According to Caren Braby at the Hopkins Marine Station, Stanford University, "Phytoplankton can only live with both nutrients and sunlight, unless you count a resting cyst stage (which occur in some phytoplankton groups) which are thought to be able to produce living cells from sand or mud-embedded resting cysts. If you count cysts, then yes they can live without sunlight. If you don't count them, no they can't."