Marine Oligotrophic Ultramicrobacteria Have a Different Physiology to Their Eutrophic Counterparts
Fitri FEGATELLA*, Julie LIM, Staffan KJELLEBERG, and Rick CAVICCHIOLI
School of Microbiology and Immunology, The University of New South Wales, Sydney, NSW 2052, Australia
The marine ultramicrobacterium Sphingomonas Sp. strain RB2256 is a dominant bacterial species isolated from Resurrection Bay, Alaska (2). This slowly growing ultramicrobacterium displays unique characteristics which may be representative of other oligotrophic and ultramicobacterial isolates. Characteristics include a small genome (1.3 x 103 kb) (2), a single copy of rRNA operon and the retention of an ultramicro-cell size (<0.08 μm3) and a constant growth rate (0.13-0.16 h-1) over 1000- fold range in glucose concentration (3). Interestingly, growing and starved cells are equally resistant to various physiological stresses (56℃, 20% ethanol, 25 mM H2O2) (3). Furthermore, cultures experiencing nutrient limitations exhibit an immediate response to maximum growth rates in response to nutrient upshift regardless of the nutrient source encountered (3). These characteristics indicate that certain physiological processes may operate constitutively, rather than being regulated by environmental stimuli.
We are currently examining cellular responses to various growth and survival conditions. In this study, changes in cellular ribosome content were examined in batch cultures (in ASW medium with 3 mM D-glucose) during growth and up to 7 days of carbon starvation. In addition, we examined the growth rate of cells in response to the sudden addition of excess glucose to log phase and starved cells.
Maximum ribosome content was observed during mid-log phase, however this maximum content was low compared to faster growing, heterotrophic bacteria (approximately 8% of fast growing E. coli, μ= 1.5 h-1) (1). In the first instance, this low ribosome content appears to correlate with the slow growth rate (μ=0.16 h-1) and the presence of a single copy of r-RNA operon, however, on the basis of cell volume, RB2256 appears to have a higher ribosome concentration than E. coli (approximately double that of E. coli with aμ= 1.5 h-1) (1). Furthermore, late-log phase and starved cells (which have only 10% of maximum levels) display an immediate response to the sudden addition of excess glucose (3 mM) in a similar fashion to mid-log cells. Clearly, the ribosome content is sufficient to allow cells to immediately respond to nutrient upshift, even when cells have been starved for carbon. This indicates that the apparent excess of ribosomes present in mid-log phase cells is not the limiting factor contributing to a slow growth, and suggests that the ribosome efficiency and
