Physiological Limitation of Phytoplankton Photosynthesis in the Eastern Equatorial Pacific Determined from Variability in the Quantum Yield of Fluorescence

On a transect study in the eastern equatorial Pacific, from the high-nutrient, low-chlorophyll tropical waters to the oligotrophic subtropical waters, we determined the variability in the maximum change in the quantum yield of chlorophyll fluorescence $(\Delta \phi_m)$ by means of a fast repetition rate fluorometer. $\Delta \phi_m$ is a quantitative measure of photochemical energy conversion efficiency in photosystem 2, the variability of which is determined by the functional organization of the photosynthetic apparatus. The results revealed that $\Delta \phi_m$ was relatively low throughout the nutrient-rich equatorial waters, providing unequivocal evidence for physiological limitation of photochemical energy conversion efficiency in the natural phytoplankton populations. Shipboard enrichment studies showed that $\Delta \phi_m$ increased following addition of nanomolar concentrations of inorganic iron as well as aerosol dust added at similar iron concentrations. This response reflects an iron-induced repair of photosystem 2 function in the enclosed bottle community. Low $\Delta \phi_m$ at the top of the Equatorial Undercurrent indicated that the amount of iron upwelled to the surface was insufficient to repair photosystem 2 function. Our results strongly suggest that iron availability limits photochemical energy the surface was photoschemical energy conversion efficiency and is the principal mechanism controlling rates of photosynthesis and growth in the nutrient-rich equatorial Pacific.