the food web by heterotrophic bacteria.
(^) (After Kirchman et al. 2009.)
(^) Realization that heterotrophic bacteria are numerous in ocean waters came late in
the development of biological oceanography (Pomeroy 1974; Hobbie et al. 1977).
This was because the dominant means up to that time for estimating their abundance
was plate culture. Seawater agar, usually with substantial nutrient enrichment
(“peptone”, beef extract, or other amendment), would be sterilized, gelled, inoculated
with a small volume of seawater collected with a sterile sampler, and allowed to
incubate for a few days. Typically, the plates would show a few hundred colonies per
milliliter of near-surface seawater, each presumed to have started with a single
bacterium. Compared to marsh water, sediment or soil, these were minuscule
numbers, so bacteria were taken to have a very limited role in the marine pelagial.
Some of the cultured forms were shown to have specific roles, including mediation of
specific steps in the marine nitrogen cycle. Others were studied for the dependence of
their growth on cold (psychrophiles) or pressure (barophiles). Lots of interesting work
was accomplished with these bacteria, but, given the vanishingly small populations, it
did not appear to have much relevance to trophic processes in the ocean.
(^) Then, in the 1970s, perhaps suggested by reading of Russian work (Sorokin 1964),
but with hints from work in the West (Jannasch & Jones 1959), the alternative
technique of direct microscopic counts was tried. Pelagic bacteria are small; cell
diameters of 0.3 to 1.0 μm are typical, the mean about 0.6 μm. An object of 0.3 μm is
near the resolution limit of light microscopy. So, special techniques are needed to
make reliable counts that distinguish bacterial cells from tiny bits of organic or
inorganic detritus. A moderately priced scheme introduced by Francisco et al. (1973)
is to strain bacteria from a water sample with a flat, black plastic filter, then to stain
the cells retained on the filter with fluorescent dyes, such as acridine orange, that bind
specifically to nucleic acids. Hobbie et al. (1977) developed the now standard
procedure utilizing Nuclepore™ filters. Cells are counted with an epifluorescence
microscope (see Box 2.2), producing what is called an “acridine orange direct count”.
Following the biological acronym habit, this became AODC. Typical numbers are 0.5