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In the Mediterranean Sea, crustose coralline algae form mesophotic reefs referred to as Coralligenous. They act as a sediment trap and may be considered as a potential source of geological information. Two coralligenous build-ups collected during the project FISR “CRESCIBLUREEF” at 36 and 37 m water depth off the coast of south-eastern Sicily reveal the presence of calcareous nannoplankton/ nannofossil specimens within the sediment infilling. This novel finding reveals the occurrence of a mixed modern and reworked nannoplankton assemblage derived from marine-snow sedimentation and land weathering, respectively. The quantitative analysis of the assemblages from both build-ups shows a difference in sedimentation exposure derived from the different local environments where the build-ups were collected, highlighting the potential role of coccolithophores as a sedimentation proxy for coastal settings.

The most important reef of the Mediterranean is the Coralligène (Coralligenous = C), including several types of calcareous algal-invertebrate build-ups growing in normal open marine conditions. We analyzed and compared two C samples from the Ligurian Sea developed in different environmental settings: 1) off Portofino on a rocky cliff, at a depth of about 40 m and 2) in front of Bogliasco, on a sub-horizontal substrate at a depth of 10 m. The maximum AMS radiocarbon dating provided an older age for Bogliasco (about 5 ka BP) than for Portofino (about 3.6 ka BP), and the mean accumulation rate of the Portofino build-up (about 80 µm y−1) was found to be higher than the one in Bogliasco (about 65 µm y−1). The different sides of each build-up showed a remarkable heterogeneity in the dominant cover by living organisms, and the comparison between the two build-ups highlighted an evident diversity in their taxonomic composition and structure, although crustose coralline algae (CCA) are the dominant framework builder and major autogenic ecosystem engineers at both localities, in the present as in the past millennia. Other major components of the structure are bryozoans and serpulids, and an important role is played by sediment filling. In Bogliasco, extreme climate events and major peaks of fine matrix and terrigenous grains are observed, lithologically related to the drainage basin of the Poggio creek and associated with charophyte occurrence and reduced CCA abundance. The occurrence of the rare Sporolithon ptychoides was observed both in Portofino at about 750 BCE and in Bogliasco. These Sporolithon phases are likely related to warm and humid spells punctuating the Holocene climate fluctuations in the Ligurian Sea. Because coralline algae are confirmed to be the most important habitat engineer of the Mediterranean reefs, they deserve more attention in the framework of any monitoring initiative aimed at C management and conservation.

Along the Mediterranean Sea shelf, algal reefs made of crustose coralline algae and Peyssonneliales are known as Coralligenous. It ranks among the most important ecosystems in the Mediterranean Sea because of its extent, complexity, and heterogeneity, supporting very high levels of biodiversity. Descriptive approaches for monitoring purposes are often aimed at assessing the surficial ephemeral canopy, which is sustained and controlled by the occurrence of the long-lasting rigid structure at the base. This practice led to the non-univocal definition of Coralligenous, sometimes indicated as “animal Coralligenous” because of the surficial dominance of these components. The quantitative assessment of the builders that actively build up the persistent structure through geological time is therefore a fundamental topic. We collected two discrete coralligenous samples in front of Marzamemi village (Sicily, Ionian Sea), the first from an area of a dense coralligenous cover (- 37 m) and the second one from an area with sparse build-ups (- 36 m). By using image analysis and computerized axial tomography, we distinguished and quantified the different components both on the surface and inside the framework. In both cases, our results confirm the primary role of crustose coralline algae as major builders of the Mediterranean Coralligenous, this aspect matching with the evidence from the Quaternary fossil record. We suggest that the role of encrusting calcareous red algae in the Coralligenous should be considered in conservation and management policies.

