Fig. 5. Nitrogen status of Ceratodictyon spongiosum in the field and in culture, as measured by the enhancement of dark carbon fixation in the presence of ammonium (20µmol l^-1 NH₄Cl). C. spongiosum was first cultured in 100µmol l^-1 NH₄Cl for 3 weeks and then either deprived of nitrogen for up to 6 weeks or provided with 100µmol l^-1 NH₄Cl for a further 6weeks (+NH₄⁺). Freshly collected C. spongiosum (`Field') was analysed within 24h. N=3 for each time point/condition. Values are means ± S.D. FSW, filtered sea water