Sample characteristics by realm and ecosystem type.

§Coastal samples were located at less than 200 nautical miles from the shoreline and less than 200 m water depth. Surface Open Ocean refers to offshore samples from the top 200 m of the water column, whereas Deep Open Ocean refers to offshore samples below 200 m water depth. Deep Seafloor refers to sediment samples that were located below 200 m water depth. Vents and Anoxic Ecosystems were identified from project descriptions, available on MICROBIS (See Material and Methods). $OTU0.03: Operational Taxonomic Unit at 3% of sequence dissimilarity. These numbers are only indicative and are not equivalent to bacterial community richness (See Material and Methods). *These samples were not kept for variation partitioning analyses due to insufficient number of samples.