Regulation of growth and energetics of a marine bacterium by nitrogen source and iron availability

This study examined the effect of nitrogen source and iron availability on growth rates, electron trans- port system (ETS) activity and growth efficiency for the marine gamma-proteobacterium Vibrio harveyi. We found that all 3 parameters increased with increasing iron concen- tration and varied as a function of nitrogen source (amino acids, ammonium, and nitrate). At low iron concentrations, growth rates were highest and lowest in the amino acid and nitrate media, respectively. Rates were similar for the amino acid and the ammonium plus glucose media with sufficient iron (>20 nM). Growth rates were lowest when nitrate was the nitrogen source, regardless of iron concentration. ETS activity per cell in the different media mostly followed the pattern in growth rates, although ETS activity for cells using nitrate was even lower than would have been predicted from the growth rate data. Growth efficiencies for iron-sufficient cells did not vary substantially among cells using the 3 nitro- gen sources (45 to 55%), but they were substantially higher than growth efficiencies of iron-depleted cells ( 2-fold lower than iron-stressed cells using ammonium or amino acids as nitrogen sources. Although results from any single cultured bacterium should be applied cautiously to natural microbial assemblages, our data may help explain the low growth efficiencies and growth rates in iron-limited oceanic regimes with high nitrate but low chlorophyll concentrations.