UTILIZATION OF DISSOLVED NITROGEN BY HETEROTROPHIC BACTERIOPLANKTON -EFFECT OF SUBSTRATE C N RATIO/

The significance of dissolved combined amino acids (DCAA), dissolved f ree amino acids (DFAA), and dissolved DNA (D-DNA) as sources of C and N for marine bacteria in batch cultures with variable substrate C/N ra tios was studied. Glucose, ammonium, alanine, and phosphate were added to the cultures to produce C/N ratios of 5, 10, and 15 and to ensure that phosphorus was not limiting. Maximum bacterial particulate organi c carbon production (after 25 h of incubation) was inversely correlate d with the C/N ratio: with the addition of identical amounts of carbon , the levels of production were 9.0-, 10.0-, and 11.1-fold higher at C /N ratios of 15, 10, and 5, respectively, relative to an unamended con trol. The bacterial growth efficiency increased from 22% (control cult ures) to 44 to 53% in the cultures with manipulated C/N ratios (C/N-ma nipulated cultures). Net carbon incorporation from DCAA, DFAA, and D-D NA supported on average 19, 4, and 3% (control cultures and cultures t o which only phosphate was added [+P cultures]) and 5, 4, and 0.3% of the particulate organic carbon production (C/N-manipulated cultures), respectively. In the C/N-manipulated cultures, a 2.6- to 3.4-fold-high er level of incorporation of DCAA, relative to that in the control cul tures, occurred. Incorporation of D-DNA increased with the substrate C /N ratio, suggesting that D-DNA mainly was a source of N to the bacter ia. Organic N (DCAA, DFAA, and D-DNA) sustained 14 to 49% of the net b acterial N production. NH4+ was the dominant N source and constituted 55 to 99% of the total N uptake. NO3- contributed up to 23% to the tot al N uptake but was released in two cultures. The studied N compounds sustained nearly all of the bacterial N demand. Our results show that the C/N ratio of dissolved organic matter available to bacteria has a significant influence on the incorporation of individual compounds lik e DCAA and D-DNA.