Inorganic nutrients, bacteria, and the microbial loop.

The realization that natural assemblages of planktonic bacteria may acquire a significant fraction of their nitrogen and phosphorus via the uptake of dissolved inorganic nutrients has modified our traditional view of these microorganisms as nutrient remineralizers in plankton communities. Bacterial uptake of inorganic nitrogen and phosphorus may place bacteria and phytoplankton in competition for growth-limiting nutrients, rather than in their traditional roles as the respective "source" and "sink" for these nutrients in the plankton. Bacterial nutrient uptake also implies that bacterivorous protozoa may play a pivotal role in the remineralization of these elements in the microbial loop. The overall contribution of bacterial utilization of inorganic nutrients to total nutrient uptake in the ocean is still poorly understood, but some generalizations are emerging with respect to the geographical areas and community physiological conditions that might elicit this behavior.