Document Type

Article

Department/Program

Virginia Institute of Marine Science

Publication Date

8-2006

Journal

Limnology And Oceanography

Volume

51

Issue

5

First Page

2290

Last Page

2299

Abstract

Humic substances are a collection of colored organic acids characterized by high molecular weight and low nitrogen (N) content that are thought to be biologically recalcitrant. We examined a suite of nonaxenic estuarine phytoplankton isolates to determine their ability to take up N-15-labeled humic substances formed in the laboratory and supplied as the sole N source. All 17 estuarine and coastal strains took up the added humic N, but the one polar isolate did not. Two of the coastal isolates (Heterosigma akashiwo and Fibrocapsa japonica) could take up the humic N in nonaxenic culture but not in axenic culture, suggesting that bacterial remineralization played a role in making humic N accessible to these species. The ability of nonaxenic phytoplankton isolates to use humics of different ages (1 week to 1 yr old) was tested using three strains capable of taking up humic N at high rates. Younger, fresher humics were taken up by the phytoplankton strains at higher rates than older, more fulvic-like compounds, and at rates higher than inorganic N uptake run in parallel. Time-course results indicate that while uptake of the inorganic N forms was sustained, high rates of humic N uptake declined after the first few hours of incubation. Additional humic substances were labeled with both N-15 and C-13, and the relative incorporation of N versus carbon (C) was used to infer potential uptake mechanisms. None of the isolates took up humic C, suggesting that uptake of the humic N followed breakdown of the humic molecule by bacteria or via extracellular enzyme cleavage of humic N. Regardless of the mode of uptake, the observation that humic N can be rapidly used by phytoplankton suggests that the importance of humic N as a source of phytoplankton N nutrition should be reevaluated.

DOI

10.4319/lo.2006.51.5.2290

Keywords

Biologically Available Nitrogen; Dinoflagellate Alexandrium Catenella; Dissolved Organic-Matter; Marine-Phytoplankton

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