Sulphate uptake in the unicellular marine red algaRhodella maculata

Sulphate uptake by the unicellular marine red algaRhodella maculata conforms to Michaelis-Menten kinetics. Two uptake systems have been found: a low affinity system with an apparentK_m of 22 mM, and a high affinity system with an apparentK_m of 63.4 M. Transition from the low to the high affinity system can occur within 2.5 min, in response to a decrease in the ambient sulphate concentration to below 10 mM. Assimilation rates in the dark are about 20% those in the light, although enhancement by light is independent of the quanlity of light supplied above 27 mol m^-2 s^-1. Use of metabolic inhibitors indicates that photophosphorylation provides the main source of energy for sulphate assimilation, through both cyclic and non-cyclic electron flow.Abbreviations used APS-kinase ATP:adenylyl-sulphate 3-phosphotransferase (E.C. 2.7.1.25) - ATP-sulphurylase ATP:sulphate adenylyltransferase (E.C.2.7.74) - DCMU [3-(3,4-dichlorophenyl)]-1,1 dimethylurea - 2,4 DNP 2,4-dinitrophenol - DBMIB Dibromothymoquinone (2,5-dibromo-3-methyl-6-isopropyl-p-benzoquinone)