THE ENERGETICS OF EXTRACELLULAR FE(III) REDUCTION BY IRON-LIMITED CHLAMYDOMONAS REINHARDTII (CHLOROPHYTA)

Fe-limited cells of the green alga Chlamydomonas reinhardtii (Fe-limited growth rate = 0.3 d⁻¹) reduced extracellular Fe(III) to Fe(II) when Fe(III) was supplied as ferricyanide or Fe(III)-EDTA; Fe(III) reduction was stimulated by light. In both darkness and during photosynthesis, ferricyanide reduction was accompanied by a decrease in cellular NADPH levels, with a concomitant increase in NADP⁺. NADH and NAD⁺ levels were not measurably altered during ferricyanide reduction. Furthermore, cellular hexose monophosphate levels declined and 6-phosphogluconate levels increased during ferricyanide reduction. Levels of most glycolytic and tricarboxylic acid cycle intermediates were mostly unaltered. Ferricyanide reduction was also associated with a decrease in cellular ATP levels, a concomitant increase in ADP and AMP, and increased extracellular acidification. The acidification was sensitive to inhibition by the H⁺-ATPase inhibitor N,N' -dicyclohexylcarbodiimide (DCCD). We conclude that the oxidative pentose phosphate pathway provides reducing equivalents for Fe(III) reduction in darkness and also contributes reducing equivalents to Fe(III) reduction during photosynthesis. The decline in ATP was likely due to activation of the plasma membrane H⁺-ATPase during ferricyanide reduction and was not directly associated with provision of reducing equivalents.