Abstract: Ecological strategies inform niche differentiation and biogeochemical impact of the microbial community and can be inferred from metagenome-assembled genomes (MAGs). However, a quantitative framework has been lacking to measure MAG population absolute abundances, limiting our understanding of the scale of the environmental impact. In this research seminar, I will present my work on quantifying MAG population absolute abundances and understanding their ecological strategies in two different systems. Metagenomes were collected and sequenced from Station Papa in the northeastern Pacific through the EXPORTS cruise and from the Galapagos Archipelago. MAGs were assembled and their volumetric abundances were estimated using internal standard spike-ins, enabling sensitive detection down to 1,000 cells L-1. At Station Papa, two Synechococcus MAG populations accounted for over 95% of all Synechococcus abundance and showed adaptations to this iron-limited high-nutrient low-chlorophyll region, suggesting that these Synechococcus potentially have a significant impact on the ecosystem. Among the 15 high-quality Flavobacteriaceae MAGs recovered from the Galapagos, different distributions across the islands were observed. Several MAG populations had increased abundance at western stations in 2016 due to strong upwelling bringing nutrient-rich deep water to the surface. In contrast, a Formosa population had increased abundance in the central archipelago where small phytoplankton dominated. Functional analyses indicate that the flavobacteria MAGs were enriched with unique sets of genes related to the uptake and storage of carbon and defensive mechanisms. Our findings link population-resolved microbial distributions with biotic and abiotic drivers, providing a better quantitative understanding of marine microbial processes and their potential ecological impacts.