Interactive effects of light and temperature on iron limitation in a marine diatom: Implications for marine productivity and carbon cycling

We examined effects of light intensity, photoperiod, and temperature on iron uptake rates, cellular iron : carbon (Fe : C) ratios, chlorophyll a, and specific growth rate in the coastal diatom Thalassiosira pseudonana. Decreasing any of these variables decreased growth and increased the cellular Fe : C ratio needed to support a given growth rate. However, because iron-limited growth rates (mu) and iron-sufficient growth rates (mu(max)) decreased by similar relative amounts, relationships between the relative degree of iron limitation (mu/mu(max)) and the mean daily concentration of biologically available dissolved inorganic iron species (mean [Fe']) varied little, as we had observed previously in three other algal species with varying light intensity. Thus, one might expect changes in light and temperature to have only a minimal effect on iron limitation in the ocean. However, because increasing light can increase [Fe'] via photolysis of ferric chelates and resulting iron redox cycling, light increases can decrease iron limitation of algal growth rate, an effect we observed here for diatom growth in media containing Fe chelates with ethylenediaminetetraacetate (EDTA). This photochemical effect is enhanced at low temperatures, which should increase algal growth rates in cold water regions of the ocean, where growth is often limited by an insufficient supply of iron. This effect could increase the ocean's biological carbon pump and thereby decrease CO(2)-linked greenhouse warming. But this cold water enhancement of iron availability may be countered by ongoing global warming, generating a potential positive feedback that further increases atmospheric CO(2) and global temperatures.