Induction of sexual reproduction and zygospore patterns in the filamentous green alga Spirogyra (Conjugatophyceae: Zygnematales)

Abstract

Morphotaxonomy, ecological preferences and biological characterization of algal taxa of order Zygnematales (Conjugatophyceae, Chlorophyta), mainly the freshwater species Spirogyra, Zygnema, and Mougeotia are still poorly understood and need further in-depth investigations. In this study, different Spirogyra strains were examined to characterize their abilities in conjugation and formation of zygospores under different environmental conditions.  It was found that 16:8-h light:dark photoperiod is the best condition to induce the conjugation and zygospore formation. Moreover, the sexual reproduction was noticed to be motivated with increasing the light intensity up to 85 µmol photons m^-2s^-1 and no conjugation was observed at 35 µmol photons m^-2s^-1. The attribute of conjugation in red and blue lights, respectively, never equalized that in the white light even with a high intensity. pH value (7.5) was the most suitable niche for induction of sexual reproduction in the studied Spirogyra strains. An increase of CO₂ in the atmosphere, provided by NaHCO₃ solution, did not enhance the sexual reproduction. Cultivation of the investigated Spirogyra strains on 0.003% CaCl₂-containing agarized Pringhsheim´s medium (1/2 conc. and without nitrogen) induced the conjugation process as in case of CaCl₂ omitted. UV radiation completely inhibited the conjugation at all growth conditions.