INCREASING THE SENSITIVITY OF A FACSCAN FLOW CYTOMETER TO STUDY OCEANIC PICOPLANKTON

Existing limitations on the sensitivity of commercially available flow cytometers do not permit the detection of extremely dim picoplankton cells, particularly Prochlorococcus marinus in near-surface oligotroph ic ocean waters. This problem was overcome by making some simple modif ications to a FACScan (Becton Dickinson) flow cytometer to increase it s sensitivity by about fivefold. The fluorescence and side scatter sen sitivity was increased by replacing the existing focusing lens and bea m-steering mirror with a 3 x expansion telescope and a slightly shorte r focusing lens, resulting in a smaller laser spot size at the interro gation point. Forward scatter sensitivity was increased by replacing t he existing photodiode with a photomultiplier tube, which has a backgr ound noise level about an order of magnitude lower. The fluorescence s ensitivity of the modified instrument, measured with a quantitative fl uorescence microbead standards kit, is 74 molecules of equivalent solu ble fluorochrome for fluorescein. The instrument can now detect extrem ely dim P. marinus from open-ocean surface waters.