Taxon-specific contributions to export production - Development of biomarker approaches

光自养微浮游生物对海洋浮游植物生物量和初级生产有重要贡献。已知最小的光自养生物是原绿球藻（Prochlorococcus sp.）。它是不寻常的，因为它的主要光合色素是二酰叶绿素a (Chl a2)，这是该生物的独特生物标志物。海洋食物网的传统观点认为，原绿球藻和其他皮自养生物吸收的碳主要在微生物网中循环利用。在东热带北太平洋的表层沉积物和沉积物捕集器材料中发现高浓度的chl2降解产物，这表明原绿球藻可能对该环境下的出口生产做出了重大贡献。这一推断基于以下假设：1)水体中Chl a2和Chl a1的浓度分别可以很好地代表原绿球藻和其他浮游植物的生物量；2)Chl a1和Chl a2在被食草动物摄入时的氯转化效率相似；3)沉积物中氯的成岩作用不会改变Chl a1和Chl a2衍生氯的比例。本提案的第一个目标是检验这些假设，将基于Chl a1/ a2比率的推论置于坚实的基础上。了解原绿球藻对出口产量的相对贡献，就可以计算出一般浮游生物的贡献。这些通量可能是由中上层被囊动物介导的，假设进入微生物循环的氯被迅速降解，并且中上层被囊动物，或至少其中一些，是唯一能够以原绿球藻为食的大型浮游动物。在实地检验这两个假设是本建议的第二个目标。拟议的工作构成了一种生物标志物方法，使我们能够衡量原绿球藻和微浮游生物对出口生产的贡献。这些工具将帮助我们了解公海出口产品的来源和控制。这将有助于我们预测环境变化对开放海洋生态系统的影响。

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.