Our oceans are dominated by microbial species that are essential for marine ecosystems and have central roles in biogeochemical cycles, influencing the climate of our planet. Yet, our understanding of spatial patterns of microbial and functional diversity, and the drivers of this diversity, is limited. Now, two recent studies report ocean microbial diversity pole to pole, using data collected from the Tara Oceans expedition, an international and interdisciplinary project that collected 35,000 samples across the world’s oceans from 2009 to 2013. Ibarbalz et al. used DNA sequencing of filtered seawater and imaging of net catches to investigate the latitudinal gradients and global predictors of diversity across bacteria, archaea, eukaryotes and major viral clades in the surface oceans. The authors observed a general decline in diversity towards the poles for most groups, mainly driven by decreases in temperature; indeed, temperature was the best predictor of diversity. To understand how global warming may affect microbial diversity, the authors modelled trends in diversity on a global scale at the beginning and the end of the 21st century and found that severe warming of the surface ocean could lead to tropicalization of diversity in temperate and polar regions. Salazar et al. used a combination of metagenomics and metatranscriptomics to survey microbial genetic composition and gene expression in the global ocean. A dataset of 370 metagenomes and 187 metatranscriptomes comprising 47 million genes was generated from 126 sites pole to pole. The authors examined gene expression changes and community turnover as the primary mechanisms shaping metatranscriptomes across latitude and depth and found that their individual contributions differ for various biogeochemical processes, including the cycling of carbon, nitrogen and sulfur, and processes involved in photosynthesis. The authors also found that the relative contribution of gene expression changes are lower in polar waters compared with non-polar regions and hypothesise that changes in community activity as a consequence of warming will be driven more by changes in composition than by gene expression.