organic nitrogen (DON) ranges from 3.5 to 7 μmol liter−1. Identified components
include amino acids and amino sugars; however, the bulk of DON is not characterized
(Benner 2002). “Bacterial biomarkers” that appear in DOC and DON are methylated
sugars, amino sugars, D-amino acids and muramic acid (Benner 2002). Using D-
amino acids and muramic acid as markers, Kaiser and Benner (2008) estimated that
25% of POC and DOC and 50% of PON and DON are derived from bacterial
residues. There is no sign that much of the refractory DOC in seawater derives from
terrestrial sources such as lignin and humic acids (Hedges et al. 1997).
Nutrient Regeneration in Microbial Food Webs
(^) An important process in microbial food webs is nutrient regeneration, especially
mineralization of organic nitrogen to ammonium and organic phosphorus to
phosphate. These originally bound nutrients can be released directly by bacteria,
released by grazers during the feeding process (sloppy feeding), or by viral lysis of
bacteria and protists. Release rates have been measured in culture with controlled
food sources and predicted by comparing elemental ratios of food source and cell
composition. Bacteria release ammonium when the C : N of the DOM utilized is <4.5,
but take up ammonium when the C : N of DOM utilized is >6.6 (Goldman et al. 1987;
Kirchman 2000). There remains some controversy over the relative roles of bacteria
and protists for ammonium regeneration. Both can release ammonium, but bacteria
collectively appear to take up as much ammonium as they release (Kirchman 2000).
Therefore, the availability of ammonium for other organisms (phytoplankton in
particular) appears to depend on the activity of heterotrophic protists.
Bacterivores, Protist Consumers of Bacteria
(^) Ocean waters contain a diverse bacterial assemblage. It is not clear what all of them
are, what they are doing, or even if all of them are metabolically active, but progress
is being made on those problems. Since the growth implied by the TdR- and leucine-
uptake rates does not fill the water solid with bacteria, the bacteria must consistently
be removed at very close to the rates at which they grow. There are three candidate
explanations for their removal. First, about half of the new cells produced at division
just die. There is no evidence for that. Second, bacteria are eaten at rates about the
same as their growth rates. Third, there is substantial and recurring cell loss due to
viral lysis. Both of these latter possibilities are part of the explanation.
(^) Ocean water harbors an elaborate array of bacterivorous protists (Sherr & Sherr
2000). These can be classified by either a phylogenetic or a functional scheme. In
practice, the two schemes overlap. The most numerous group, especially well out to