Enhancement of heat tolerance by salt stress in Tetraselmis striata CTP4: impacts on HSP gene expression, pigments, and proximal composition

Abstract

As the world average temperature is on the rise and heat waves are becoming more prevalent, microalgal producers have been facing significant challenges regarding the time periods during which they are able to grow less thermotolerant microalgae in outdoor production facilities. Therefore, it is urgent to explore how microalgae cope with thermal stress and under which growth conditions tolerance to temperatures close to or higher than their maximum thermal threshold is induced. For this purpose, Tetraselmis striata CTP4, a euryhaline microalga known for its thermotolerance, was selected. Tetraselmis striata belongs to the Chlorodendrophyceae, a clade that branched off early from other "core chlorophyte" clades, usually comprised of microalgae able to colonise freshwater habitats. Here, we present compelling evidence that the ability of this microalga to withstand otherwise lethal thermal upshifts to 40 °C is induced by exposure to higher salinity (35 ppt). In contrast, this response is abrogated at lower salinities. Concomitantly, the expression of genes encoding HSP70 and HSP100, two heat shock proteins known to mediate thermotolerance and tolerance to other stresses (e.g., salt stress) in fungi, animals and plants, was enhanced when exposed to both heat stress and higher salinities. This suggests that cross-protective mechanisms against abiotic stress appeared early during the evolution of the core chlorophytes and of Archaeplastida in general. This knowledge can be used to select novel strains and growth conditions that promote thermotolerance in microalgae that are grown in outdoor industrial production facilities in environments where heat waves are expected.

Bibliographic Information