The majority of biodiversity of life on Earth consists of poorly-understood groups of “microbial dark matter” (MDM) that play important roles in regulating the climate and influencing the physiology of multicellular organisms (Rinke et al, Nature, 2012). Understanding ecosystem processes and global biogeochemical cycles requires a deeper knowledge of these organisms within their natural environments; however, their cryptic metabolisms make them difficult and at times impossible to study in the laboratory. In this project, we are using transcriptomic sequencing to analyze mRNA transcripts produced by members of the Marinimicrobia, an abundant group of MDM that lives in ocean waters around the globe and plays important roles in biogeochemical cycling (Hawley et al, Nat. Comm, 2017). We focus on monitoring gene expression of Marinimicrobia in both coastal and open ocean environments and analyzing the correlation between species within the phylum and how their interactions affect their surrounding ecosystems. On a global spectrum, it is vital that the mechanisms of microbial communities within the oceans are understood to know how the systems will react to climate change caused by human activities. Given the importance of MDM, this project will help advance our understanding of the role that the Marinimicrobia play in regulating these key global biogeochemical cycles. Thus far, I have analyzed mRNA transcripts from various time points sampled in the North Pacific Subtropical Gyre and found that there were transcripts present from 13 genomes which belong to 3 distinct clades of Marinimicrobia.