Fig. 6

Model of the distribution, metabolic redundancy and adaptations of Campylobacterota MAGs along the Kebrit Deep BSI. The three Kebrit Deep Campylobacterota MAGs—Sulfurimonas Kbt_01 (green), Sulfurovum Kbt_02 (orange), and Sulfurovum Kbt_03 (blue)—exhibit distinct metabolic and salinity adaptation strategies. Bacterial cells represent their relative abundance along the vertical BSI stratification, with grey cells representing other microorganisms potentially interacting with Campylobacterota and utilising their metabolic by-products (small black dots), such as organic carbon (indicated as C) fixed via rTCA. Kbt_01 and Kbt_02 can gain energy from the oxidation of reduced sulfur forms (indicated as S) using oxygen as the final electron acceptor through pathways involving the SOX system and sulfide:quinone oxidoreductase (SQR). In contrast, Kbt_03 performs nitrate reduction (indicated as N) coupled with sulfur compound oxidation via sqr (S) and does not encode for the SOX system. Given the low oxygen concentration along the BSI and in accordance with the energetic metabolisms, the electron transport chain in the three bacteria includes high-affinity cbb3-type (black rectangles) cytochromes for facilitating the reduction of oxygen, even under low oxygen concentrations. Additionally, Kbt_01 and Kbt_02 encode another type of high-affinity cytochromes, the bd-type (grey rectangles), while Kbt_03 encodes periplasmic nitrate reductase (nap) and nitrous oxide reductase (nos) for nitrate reduction (light-blue rectangle). The decrease in electron acceptors and the increase in salinity along the BSI explain the observed reduction of the abundance of all three Campylobacterota MAGs, but the ability to synthesise osmoprotectants, such as ectoine, alongside proline and glutamate/glutamine (Glu/Gln), enable the two Sulfurovum MAGs to persist down to the brine body