Photoautotrophic picoplankton contribute significantly to phytoplankton biomass and primary production in the ocean. The smallest known photoautotroph is Prochlorococcus sp. It is unusual because its primary photosynthetic pigment is divinyl-chlorophyll a (Chl a2), a unique biomarker for this organism. The traditional view of the marine food web suggests that carbon assimilated by Prochlorococcus, and other picoautotrophs, is primarily recycled within the microbial web. High concentrations of Chl a2-degradation products found in surficial sediments and sediment trap material from the Eastern Tropical North Pacific suggest that Prochlorococcus may nonetheless contribute significantly to export production in that environment. This inference depends on the assumptions that 1) water column concentrations of Chl a2 and Chl a1 are equally good proxies for the biomass of Prochlorococcus and other phytoplankters, respectively, 2) the chlorin-conversion efficiencies of Chl a1 and Chl a2, when ingested by herbivores, are similar and 3) the diagenesis of chlorins in sediments does not alter the ratio of Chl a1 and Chl a2-derived chlorins. The first objective of this proposal is to test these assumptions to place inferences based Chl a1/ a2-ratios on a firm basis. Knowing the relative contributions of Prochlorococcus to export production would allow one to calculate the contributions of picoplankton in general. These fluxes are likely mediated by pelagic tunicates, assuming that chlorins entering the microbial loop are quickly degraded and that pelagic tunicates, or at least some of these, are the only macrozooplankters capable of grazing on Prochlorococcus. Testing these two assumptions in the field is the second objective of this proposal. The proposed work constitutes a biomarker approach that will enable us to measure the contribution of Prochlorococcus and picoplankters to export production. These tools will help us understand the sources and controls of export production in the open ocean. This added understanding will contribute to our ability to predict the effects of environmental change on open ocean ecosystem.