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.
Bibliographical noteFunding Information:
This study was supported by NSF grant BSR-8019918 to C.R.G. and an NSF predoctoral fellowship to J.P.Z. Research support of the Castle Lake group was invaluable. Special thanks are due J.C. Meeks for stimulating discussion, and J.E. Reuter, H.W. Paerl and an anonymous reviewer for critical reviews of the manuscript. M. Smith and G. Malyj typed the manuscript.