Massive multiplication of genome and ribosomes in dormant cells (akinetes) of Aphanizomenon ovalisporum (Cyanobacteria)

Abstract

Akinetes are dormancy cells commonly found among filamentous cyanobacteria, many of which are toxic and/or nuisance, bloom-forming species. Development of akinetes from vegetative cells is a process that involves morphological and biochemical modifications. Here, we applied a single-cell approach to quantify genome and ribosome content of akinetes and vegetative cells in Aphanizomenon ovalisporum (Cyanobacteria). Vegetative cells of A. ovalisporum were naturally polyploid and contained, on average, eight genome copies per cell. However, the chromosomal content of akinetes increased up to 450 copies, with an average value of 119 genome copies per akinete, 15-fold higher than that in vegetative cells. On the basis of fluorescence in situ hybridization, with a probe targeting 16S rRNA, and detection with confocal laser scanning microscopy, we conclude that ribosomes accumulated in akinetes to a higher level than that found in vegetative cells. We further present evidence that this massive accumulation of nucleic acids in akinetes is likely supported by phosphate supplied from inorganic polyphosphate bodies that were abundantly present in vegetative cells, but notably absent from akinetes. These results are interpreted in the context of cellular investments for proliferation following a long-term dormancy, as the high nucleic acid content would provide the basis for extended survival, rapid resumption of metabolic activity and cell division upon germination.

Evaluating rRNA as an indicator of microbial activity in environmental communities: limitations and uses

“…Again, since growth activity likely accounts for much of total activity, these results indicate that using rRNA concentration to assess current activity or changes in activity over time is problematic. Further evidence showing potential for misleading environmental interpretations includes significant increase in cellular rRNA in Aphanizomenon ovalisporum cells transitioning from vegetative to dormant state (Sukenik et al, 2012). These results indicate that a measurable increase in rRNA abundance does not necessarily indicate an increase in activity.…”

“…Commonly, RNA or rRNA concentration is normalized to the number of cells using DNA concentration to calculate the RNA:DNA or an rRNA:rRNA gene ratio (for example, Kemp et al, 1993;Kerkhof and Ward, 1993;Poulsen et al, 1993;Muttray et al, 2001), since DNA concentration per cell is generally more stable than RNA concentration. Note, however, that while cell genome content commonly varies less than RNA content, genome abundance per cell can vary significantly and therefore could influence RNA:DNA measurements (Schaechter et al, 1958;Cooper and Helmstetter, 1968;Sukenik et al, 2012), but this issue will not be addressed here.…”

“…Dormant organisms contain measurable amounts of rRNA (Chambon et al, 1968) and in some cases can contain significantly more rRNA in dormancy than in a vegetative state (Sukenik et al, 2012). Detectability of rRNA in dormant cells can be affected more by methodology (due to changes in cell structure) than by low levels of rRNA (Filion et al, 2009).…”

“…For example, the relationship between microbial activity and measurable rRNA can be influenced by heterogeneity of cell physiology within a population (Licht et al, 1999), changes in the ratio of non-growth to growth-specific metabolic activity, life history (Oda et al, 2000), life strategy (Flärdh et al, 1992;Lepp and Schmidt, 1998;Mitchell et al, 2009;Sukenik et al, 2012), sample heterogeneity, changing environmental conditions and of course fundamental enzyme kinetics (that is, substrate concentration). Additionally, the concentration of rRNA in a cell at a given point in time is the net result of rRNA synthesis (that is, transcription) and degradation rates (Gausing, 1977), each of which may be under distinct controls.…”

“…3. Dormant cells can contain high numbers of ribosomes; therefore, in environments that could likely contain dormant cells, employing rRNA to identify current activity is highly problematic (Chaloupecky, 1964;Bishop and Doi, 1966;Chambon et al, 1968;Filion et al, 2009;Sukenik et al, 2012). 4.…”