Does carbonate ion control primary productivity in the iron-limited HNLC?

Jeff McQuaid (J Craig Venter Institute)

Abstract: In high-nutrient, low chlorophyll (HNLC) regions, the scarcity of iron drives biogeochemical cycles, controls ocean carbon flux and constrains marine productivity.  In this context, carbonate has never been considered a limiting nutrient.  However, we recently showed that phytoplankton access dilute concentrations of labile ferric iron using a transferrin-like mechanism.  These ‘phytotransferrins’ have a second-order dependency on carbonate ions, raising the intriguing possibility that the concentration of carbonate controls the bioavailability of inorganic iron.  In this presentation, we will review the chemical and biomolecular constraints behind sub-picomolar iron acquisition, revealing how a carbonate-dependent uptake mechanism became widely distributed among marine phytoplankton and across HNLC environments.  We will examine data from ocean acidification experiments which support the hypothesis that carbonate co-limits iron uptake, and we will discuss strategies phytoplankton use to escape the restrictions of second-order binding kinetics, including the surprising technique of altering cell-surface pH.  Because phytotransferrin-mediated iron uptake represents a direct biomolecular link between atmospheric CO2, iron bioavailability and ocean productivity, we will conclude with a brief discussion of the implications in the context of past and current ocean acidification events.