This story began during the fall (norther hemisphere) of 2020, when we all lived the spread of COVID-19 pandemic. I was in Northern Italy, at home, in what was one of the epicenters of the virus, at least apparently. My colleagues, in the midst of thousand of limitations instead, were preparing to live one of the most formative experiences of their lives. In fact, the first Ocean-X expedition in the Red Sea (Deep Blue Expedition aboard the M/V OceanXplorer, between October and November 2020) was about to take off, in the waters of the Saudi Arabia NEOM development giga-project. They were preparing to explore the se floor every day with submersibles and a remotely operated vehicle (ROV) down to 500 meters deep to discover for the first time ever the aphotic and mesophotic environments of the Saudi coasts of the Red Sea. A completely uncharted territory .

After the first exploratory dives, discovering an incredible world, my colleagues began to identify recurring benthic assemblages responsible for a much more varied habitat diversity than expected. Among these, on the external portion of the shelf, there were extensive aggregations of what looked like pétanque balls, usually ranging from red to brown in color. Each of these balls housed a sort of flattened umbrella, one or more specimens of solitary zooxanthellate coral of the genus Leptoseris. They looked so nice and undescribed!

It was at this moment that I got a phone call and entered the game. Maybe, they found a bed of rhodoliths, my topic. Rhodoliths are nodules made of crustose coralline algae, algae that have mineralized-cell and thalli, which can develop rigid structures at the seafloor creating nodules.

So, I organized the first of a series of trips to the King Abdullah University of Science and Technology in Thuwal, Saudi Arabia, where my colleagues work. Let me say that these trips were not only a moment of science, but also a great opportunity of human growth, because I was in contact with a country so different from mine, a country with a profound and beautiful history.

There, I got the possibility to put my hands on these nodules!

Right from the start, from the video analysis of the submarines and ROV, along transects covering tens of kilometers of unexplored Red Sea seabed, I realized that we were dealing with something really peculiar and unexpected. The images spoke for themselves. Between 50 and 130 m depth, there were areas entirely covered by these very-rounded nodules, partially buried into sediment, which at the same time served as a substrate for many other upright organisms, from sponges to corals. They had a spheroidal to sub-spheroidal shape. Something similar are rhodoliths, but these were not. The evidently pink crusts of calcareous algae were too sporadic to be the main builders of these structures.

My colleagues had managed to amass quite the collection of these nodules thanks to the skillful OceanX ROV and submersible pilots. This allowed their direct observation and analyses. Optical and electron microscopy allowed us to discover the nodules actual nature. They were macroids, sub-concentric, built by the laminar growth of encrusting foraminifera (Acervulina cf inhaerens), crustose coralline algae, scattered bryozoans and serpulids. Therefore, they were foraminiferal-algal nodules (FANs) and not rhodoliths. Radiocarbon dating of 5 FANs gave an age dating back to more than 2300 years BP, which means that they have a very low accretion rate (less than 0.02 mm/year) and developed in a mesophotic environment. FANs therefore represent a habitat closely linked to these depths and this environment. Interestingly, they formed beds just like the more widely studied rhodoliths. Aggregating at the external portion of the shelf, we observed FANs fall down the slope and we collected them dead at 425 m depth, partially buried into the mud. We could not assume that this is the fate for all of them, but at least for some of them this looked like their burial place.

But how many FANs are there? And how much carbonate do they produce? We did not ask ourselves these questions only as scientists, but also as citizens of the world and in the world. Because if on the one hand FAN beds represent a biodiversity hot-spot unknown to date, on the other hand it is necessary to convey this discovery in an appropriate way, so that it can be duly taken into account in the context of sustainable management and development of human activities both on land and at sea.

Video analyses allowed us to estimate that the nodules had a coverage ranging from 25 and 100% of the sea floor, with a number of nodules per square meter that varied from 50 to 287, a volume that varied between 10.8 and 433.3 cm3, a mass that varied between 23.9 and 544.4 g and a density ranging between 0.9 and 2.2 g cm⁻³. Volume and mass were well correlated which suggests that the inner features are similar, which was confirmed by cutting the nodules and observing the inner structure. Basically they were fully made by calcium carbonate of foraminifers and other minor contributors. Considering both our occurrence data and a large constrain dataset derived from bathymorphology and physical oceanography, we built a habitat suitability model for the nodules and calculated that about 6% of the shelf in the considered area has all the characteristics for accommodate nodules. Which means an area of ​​just over 6 km² in our study area alone.

Little? Meanwhile, something like 156.3 kg m² of calcium carbonate is unexpectedly deposited in the area, with a production rate ranging between 0.5 and 66 g m² yr⁻¹. Meaning that the total calcium carbonate currently deposited by FANs in the NEOM area we explored can reach 979.6 megatons. FANs are a carbonate factory, and therefore they must henceforth be consider when modeling marine carbonate budget in the region and its possible implication under the scenario of climate change. This kind of results are fundamental for understanding how carbonate deposition responds to environmental condition changes trough time, and due to their occurrence and dynamic how they contribute to the burial of C_inorg.