Photosynthetic response and DNA mutation of tropical, temperate and polar Chlorella under short-term UVR stress

Abstract

Changes in photosynthetic efficiency of Photosystem II can be used as an early stress indicator in phototrophs. In this study, chlorophyll fluorescence, measured by a Pulse-Amplitude Modulated Fluorometer (PAM), was used to determine the photosynthetic performance of tropical, temperate, Antarctic and Arctic Chlorella in response to short-term acute ultraviolet radiation (UVR) stress, with measurements of maximum quantum yield (Fv/Fm), photosynthetic efficiency (α), maximum electron transport rate (rETRm) and photoadaptive index (Ek). Three light treatments were conducted over a continuous, five-hour duration: (i) control subjected only to photosynthetic active radiation (PAR), (ii) PAR + UVA (UVA) and (iii) PAR + UVA + UVB (UVR). Tropical Chlorella showed better adaptive ability to UVA stress compared to strains from temperate and polar regions. UVB stress caused significant photosynthetic dysfunction in all samples, with polar strains showing a lower inhibition (about 40%) compared to the tropical strain (about 98%). Photosynthetic responses in Chlorella towards UVR are possibly origin dependent. DNA mutation induced by both UVA and UVR treatments was revealed by Random Amplified Polymorphic DNA (RAPD) analysis. Out of sixty RAPD primers tested, two primers: S33 (polymorphism degree 44.83%) and S90 (polymorphism degree 38.71%) were chosen as potential primers to conduct genomic study of UV stress in microalgae.