native bacteria were diluted with ultrafiltered (0.2 μm), autoclaved seawater to
densities of ∼22 bacterial cells ml−1 and enriched with 1 μM ammonium and 0.1 μM
phosphate. Some samples were also given 0.001% (w/v) of simple organic molecules
(e.g. sugars and amino acids). After 12 days at sea-surface temperatures, the cells
appeared above the detection limit in DAPI counts, 3000 cells ml−1, and then
increased at rates from 0.40 to 0.58 d−1, reaching apparently resource-limited levels of
350,000 cells ml−1 after 27–30 days. Cultured cells were comma-shaped and very
small, 0.2 × <0.9 μm. They could be enumerated with a SAR11 fluorescent probe, and
SSU rRNA sequence variations were no greater than usually associated with SAR11
rRNA from the field.
Fig. 5.5 Frequencies of bacterial types identified among 578 DNA clones of SSU
rRNA. SAR stands for Sargasso Sea, where Mullins et al. (1995) first began defining
types from sequences. Each type is narrowly defined, not a broad class of bacteria.
Thus, SAR11 is the most common “species” of bacteria in the marine pelagial.
(^) (After Giovannoni & Rappé 2000.)
Having cultured SAR11, Rappé et al. (2002) suggested the “candidate” name
Pelagibacter ubique. Its growth was inhibited altogether by a very dilute addition of
peptone. Thus, the characterization that abundant, hard-to-culture bacterioplankton
are oligotrophs, inhibited by any but very dilute organic substrates, appears to be
confirmed. Pelagibacter ubique has one of the smallest genomes of any free-living
organism, and appears to be adapted for efficient growth under limited nutrient
resources (Giovannoni et al. 2005). Comparison of proteins expressed during
exponential and stationary growth, shows that Pelagibacter ubique adapts to growth
limitation by increased synthesis of a small suite of proteins which allow quick
response to fluctuations in nutrient supply (Sowell et al. 2008).