defining the biological units that are the subject of scientific investigation,” Kato explains. “It is not limited to [simply] assigning names based on DNA sequence differences; rather, it involves critically evaluating morpho-anatomical characters that distinguish species and genera, and formally describing new taxa to establish a coherent system for understanding biodiversity.”
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Rhodolith samples were collected by dredging the seafloor (30–40m deep) around two locations (Station 1 and Station 2) approximately 10 km off the western coast of Tanegashima Island.
Image Credit: Min-Khant-Kyaw, et al. Journal of Phycology
Coralline Algae Species Composition Changes Dramatically with Depth
Based on comparisons with other rhodolith beds in Japanese waters, the research team hypothesizes that warmer seawater close to the seafloor and the higher level of sunlight penetration due to clear waters may contribute to the number and diversity of coralline algae on the seafloor off Tanegashima Island.
“A striking finding of this study is that completely different biological communities can exist at different depths within the same marine area,” says Min Khant Kyaw, first and corresponding author of the paper, and postdoctoral researcher at Hiroshima University’s Seto Inland Sea Carbon-neutral Research Center at the time of research (now at Micron Memory Japan KK, Higashi-Hiroshima). “We identified twelve species of coralline algae forming rhodoliths off Tanegashima. However, only three of these species were shared with the nearest shallow rhodolith bed (1-meter depth). In contrast, studies of other fleshy red algae in the same region have shown that about 80 percent of the eighty-five species recorded in the mesophotic habitat are also found in shallower waters in Japan.”
“This contrast reveals that while many seaweeds exhibit continuous distributions across depth gradients, coralline algae show a distinct pattern in which species composition changes dramatically with depth,” Min Khant Kyaw elaborates. “Such patterns likely reflect not only physical factors such as light and temperature but also ecosystem characteristics, dispersal barriers, and biotic interactions. Understanding these mechanisms will be essential for explaining the unique distribution patterns of coralline algae.”
The next step is to confirm the endemism of the four new species to offshore Tanegashima Island by further molecular assessments of coralline algae species from nearby localities. Additionally, the coverage and abundance of rhodoliths in the waters off Tanegashima Island remain unknown.
“We aim to reassess the species diversity of coralline algae, focusing on Japanese taxa, using molecular and morpho-anatomical data,” Kato concludes. “Although more than seven hundred species of coralline algae have been recognized worldwide, many of them were classified based solely on morpho-anatomical characters, and only a limited number have been validated using genetic evidence. We ultimately aim to build a comprehensive understanding of coralline algal diversity and strengthen the foundation of marine biodiversity research.”
Funding: This research was supported by the Japan Society for the Promotion of Science (JSPS) KAKENHI (26850123, 24K09572).
Published in journal: Journal of Phycology
Authors: Min-Khant-Kyaw, Masasuke Baba, Ryuta Terada, and Aki Kato
Source/Credit: Hiroshima University
Edited by: Scientific Frontline
Reference Number: bot060926_01
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