Genome copy number of a thermophilic cyanobacterium Thermosynechococcus elongatus E542 is controlled by growth phase and nutrient availability.

ABSTRACT

Recently isolated thermophilic cyanobacterium Thermosynechococcus E542 is a promising strain for the fundamental and applied research. Here, we used several improved ploidy estimation approaches, which include qPCR, spectrofluorometry and flow cytometry to precisely determine the ploidy level in Thermosynechococcus E542 across different growth stages, nutritional, and stress conditions. The distribution of genome copies per cell among the populations of Thermosynechococcus E542 was also analysed. The strain tends to maintain 3 to 4 genome copies per cell in lag phase, early growth phase or stationary phase under standard conditions. Increased ploidy (5.5 ± 0.3) was observed in exponential phase; hence ploidy level is growth phase regulated. Nearly no monoploid cells were detected in all growth phases, and prolonged stationary phase could not yield ploidy level lower than 3 under standard conditions.

During the late growth phase, significantly higher ploidy level was observed in the presence of bicarbonate (7.6 ±0.7) and high phosphate (6.9 ±0.2) at the expense of the reduced percentage of di- and triploid cells. Meanwhile, the reduction in phosphates decreased the average ploidy level by increasing the percentage of mono- and diploid cells. In contrast, temperature and antibiotic stress reduced the percentage of mono-, di and triploid cells, yet maintaining average ploidy. The results indicate a possible causality between growth rate, stress and genome copy number across the conditions tested, but the exact mechanism is yet to be elucidated. Furthermore, the spectrofluorometric approach presented here is a quick and straightforward ploidy estimation method with reasonable accuracy.

Importance Present study revealed the genome copy number (ploidy) status in thermophilic cyanobacterium Thermosynechococcus E542 is regulated by growth phase and various environmental parameters to give us a window into understanding the role of polyploidy. Increased ploidy level is found associated with the higher metabolic activity and increased vigor by acting as backup genetic information to compensate for damage to the other chromosomal copies. Several improved ploidy estimation approaches are presented in this work that may upgrade the ploidy estimation procedure for cyanobacteria in the future. Furthermore, a new spectrofluorometric method presented here is a rapid and straightforward method of ploidy estimation with reasonable accuracy compared to other laborious methods.