Earth's most needed uncultivated aquatic prokaryotes

Aquatic ecosystems house a significant fraction of Earth's biosphere, yet most prokaryotes inhabiting these environments remain uncultivated. While recently developed genome-resolved metagenomics and single-cell genomics techniques have underscored the immense genetic breadth and metabolic potential residing in uncultivated Bacteria and Archaea, cultivation of these microorganisms is required to study their physiology via genetic systems, confirm predicted biochemical pathways, exploit biotechnological potential, and accurately appraise nutrient turnover. Over the past two decades, the limitations of culture-independent investigations highlighted the importance of cultivation in bridging this vast knowledge gap. Here, we collected more than 80 highly sought-after uncultivated lineages of aquatic Bacteria and Archaea with global ecological impact. In addition to fulfilling critical roles in global carbon, nitrogen, and sulfur cycling, many of these organisms are thought to partake in key symbiotic relationships. This review highlights the vital contributions of uncultured microbes in aquatic ecosystems, from lakes and groundwater to the surfaces and depths of the oceans and will guide current and future initiatives tasked with cultivating our planet's most elusive, yet highly consequential aquatic microflora.