The authors have used a novel molecular based approach,
using cellular ribosomal RNA (rRNA), to measure specific in situ activity of
two strains of eHL-11 clade. By
measuring side scatter (SSC), as an operational index of cell size, at
different light levels specific growth rates was found. Using this data and
plotting it against rRNA cell content (rRNA cell⁻¹) resulted in a linear
correlation, a clade eHL-11 specific response. Though the correlation differed
between strains, once data was normalised for cell size both strains showed the
same slope thus showing a conserved relationship between rRNA cell⁻¹ SSC⁻¹ and
growth rate (rRNA content and activity). From this the authors were able to
assess the strain specific in situ activity and the response to environmental
factors.
Diel variation
The patterns found are consistent with other Prochlorococcus strains in that many cell responses, both genetic and physiological, are tightly coupled with light intensity, showing a substantial diel cycle in order to optimise growth. Cell division was found to occur at night whilst both rRNA synthesis and biomass accumulation increased during the day owing to the observed increase in SSC and rRNA cell⁻¹ (similar in magnitude as driven by same mechanisms). The accumulation of biomass requires more ribosomes for protein synthesis, the energy for which is provided by photosynthesis, requiring light.
Effect of environmental variables
The authors sampled two geographically close stations, yet each with a distinct combination of environmental variables. At both sites: eHL-11 was the dominant clade of Prochlorococcus, the number of cells decreased rapidly once below the mixed layer (despite depth differences of the mixed layer), the cell size was small and constant within the mixed layer and growth rates were similar and ranged from 0.2-0.4 day⁻¹ within the upper 50m.
The patterns found are consistent with other Prochlorococcus strains in that many cell responses, both genetic and physiological, are tightly coupled with light intensity, showing a substantial diel cycle in order to optimise growth. Cell division was found to occur at night whilst both rRNA synthesis and biomass accumulation increased during the day owing to the observed increase in SSC and rRNA cell⁻¹ (similar in magnitude as driven by same mechanisms). The accumulation of biomass requires more ribosomes for protein synthesis, the energy for which is provided by photosynthesis, requiring light.
Effect of environmental variables
The authors sampled two geographically close stations, yet each with a distinct combination of environmental variables. At both sites: eHL-11 was the dominant clade of Prochlorococcus, the number of cells decreased rapidly once below the mixed layer (despite depth differences of the mixed layer), the cell size was small and constant within the mixed layer and growth rates were similar and ranged from 0.2-0.4 day⁻¹ within the upper 50m.
Though no subpopulation of Prochlorococcus were found
throughout both of the water columns vertical trends in specific activity (rRNA
cell⁻¹ SSC⁻¹) and inferred growth rates vary substantially between sites and do
not follow the same trend as with cell concentrations. These differences could
be attributed by many mechanisms including temperature, light and nutrient
availability. It has been suggested by the authors that the variability in
specific activity despite relatively constant cell counts may be due to the
distribution of cells due to mixing, or abiotic pressures of viral lysis and
grazing that specifically target fast growing or highly active cells.
I believe this approach has potential, particularly in it’s application on other clades as the rRNA sequences of bacteria are conservative and sufficiently diverse allowing for oligoprimers targeting specific clades. However there are certain limitations e.g. 1) the authors state that linear relationship between rRNA cell⁻¹ and specific growth rate is clade specific to eHL-11 in which case, would this impede the use of this method on other clades? 2) rRNA content can be affected by environmental variations and cellular physiological characteristics. The clade tested is relatively well understood yet many inferences and presumptions have been made from the observed in situ activity due to the variability in the responses to several environmental factors. For that, whilst useful for measuring in situ activity within this clade, I feel it’s difficult to deduce any rational behind the observed activity.
Lin Y., Gazsi K., Lance V. P., Larkin A. A.,
Chandler J. W., Zinser E. R. and Johnson Z. I. (2013) In situ activity of a
dominant Prochlorococcus ecotype (eHL-11) from rRNA content and cell size.
Environmental Microbiology, 15(10), 2736-2747
I believe this approach has potential, particularly in it’s application on other clades as the rRNA sequences of bacteria are conservative and sufficiently diverse allowing for oligoprimers targeting specific clades. However there are certain limitations e.g. 1) the authors state that linear relationship between rRNA cell⁻¹ and specific growth rate is clade specific to eHL-11 in which case, would this impede the use of this method on other clades? 2) rRNA content can be affected by environmental variations and cellular physiological characteristics. The clade tested is relatively well understood yet many inferences and presumptions have been made from the observed in situ activity due to the variability in the responses to several environmental factors. For that, whilst useful for measuring in situ activity within this clade, I feel it’s difficult to deduce any rational behind the observed activity.
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