5M DCF-DA dissolved in 0.2 m filtered seawater for 1 h at
15 ◦C. After incubation, samples were rinsed in seawater, blotted
dry, weighed, and frozen in liquid nitrogen. Fluorescence of
DCF was determined in a Perkin-Elmer LS-5 spectrofluorometer
at an excitation wavelength of 488 nm and at an emission
wavelength of 525 nm.