Mn and Cu speciation in the Tropical Atlantic: influences of Saharan dust, phytoplankton exudation and ROS (H2O2 and O2)

铜(铜)和锰(锰)是两种痕量金属，它们在表层海洋生物地球化学循环中表现出截然不同的行为，但仍然通过共同的过程紧密联系在一起。虽然铜对海洋浮游植物是一种剧毒元素，特别是在热带少营养地区占主导地位的微浮游植物，如原球藻和聚球藻，但由于锰在光系统II和许多氧化还原酶中的作用，它是所有浮游植物所必需的元素。在大多数浮游植物中，Mn2和Cu2具有相同的金属吸收系统，有毒的Cu2被非特异性转运体强烈地偏爱于Mn2。在远洋海水中，铜和锰的形态差异很大，其中锰主要以Mn2水离子形式存在，而铜主要以有机铜配位配体形式存在，可能是浮游植物对铜胁迫的响应。这两种金属在表层海洋中的氧化还原循环也不同，因为微粒MnO2可以被光还原生成可溶性Mn2，再通过Mn(III)慢慢氧化回Mn(IV)。而Cu2络合物可以直接通过光化学或通过与光生过氧化氢反应还原为铜，后者被迅速氧化回Cu2，可能有一小部分通过与海水中存在的硫醇配体络合而稳定为铜。在这项提案中，我们将通过实地工作和实验室研究相结合的方式，研究受撒哈拉沙漠沙尘严重影响的佛得角群岛附近少营养表层热带海洋中铜和锰的形态和氧化还原循环。本研究旨在阐明光氧化还原循环对热带表层海洋中铜、锰的分布和生物地球化学循环的影响。

Copper (Cu) and Manganese (Mn) are two trace metals that show contrasting behaviours in their surface ocean biogeochemical cycles, but are still strongly linked through shared processes. While Cu is an extremely toxic element to marine phytoplankton, particularly pico-phytoplankton such as Prochlorcoccus and Synechococcus that dominate oligotrophic tropical regions, contrastingly Mn is a required element for all phytoplankton due to its role in photosystem II and in many redox enzymes. Mn2+ and Cu2+ share the same metal uptake system in most phytoplankton with the toxic Cu2+ strongly favoured over Mn2+ by non-specific transporters. Cu and Mn differ greatly in their speciation in open ocean seawater with Mn present mostly as the Mn2+ aquo ion, while Cu is dominated by strong organic Cu complexing ligands that are probably produced by phytoplankton in response to Cu stress. The two metals also differ in their redox cycling in the surface ocean as particulate MnO2 can be photoreduced to produce soluble Mn2+ which is slowly oxidized back to Mn(IV) via Mn(III). While Cu2+ complexes may be reduced directly by photochemistry or via reaction with photoproduced H2O2 to Cu+, which is rapidly oxidized back to Cu2+ with possibly a small fraction stabilised as Cu+ via complexation with thiol ligands present in seawater. In this proposal, through a combination of field work and laboratory studies, we will investigate the speciation and redox cycling of Cu and Mn in the oligotrophic surface tropical ocean near the Cape Verde islands which are heavily impacted by Saharan dust. This study seeks to elucidate the role of photoredox cycling on the distribution and biogeochemical cycling of Cu and Mn in the surface tropical ocean.