The Amazon river is the largest river in the world. It drains the entire Amazon rainforest, sending leftover nutrients, detritus, and minerals from the South American jungle out into the tropical Atlantic ocean. This runoff forms a freshwater plume, hundreds of miles across, that profoundly affects the ocean underneath it. In this study, we focus on the outer edge of the plume, where a unique microbial partnership takes place: Here, a microscopic plant called a diatom teams up with a microbe called a cyanobacterium; together, they are able to grow rapidly and sequester carbon in the ocean. We measured their impact on the nutrients and gases dissolved in the ocean, and e find that the partnership is unusually efficient, sequestering carbon 2-3 times more efficiently than microbes elsewhere in the plume. By themselves, they may provide more than half the food in this region.