Figure 2.

Marine microbial eukaryotes known to take up ambient nitrate for intracellular storage and/or dissimilatory nitrate reduction drawn in a conceptual scheme of a natural environment with oxic and anoxic compartments. Organisms and environmental compartments are stylized and not to scale. Scenarios: (1) Benthic diatoms and foraminifers migrate actively up and down between oxic and anoxic sediment layers, or are buried in deep, anoxic sediment layers by e.g., macrofaunal activities, (2) Foraminifers move through different sediment layers and might re-fill their nitrate stores at “hotspots” of nitrate in deeper sediment layers, e.g., macrofaunal burrows, (3) Gromiids reside at the sediment surface or in anoxic subsurface layers, (4) Fungi grow in various sediment layers, (5) Pelagic diatoms sink onto the sediment after phytoplankton blooms and are re-suspended due to spring storms or macrofaunal activities, and (6) Pelagic diatoms are exposed to hypoxic or anoxic conditions inside sinking diatom-bacteria aggregates. Aside from the spatial separation into oxic and anoxic compartments, temporal variation of oxygen availability in the bottom water or inside macrofaunal burrows causes sudden shifts from oxic to anoxic conditions (and back) that may influence nitrate uptake and dissimilatory nitrate reduction by microbial eukaryotes (not shown).