Bacteria : Nutritional limitations

JP Torréton, IRD

In most of the atoll lagoons, the bacteria constitute a large biomass, which is 1 to 3 times as large as that of the phytoplankton, whereas their productivity is low, amounting on the average to only 30% of the total primary production.The rates of bacterial renewal are therefore long, although they are shorter than those of the phytoplankton. These findings, together with the very low growth rates (which are among the lowest ever published in the literature) and the small proportion of active bacteria indicate that a bottom-up process is responsible for the limitation of the bacterial production. This conclusion was confirmed by bioassays using various nutrients (carbon, nitrogen and phosphorus). The results of these assays showed that on most of the atolls studied, the limitation of the bacteria depends either on  the nitrogen supply alone or on nitrogen and carbon together (see the accompanying figure). These results were confirmed on the one hand by the relations between the dissolved nutrients and the bacterial responses to these nutrients, and on the other hand, by the relations between the nutrients and the bacterial growth rates.