BRC-BIO: Exploring multi-domain syntrophic symbioses in marine oxygen-depleted environments

About This Grant

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, cellular functions, and the evolution of eukaryotic anaerobiosis. The project will create new research opportunities for undergraduate and graduate students, along with a seminar series and microscopy workshops for local schools, introducing the world of microbial symbioses and the possibility of ocean life existing under increasing ocean hypoxia. The project will map the diversity of marine anaerobic ciliates and consortia of their prokaryotic symbionts and characterize the cell-to-cell dynamics among these interactions. Following intensive sampling of oxygen-depleted waters and sediments in Puerto Rico, the detected marine anaerobic ciliates will be identified and screened for the presence of dominant prokaryotic symbionts using rRNA gene sequencing and microscopy. Single cell genomic and transcriptomic sequencing methods will be used to perform phylogenomic analyses of symbiotic prokaryotes compared to free-living prokaryotes in the same habitats and to assess interactions between methanogenic and SRB symbionts to help determine selective pressures impacting the structure and fidelity of these consortia. Comparative genomics analysis will be conducted to identify factors influencing symbiont adaptations and specificity. Cultivation experiments will be conducted to elucidate the SRB mode of transmission and to test if SRB ectosymbionts may increase the host's metabolism rate when methanogenic endosymbionts are absent. This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.