Heterotrophic mineralization of amino acid nitrogen in subalpine Castle Lake, California

Heterotrophic mineralization of dissolved organic nitrogen (DON) can be a major source of N for primary producers. During the summer growing season in mesooligotrophic Castle Lake, dissolved inorganic nitrogen (DIN) is often below detection (<0.1 μg-atom N l⁻¹) whereas DON can vary between 7 and 17 μg-atom N. The heterotrophic metabolism of glutamate resulted in the recycling of both carbon and nitrogen, but not at the same rate. The uptake of ¹⁵N- and ¹⁴C-labeled glutamate showed that C was preferentially assimilated relative to N resulting in an N-mineralization rate of 2.5–3.0 ng-atom N l⁻¹ h⁻¹. This suggests that heterotrophs in Castle Lake are not N-deficient, and metabolism of organic compounds results in the release of DIN which is available for primary production.