Marine nitrogen cycle and its transition control primary production, and therefore contribute to the concentration of atmospheric carbon dioxide, which is the essential information to understand the biogeochemical cycle and the global climate system. However, the spatial and continuous observation of nitrogen dynamics at surface ocean has been scarce. Reef coral skeleton could be used to reconstruct past nutrient dynamics in tropical and subtropical ocean with decades to millennia. Nitrogen isotope of organic matter in the coral skeleton could vary with change in nitrogenous sources and mainly capture nitrogen isotopes of marine nitrate. In this paper, I reviewed the progress of nitrogen isotope proxy in reef coral skeletons together with the introduction of our studies. Nitrogen isotopes in reef coral skeletons can be a high-resolution recorder of nitrate dynamics in oligotrophic ocean.

Chemical speciation studies of trace metals in seawater have been performed for more than five decades because chemical forms of trace metals in seawater are closely related to their concentrations and biogeochemical roles in the ocean. Although some of the metals, especially iron, copper, zinc and others, are essential for phytoplankton growth, their levels in oceanic water are extremely low. On the other hand, such metals as copper show toxicity to marine microorganisms in increasing concentrations of free ion. Many researches have revealed that these trace metals in seawater form complexes with organic ligands, which depends on kinds of metal and oceanic conditions. Recently, biogeochemical behaviors of iron in seawater, which is related to controlling factor of primary production, have been focused. In the 2000s, ocean biogeochemical modeling including speciation of iron has been developed. Moreover, ocean acidification due to increasing atmospheric CO₂ is one of new topics in the 2000s because it affects not only ecosystem but also chemical forms of trace metals in seawater. Development of chemical speciation of trace metals including chemical equilibrium model is required for better understanding of biogeochemical behaviors of trace metals and their ecological roles in oceanic water.