Dissolved free amino acids, combined amino acids, and DNA as sources of carbon and nitrogen to marine bacteria

Utilization of naturally-occurring dissolved free and combined amino acids (DFAA and DCAA) and dissolved DNA (D-DNA) was studied in batch cultures of bacteria from 2 shallow marine environments, Santa Rosa Sound (SRS), Florida. USA, and Flax Pond (FP), Long Island, New York, USA. In addition to control experiments, cultures were enriched with various sources of C and N. In the SRS cultures, net C assimilation of DCAA, DFAA and D-DNA made up from 4 2 to 60 %of net bacterial C production. The average, relative incorporation of C from DCAA, DFAA and D-DNA was 0.13, 0.77 and 0.10, respectively. The corresponding values of N were 0.10, 0.81 and 0.09. Addition of NH,', DFAA, or both, increased the incorporation of DCAA, DFAA and D-DNA. Incorporation of N from DCAA, DFAA and D-DNA equaled 11 1 to 219 % of the bacterial N production. Including incorporation of NH,' and release of NO,-, calculated net bacterial N incorporation made up 4 8 and 290 %of bacterial N production. In the FP experiment, net assimilation of DCAA. DFAA and D-DNA equaled 66 to 91 % (average 77 %) of net bacterial C production. Largely all C incorporation was due to assimilation of DFAA. Enrichment of the cultures with NH4+, glucose, methylamines, and high-molecular-weight dissolved organic matter all increased bacterial production. Net N incorporation of DCAA, DFAA and D-DNA corresponded to between 100 and 180 % of net bacterial N production. In 3 cultures DFAA were the dominant (> 50 %) bacterial N source. In the other cultures, DFAA and NH,' were similar in importance as an N source, and together they accounted for 80 to 99 % of bacterial N incorporation. Including uptake of NH4' and uptake or release of NO3-in the N budgets, bactenal N incorporation equaled 155 to 413 % of bactenal N production. The reason for this apparently excessive N uptake in both sets of cultures may be the release of organic nitrogen compounds. 'Addressee for reprint orders ,sources of N to aquatic bacteria. In marine bacteria, DFAA and NH,' have been found to sustain from 8 to about 50 %, and from 22 to 88 %, respectively, of the bacterial N demand (Tupas & Koike 1990, Goldman & Dennett 1991, Keil & Kirchrnan 1991a. Simon 1991). Keil & Krchrnan (1991a) observed that DFAA and NH4+ were often equally important, sustaining 90 % or more of the bacterial N requirement. Whether NH4+ or DFAA is the dominant N source appears to depend upon the composition of the available dissolved organic compounds (Goldman & Dennett 1991). Q Inter-Research 1993 ' Bacterial N production based on incorporation of thymidine, assuming a C/N ratio of 5:l (Table 4) Calculated as sum of individual contributions of DCAA, DFAA, D-DNA, NO3-and NHqt (Fig. 6)