Probing Nanoscale Chemical Environments of Zinc in Diatoms

Diatoms impact the biogeochemical cycling of zinc (Zn) due to elevated cellular Zn levels over other phytoplankton, and their strong biological uptake of Zn in the Southern Ocean sets the global distribution of Zn in ocean water columns. Past studies have revealed the abundance and spatial distribution of Zn in individual diatoms, while others have shown diverse Zn chemical species in whole-cell aggregates. However, intracellular and intercellular variations in Zn chemical forms remained unknown. For the first time, we applied a synchrotron X-ray nanoprobe to cultured diatoms (<i>Phaeodactylum tricornutum</i> and <i>Chaetoceros muelleri</i>) and resolved spatial heterogeneities of Zn chemistry. The result shows the partitioning of Zn between intracellular contents (soft parts) and frustules (mainly in the intracellular contents for <i>P. tricornutum</i> and similar Zn concentrations in the intracellular contents and frustules for <i>C. muelleri</i>). We found multiple Zn chemical species unevenly distributed within individual cells, where Zn-phosphoryl complexes were the most abundant followed by cysteine, histidine, biogenic silica, and carboxyl complexes. Cellular Zn species varied between individual organisms and were influenced by Zn availability. To our knowledge, this work presents the first X-ray measurements of trace metal speciation at sub-100 nm resolution in biological samples in their natural state. This research improves our ability to examine the biogeochemistry of Zn on the nanoscale and can help us understand the role of Zn in plankton growth and the Zn cycle in ocean waters.