Growth of marine bacteria in batch and continuous culture under carbon and nitrogen limitation

In a combination of batch and continuous culture experiments involving natural assemblages of marine bacteria we examined the impact of varying both the carbon and nitrogen substrates and the substrate C:N ratio (C:N-S) on the bacterial C:N ratio (C:N-B), the gross growth efficiency (GGE), and the ability of marine bacteria to cometabolize amino acids and NH4+. We varied the C:N-S ratio from 5 : 1 (carbon limitation) to 30 : 1 (nitrogen limitation) with varying combinations of glucose, glutamate, and NH4+ (and in some cases with an amino acid mixture and with other simple carbon compounds). We found that GGE was unaffected by growth rate, the C:N-S ratio, or different combinations of carbon and nitrogen substrates, averaging similar to 50%. However the 95% confidence interval was as high as +/-20%, suggesting that experimental error may be a common problem in determining GGE in laboratory studies and a far more serious problem in making such estimates in field studies. The C:N-B ratio was invariant at similar to 4.5:1 (by atoms) under all conditions when carbon was limiting growth, but increased to similar to 7 : 1-9. 1 when reaching the stationary phase during batch growth and up to 13 :1 at very low growth rates in continuous culture. These results indicate that the C:N-B ratio has some value as a diagnostic tool for determining the physiological state of bacterial populations, although there are some concerns about its general applicability. Finally, there was absolutely no evidence that NH4+ uptake was inhibited in the presence of amino acids. The key to NH4+ uptake is the presence of a readily assimilated carbon source to balance nitrogen uptake stoichiometrically. These findings have some bearing on the recent suggestions that gradients exist in both GGE and nitrogen preference of marine bacteria from productive inshore regions to the oligotrophic open ocean.