Cottrell and Kirchman (2000) found that, in the Delaware Bay estuary, no one group
dominated the consumption of all DOM, but that a diverse assemblage of bacteria
appears to be necessary for the degradation of the composite mixture of DOM present
in the water. Cottrell and Kirchman (2003) also used MICRO-FISH to compare the
TDR and leucine incorporation by the major phylogenetic groups of bacteria present
in Delaware Bay, and found that alphaproteobacteria were the dominant substrate-
active group at salinities > 9 psu, whereas betaproteobacteria were more important in
fresh water. A review of studies of the substrates commonly used by the most
abundant groups of bacteria (del Giorgio & Gasol 2008) showed that in general
between 10% and 60% of each phylogenetic group is actively incorporating a given
substrate. Thus, at any given time only a small fraction of all bacteria are actively
utilizing a specific substrate.
Bacterial Respiration and Growth Efficiency
(^) Early and recent studies with size-fractionated samples showed that the majority of
total marine community respiration occurs in the <1 μm size fraction. Most of this
respiration is due to the activity of heterotrophic bacteria. How much of autotrophic
primary production is consumed by heterotrophic bacteria? An obvious comparison is
between estimates of bacterial production from TdR-uptake or leucine uptake and
primary production estimated by carbon uptake. If both measures are made at the
same station using the same water samples, the importance of bacterial growth can be
scaled to the autotrophic activity of the ecosystem. Ducklow (2000) assembled a
number of open ocean euphotic zone comparisons (most of them his; Table 5.1)
meeting this standard and that provided multiple estimates to allow some averaging.
The conclusion is that the ratio of bacterial production to primary production is 10 to
25% in a wide range of habitats. Bacterial growth efficiency (BGE) is the growth
yield or the amount of biomass produced (BP) relative to the total carbon required for
growth which is calculated as the sum of BP and bacterial respiration (BR). Thus,
BGE = BP/(BP + BR). The values of BGE efficiency can be determined by
simultaneous measurements of bacterial respiration and bacterial net production in
relatively short (<36 h) incubations or in dilution cultures (see Box 7.5). Values for
BGE in natural aquatic systems generally range from 0.05 to 0.5. There is a strong
positive correlation between BGE and phytoplankton production (del Giorgio & Cole
2000). In oligotrophic waters, BGE < 0.15, while in productive environments BGE
approaches 0.5.
Table 5.1 Bacterioplankton and phytoplankton properties in the open sea. All stock
estimates are based on 20 fgC per cell.
(From Ducklow 2000.)