Amino acid profiling in natural organic matter isolated by reverse osmosisfrom eight different boreal freshwaters

Hydrolyzable amino acids (HAA) were analysed in reverse osmosis (RO) isolat ed natural organic matter (NOM) from eight different boreal freshwaters in Norway. Dissolved organic carbon (DOC) contents in those RO isolated NOM (R O-NOM) samples varied between 3 to 22 mg C L-1 (average 8+/-7 mg C L-1) and colour between 13 to 152 mg Pt L-1 (average 62+/-55 mg Pt L-1). DOG, colou r, W-absorbance at 254 nm and iron were significantly correlated (r=0.88-0. 98, p<0.05) in those samples. C:N ratios in RO-NOM samples varied between 9 :1 to 53:1 and C:H:N ratios varied between 9:1:1 in a clear water lake and 53:5:1 in a meso-humic lake. However, C:H ratios varied only between 8:1 to 12:1 in all samples. HAA carbon (C-HAA) varied between 38 to 124 mu g C-HA A mg(-1) C-org (average 64+/-26 mu g C-HAA mg(-1) C-org) in the DOC pools. HAA nitrogen (N-HAA) varied between 5-16 mu g N-HAA mg(-1) N-org (average 9 +/-3 mu g N-HAA mg(-1) N-org). Amino acids with aliphatic- and acid-functio nal groups were the dominating amino acid fractions and contributed an aver age of 38 to 39% of their C-HAA to the total C-HAA pools. The C-HAA contrib uted on average 6% to the DOC and correlated only significantly with the C: N ratios in the RO-NOM samples (r=0.83, p<0.05). N-HAA contributed on avera ge only about 1% to the dissolved organic nitrogen (DON), but correlated in a broader scale with other parameters such as with catchment area (r=0.839 ), water retention time (r=0.827), N-NO3 (r=0.856), and C:N ratio (r=-0.693 ). Dominating HAA were Glu, Asp, Phe, Ser, His, Thr, and Leu, which all sho wed significant variation (p<0.05, from analysis of variance (ANOVA)) betwe en the eight RO-NOM samples. Although the total C-HAA and N-HAA contributed only small amounts (4-12% of DOC and 0.5-2% of DOW) to the DOC/DON pools, they are considered as significant constituents of NOM and may be used as r eliable bio-markers to characterise NOM from boreal freshwaters. Both C-HAA and N-HAA may have reflected an important part of the biological labile fr action in the RO-NOM samples, which function as potential substrates and nu trients for the indigenous micro-heterotrophs. (C) 1999 Elsevier Science Lt d.