THE ROLE OF IRON IN PHYTOPLANKTON PHOTOSYNTHESIS, AND THE POTENTIAL FOR IRON-LIMITATION OF PRIMARY PRODUCTIVITY IN THE SEA

Iron supply has been suggested to influence phytoplankton biomass, gro wth rate and species composition, as well as primary productivity in b oth high and low NO, surface waters. Recent investigations in the equa torial Pacific suggest that no single factor regulates primary product ivity. Rather, an interplay of bottom-up (i.e., ecophysiological) and top-down (i.e., ecological) factors appear to control species composit ion and growth rates. One goal of biological oceanography is to isolat e the effects of single factors from this multiplicity of interactions , and to identify the factors with a disproportionate impact. Unfortun ately, our tools, with several notable exceptions, have been largely i nadequate to the task. In particular, the standard technique of nutrie nt addition bioassays cannot be undertaken without introducing artifac ts. These so-called 'bottle effects' include reducing turbulence, isol ating the enclosed sample from nutrient resupply and grazing, trapping the isolated sample at a fixed position within the water column and t hus removing it from vertical movement through a light gradient, and e xposing the sample to potentially stimulatory or inhibitory substances on the enclosure walls. The problem faced by all users of enrichment experiments is to separate the effects of controlled nutrient addition s from uncontrolled changes in other environmental and ecological fact ors. To overcome these limitations, oceanographers have sought physiol ogical or molecular indices to diagnose nutrient limitation in natural samples. These indices are often based on reductions in the abundance of photosynthetic and other catalysts, or on changes in the efficienc y of these catalysts. Reductions in photosynthetic efficiency often ac company nutrient limitation either because of accumulation of damage, or impairment of the ability to synthesize fully functional macromolec ular assemblages. Many catalysts involved in electron transfer and red uctive biosyntheses contain iron, and the abundances of most of these catalysts decline under iron-limited conditions. Reductions of ferredo xin or cytochrome f content, nitrate assimilation rates, and dinitroge n fixation rates are amongst the diagnostics that have been used to in fer iron limitation in some marine systems. An alternative approach to diagnosing iron-limitation uses molecules whose abundance increases i n response to iron-limitation. These include cell surface iron-transpo rt proteins, and the electron transfer protein flavodoxin which replac es the Fe-S protein ferredoxin in many Fe-deficient algae and cyanobac teria.