ABSTRACT Understanding of the complex network of microbial interactions in the microbiome is integral to human health. While most microbiome studies have focused on bacterial communities, recent metagenomic analyses report that archaea comprise a significant percentage of the microbes living in and on our bodies. Yet, little is known about the impacts of archaea and archaeal signaling in the human microbiome. In this proposal, I aim to study cell signaling in the model archaeon Haloferax volcanii. In culture, H. volcanii cells at early-log phase are rod- shaped, which then transition to pleomorphic disks in mid- to late-log phase. Cells stab-inoculated into soft agar plates can form swimming motility halos, with motile rod-shaped cells at the edge and sessile disk-shaped cells at the center. Applying cell-free conditioned media (CM) from a late-log culture to a fresh H. volcanii culture results in exclusively disk-shaped cells at early-log phase. Moreover, when CM is incorporated into soft agar plates, wildtype H. volcanii does not form a motility halo and instead grows as a nonmotile colony at the site of stab-inoculation. These results suggest that a secreted signaling molecule in the CM mediates the shape change and inhibits motility. I have coined the term Disk Forming Signal (DFS) to describe this signaling molecule. Additionally, mutant strains lacking the protein CirA seem to bypass DFS, remaining as rods throughout growth in culture and forming motility halos in soft agar supplemented with CM. Interestingly, several mutant strains that have a disrupted cirA also harbor point mutations in arlI and arlJ, two biosynthesis genes of the archaella, the archaeal flagella analog. Since cirA is in the same genomic region as arlI and arlJ, a potential link could exist between CirA and the archaella in motility regulation in response to DFS signaling. In the proposed study, I aim to discover components of a novel H. volcanii signaling mechanism. I will identify the structure of DFS via analytical chemistry techniques and investigate the genes involved in DFS biosynthesis by screening an H. volcanii transposon library. Additionally, I will characterize the role of CirA in the DFS signal transduction pathway via quantitative proteomics and protein-protein interaction studies between CirA, ArlI, and ArlJ. Discovering the identity of a signaling molecule produced by H. volcanii, along with defining the proteins involved in its synthesis, recognition, and response will broaden our understanding of microbial communities in the healthy human microbiome, begin to unravel the intricacies of signaling among all three domains of life, and lead to applications of archaeal physiology in medicine and industry.