Specific gravity was measured as described previously for microalgae. CsCl2 was dissolved in water to a concentration of 100% w/v, then diluted to 90%, 80%, and 70%. Blue dye was added to the 90% and 70% solutions, and yellow dye added to the 80% solution. Specific gravity of each solution was measured by carefully pipetting 2mL into a pre-weighed centrifuge tube. The weight of the solution was divided by 2mL to get specific gravity in g/L. Each solution was chilled to 4˚C, then 250µL was carefully layered into a 2mL centrifuge tube such that the solutions did not mix. 200µL of biomass was pipetted on the top layer and the tubes were centrifuged at 1,000 g for 30 minutes at 4˚C.