The impact of macrofauna on nitrogen and carbon mineralization was investigated in sediment of the shallow water Bering Sea Shelf. The main effort was focused on the probable role of macrofauna in the production of urea and the significance of urea turnover in the production of NH,+ Macrofaunal biomass was regulated by the quality and quantity of organic nitrogen available for degradation. This was illustrated by a low macrofaunal biomass in the sediment underlying the low productive Alaska Coastal water and a high macrofaunal blornass below the highly productive Bering Shelf/Anadyr water. A high macrofaunal biomass was correlated with high rates of urea gross production, high concentrations of urea and NH,+, and high sediment-water exchange rates of urea and NHdf. Based on a conceptual model of nitrogen mineralization in the Bering Shelf/Anadyr sediment, it was suggested that urea hydrolysis could b e responsible for u p to 80 % of the gross production of NH4+ The model intimated that a substantial part of the NH4+ produced (44 %) could have been cycled within the sediment.

ABSTRACT. The potential of adenosine 5'-monophosphate (AMP), cytidine 5'-monophosphate (CMP), 1 6 s ribosomal RNA, and a protein (bovine serum albumin) to serve a s substrates for bacterial urea production was evaluated in a defaunated, anoxic marine sediment. AMP, CMP and RNA stimulated urea production and urea turnover, but CMP to a lesser degree than AMP and RNA. The increase in urea production and turnover rates took place immediately after AMP, CMP, and RNA were added to the sediment. The rapid response in urea production and turnover rates suggests that the necessary uptake mechanisms and enzymes to utilize the substrates were present constitutively. Addition of the protein mixture did not result in any measurable changes in the urea pool size, urea turnover rate, or urea production rate during the 165 h of incubation. However, an ~ncreased and continuous net NH,' production in the protein-amended sediment relative to the control sediment indicated that the added protein mixture was accessible for bacterial degradation. The results showed that purines and pyrirnidines were substrates for the bacterial urea production in the marine sediment, whereas protein was not important for urea production.

A seasonal study of sediment urea turnover rates was carried out in a shallow Danish estuary. Turnover rates decreased with sediment depth (0 to 16 cm) and were within the range 1.2 to 424.8 nrnol N cm-3 d-'. Integrated (0 to 16 cm depth) turnover rates varied between 1.5 and 16.9 nun01 N m-' d-' with a maximum in July and a minimum in January. Urea turnover followed first-order rate kinetics, as the urea turnover rate constant, k,,,,, was independent of the urea concentration, and the urea turnover rates were positively related to the latter. The urea turnover rate and rate constant were positively related to temperature. Urea production was stimulated by availability of high quality organic material (low C/N) and temperature. Urea production rates and urea concentrations in the sediment reached a maximum in July, when C-mineralization, temperature, and the quality of organic material were maximal, and there was a high benthic macrofaunal biomass. Secondary maxima in urea concentrations matched major phytoplankton sedimentation events in spring and autumn. Urea-N accounted for 7 to 55 % of the total N pool (urea + NH,+ + NO,-+ NO,') in the sed~ment surface (0 to 1 cm). The NH,+ pool was positively related to the urea turnover rate (CO to 16 cm). Urea turnover accounted for a major part of the NH,+ production in the sediment.

The present experiment showed that the marine bacteria Delaya venusta and Pseudomonas stutzen produced urea when grown in a minimal medium without an external supply of organic nitrogen. The urea production rate depended on the bacterial state of growth, and the highest urea accumulation rates in the medium were found in the growth deceleration phase and in the beginning of the stationary phase. Urea did not accumulate in D. venusta cells, whereas the intracellular accumulation of urea in P. stutzeri cells exceeded urea accumulation in the medium during exponential growth. Further, D. venusta could, in contrast to P. stutzen, hydrolyse urea. We suggest that intracellular purines and pyrimidines (in particular RNA) were potential sources for the observed urea production.

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