In the Mediterranean Sea, crustose coralline algae form endemic algal reefs known as Coralligenous (C) build-ups. The high degree of complexity that C can reach through time creates notable environmental heterogeneity making C a major hotspot of biodiversity for the Mediterranean basin. C build-up can variably modify the submarine environment by affecting the evolution of submerged landforms, although its role is still far from being systematically defined. Our work proposes a new, ad-hoc semi-automated, GIS-based methodology to map the distribution of C build-ups in shallow coastal waters using high-resolution bathymetric data, collected on a sector of the southern Apulian continental shelf (Southern Adriatic Sea, Italy). Our results quantitatively define the 3D distribution of C in terms of area and volume, estimating more than 103,000 build-ups, covering an area of roughly 305,200 m2, for a total volume of 315,700 m3. Our work firstly combines acoustic survey techniques and geomorphometric analysis to develop innovative approaches for eco-geomorphological studies. The obtained results can contribute to a better definition of the ocean carbon budget, and to the monitoring of local anthropogenic impacts (e.g., bottom trawling damage) and global changes, like ocean warming and acidification. These can affect the structural complexity and total volume of carbonate deposits characterizing the Mediterranean benthic environment.

In the framework of the Italian multidisciplinary research project “Mud volcanoes Ecosystem study - Sicily Channel” cores MV02_GC01 and MV03_GC02 were recovered from a tabular relief at 140 m water depth, to investigate the nature of such uncommon shallow seep-related morphology trough micro- and macro-paleontological, mineralogical, sedimentological and geochronological analyses. Both cores record muddy surficial sedimentation, hosting a strictly circalittoral molluskan death assemblage typical of mobile substrate, coherent with the present-day seafloor depth and environment. The core bottom recovered a biogenic framework, in which the molluskan fossil assemblage is characteristic of shallow-water, possibly infralittoral, representing a past sea level low stand. The biogenic framework shows structure and composition that are similar to present day algal reefs indicated as coralligenous build-ups. Both cores recorded the Holocene transgression. Core MV02_GC01 is longer and its basal section records cobbles and pebbles of both terrigenous and biogenic origin partially covered by a dark patina, and finely preserved Halimeda sp. fragments. The dark patina reveals a fibrous nature, often containing framboid pyrite crystals, ankerite, but no organic matter. We interpret the dark patina level as the last lowstand, possible forming in a lagoon-like paleoenvironment. Below such level, another coralligenous build-up occurs, U/Th dated to MIS 6, in agreement with the available seismic interpretation. The tabular relief lays up to 10 m above the surrounding seafloor and reveals a complex inner architecture, in which at least two generations of coralligenous build-ups shaped its geomorphological development, and the local tectonic activity contributed to its uplift.

Coralligenous is the most volumetrically significant type of autochthonous carbonate build-up in the Mediterranean, but its range of morphological variability at the macroscale, and potential link with environmental factors have not been fully documented. Well-preserved examples of coralligenous are exposed in shallow marine carbonate deposits from the Upper Pleistocene Le Castella marine terrace (Southern Italy). The coralligenous build-ups occur in the lower part of an unconformity-bounded, transgressive-regressive sequence. They show high structural and morphological heterogeneity in terms of framework type and build-up size, over a distance of several hundred meters. In northern exposures, coralligenous forms banks up to 4 m high, with a dense, coralline-dominated framework, laterally adjacent to paleo-channels devoid of build-ups and filled by coeval infra-circalittoral biogenic packstone. To the south, build-ups are smaller (up to 1.8 m high), with an open algal framework and local predominance of the coral Cladocora caespitosa as primary framework builder. At both locations, the most important framework-building coralline species is Mesophyllum philippii; Titanoderma pustulatum also occurs. Analysis of sedimentary structures, paleo-topographic indicators, faunal composition of associated mollusk assemblages, and predominance of M. philippii suggest the algal builds-ups grew within a shallow-water, possibly infralittoral setting, competing for space with other biocoenoses. Suitable substrate availability in the form of conglomerate blocks or shell accumulations was the primary factor controlling the inception and spatial distribution of coralligenous. Differences in hydrodynamism and water turbidity determined variability in size and internal composition between the northern and southern locations. To the south, abundant supply of fine suspended sediment resulted in smaller build-ups with more open framework, the growth of which was terminated abruptly by sedi