Fig. 4. Arginine kinase activity in primmorphs from S. domuncula in response to silicic acid. The primmorphs were incubated for 0-5 days in the absence (white bars) of presence of 60 µmol l^-1 silicic acid (SA; black bars). In a parallel series of experiments the primmorphs were coincubated with 100 µmol l^-1 of DIDS in assays without (hatched) and with SA (cross-hatched). The enzyme activity is given in nmoles ATP generated min^-1 10 µg^-1 protein. Values are means ± S.E.M., N=5.