Abstract
We analyzed the elemental composition (C, N, P, S, K, Mg, Ca, Sr, Fe, Mn, Zn, Cu, Co, Mo, and Ni) of five marine phytoplankton species representing the four major marine phyla grown over a range of growth irradiances. We found substantial variability in the elemental composition between different species, which is consistent with previously reported differences associated with evolutionary history. We found that many elements (Fe, Mn, Zn, Cu, Co, and Mo) are enriched relative to phosphorus by about two to three orders of magnitude under irradiances that are limiting for growth, and net steady‐state uptake of element : P is often elevated under lower irradiances. For most elements, the variability in element : P due to irradiance is comparable to the variability due to phylogenetic differences at any irradiance, but often the interaction between genetic differences and the phenotypic response to irradiance amplifies the differences in elemental composition between species. The fractionation of trace elements relative to phosphorus into phytoplankton biomass under low light is consistent with depleted levels of Cu 2+ and Mn 2+ in deep chlorophyll maxima, suggesting that the export of lowlight‐ acclimated phytoplankton is a major source of trace element flux to the deep ocean and an important factor in the biogeochemical cycles of many of the biologically limiting elements in the oceans.