# The Marine Bacterial Genus _Euzebya_ Is Distributed Worldwide in Terrestrial Environments: A Review
## 2 Metagenomic Detection of _Euzebya_ in the Environment: Caves  
Caves are mineral environments, often oligotrophic in nature. Rocks, speleothems, and mineral deposits, such as moon milk, are colonized by microbial communities, which develop as colored biofilms [18; 19; 20]. To our knowledge, the first report on the occurrence of _Euzebya_ in caves was in a study by [PERSON] et al. [18]. In Altamira Cave, Spain, sequences with 82-92% similarity to the nearest relative _Euzebya tangerina_ were retrieved from grey biofilms, suggesting that they probably represented an unknown species. _Euzebya_ represented 72.8% of the clones retrieved from the grey biofilms [18]. [PERSON] et al. [19] recovered representatives of _Euzebyales_ from colored microbial mats found in volcanic caves in the Azores, Hawai'i, and New Mexico, and stated that the different clades obtained suggested a significant diversity within the sequences found. Other papers reported _Euzebya_ sequences from caves in different geographical regions [21; 22; 23; 24; 25; 26; 27; 28; 29; 30; 31; 32; 33; 34; 35; 36] (Table 1). The composition of microbial communities was found to be dependent on geochemical and microclimatic parameters. In this context, [PERSON] [29] reported the high relative abundance of _Euzebya_ (up to 30.7%) in one cave and its negligible occurrence (up to 0.2%) in another cave from mineralogically similar formations located 65 km apart. The difference in abundance was attributed to flooding and clay deposition in the _Euzebya_-rich cave. Also interesting was the abundance of _Euzebyaceae_ in pink biofilms from Pindal Cave, Spain (7.0-16.0%), coincident with the pink color of the three described _Euzebya_ species [36]. A recent study was conducted on the biofilms present in Covadura Cave, located in the gypsum karst of Sorbas, Almeria, Spain. The karst comprises over 100 km of passages within the six most important caves (Covadura Cave, GEP Complex, C3 Cave, Gypsum Cave, Treasure Cave, and Water Cave), which are subject to condensation-dissolution mechanisms. Water condensation on the cooler walls of Covadura Cave takes place mainly during the dry period (July to October) and the biofilms show water droplets on their surface. Biofilm proliferation has been associated with the strong condensation existing in some caves [36], as condensation favors the colonization of cave walls by microorganisms [18; 19]. The data revealed that _Euzebyaceae_ were abundant in Covadura Cave white biofilms collected in 2010, but their relative abundance was drastically reduced in the 2022 sampling (Table 1). This could be associated with the severe droughts, the last of which occurred between 2017 and 2018, and which continue until now. In the yellow biofilms, the decrease in abundance was lower. _Euzebya_ was also abundant in volcanic caves. The genus was found in caves in the Azores, Canary Islands, Galapagos, Hawai'i, Idaho, Tennessee, and Mexico [19; 20; 24; 26; 28; 29; 30; 34]. [PERSON] et al. [24] stated that yellow biofilms from a cave on the Canary Island of La Palma were dominated by metabolically active _Euzebya_ (43.9% RNA clones vs. 26.0% DNA clones). In the other three La Palma caves [26], the relative abundance of _Euzebya_ was lower (Table 1). La Palma caves are characterized by the leaching of dissolved organic matter from the soil surface, and its transport inside the cave is favored by the low thickness and high porosity of the volcanic rock [37]. _Euzebya_ colonizes the mineral formations and deposits in caves, usually at an alkaline pH (8-10), where it develops as biofilms of different colors. The wide distribution in karstic, volcanic, and gypsum caves all over the world indicates that terrestrial _Euzebya_ can thrive in these humid, largely mineral environments [38].