After density gradient centrifugation, a differential centrifugation washing step was used to remove any remaining density gradient medium from the high-density and low-density isolates (Figure , steps 5 -7). Differential centrifugation is a common technique for separating particles in aqueous solutions based on sedimentation velocity. The high-density or low-density material resuspended in ultrapure water is centrifuged for a prescribed amount of time, and the materials with sufficiently high sedimentation velocities in water at the given centrifugal force will sediment to form a pellet at the bottom of the centrifugation tube, while the rest of the material remains suspended in the water. Unlike density gradient centrifugation, differential centrifugation does not involve the addition of any density gradient media, and only involves sedimentation through the solvent (water). Here, samples were centrifuged (Beckman Coulter Microfuge 20) for 10 minutes at 10,000 × g (Figure , step 5). Calculations illustrate that this centrifugation force and time is sufficient for organic particles with sizes greater than ~0.3 μm and inorganic particles with sizes greater than ~0.08 μm to pass through the water and sediment at the bottom of the tube (See SI, Section S2, Table ). After centrifugation, the water and any material that did not pellet at the bottom of the tube INSs in the unprocessed sample if no material is lost.