Life and death of Pseudokirchneriella subcapitata: physiological changes during chronological aging

Abstract

The green alga Pseudokirchneriella subcapitata is widely used in ecotoxicity assays and has great biotechnological potential as feedstock. This work aims to characterize the physiology of this alga associated with the aging resulting from the incubation of cells for 21 days, in the OECD medium, with continuous agitation and light exposure, in a batch mode. After inoculation, cells grow exponentially during 3 days, and the culture presents a typical green color. In this phase, ``young'' algal cells present, predominantly, a lunate morphology with the chloroplast occupying a large part of the cell, maximum photosynthetic activity and pigments concentration, and produce starch as a reserve material. Between the 5th and the 12th days of incubation, cells are in the stationary phase. The culture becomes less green, and the cells stop dividing (\thinspace99\% have one nucleus) and start to age. ``Old'' algal cells present chloroplast shrinkage, an abrupt decline of chlorophylls content, and photosynthetic capacity (Fv/Fm and PSII), accompanied by a degradation of starch and an increase of neutral lipids content. The onset of the death phase occurs after the 12th day and is characterized by the loss of cell membrane integrity of some algae (cell death). The culture stays, progressively, yellow, and the majority of the population (\textasciitilde93\%) is composed of live cells, chronologically ``old,'' with a significant drop in photosynthetic activity (decay\thinspace>\thinspace75\\% of Fv/Fm and PSII) and starch content. The information here achieved can be helpful when exploring the potential of this alga in toxicity studies or in biotechnological applications.