Sulphate Uptake inFucus serratus

Abstract

The brown alga Fucus serratus Linn has been shown to possess an active sulphate uptake system, which is blocked by the metabolic inhibitors DNP and DCCD, but not by the photosystem II inhibitor DCMU. The affinity of the system for sulphate was 5.9 × 10⁻⁵ M, and a monophasic response to external sulphate concentration was found in the range 10⁻⁶–10⁻³ M. Sulphate uptake was also inhibited by the Group VI anions selenate and chromate. Short-term kinetic experiments showed an initial hyperbolic uptake of $S35O42-$ for 20 s probably representing diffusion into the intracellular free spaces of the alga, after which sulphate uptake was linear. The intermediates of the sulphate activating system, APS and PAPS, appeared in the alga after 20 s and cysteine after 1 min. The sulphate intermediary metabolic pools were saturated after 2 min, and radioactivity was linearly incorporated into the sulphated fucan, fucoidan, after 2 min. A chase period of 15 min confirmed that the sulphate activating system $SO42-$ →APS→PAPS occurs in Fucus serratus. The principal metabolic fate of the assimilated sulphate was esterification to fucoidan (80% of the total ³⁵S present after a 1 h pulse) and assimilatory sulphate reduction to the sulphur amino acids was a minor pathway in this alga. The turnover of sulphate in fucoidan was rapid (t_0.5 20 h) and was much higher than the corresponding turnover of carbon under optimal photosynthetic conditions. This suggested that a preexisting polymer was sulphated; there was also no evidence for sulphated sugar nucleotides in the short-term kinetic experiments.