Decreasing concentrations of dissolved oxygen in the ocean are leading to the expansion of oxygen-depleted marine areas worldwide. While often referred to as dead zones, these environments thrive with microbial life, including complex single-celled eukaryotes that have adapted their metabolism to function in the absence of oxygen. Ciliates, an important group playing a pivotal role in aquatic food webs, acquired an advantage alleviating the lower efficiency of anaerobic energy production – through symbioses. Anaerobic ciliates simultaneously harbor methanogenic archaea and sulfate-reducing bacteria (SRB), which use the host's metabolic waste to synthesize methane, a primary component of natural gas, or hydrogen sulfide, toxic to most complex life. In turn, these symbionts increase the host's metabolic rate. Despite their important role in increasingly frequent conditions, the organisms on both sides of these symbioses are severely understudied, and for most there are scarce genomic data, uncertain identity and function(s). This project will investigate consortia of symbiotic ciliates, bacteria, and archaea from diverse tropical marine, but oxygen-depleted, sediments, integrating molecular, microscopic, and cultivation approaches, to identify their role in these ecosystems, and to shed light on the nature and evolution of these interactions. The work will generate a collection of cultures available to other researchers and a resource for exploring novel metabolic adaptations