Effects of copper on wild and tolerant strains of the lichen photobiont Trebouxia erici (Chlorophyta) and possible tolerance mechanisms

The influence of copper was assessed on wild and tolerant strains of the lichen photobiont Trebouxia erici and shown to have multiple toxic effects. Addition of 4 mM copper chloride into Trebouxia media reduced growth rates of the wild type to less than 1% of control levels. It also injured cell membranes, inhibited dehydrogenase activity, altered pigment composition, and reduced activity of photosystem II. Membrane damage was assessed through measuring electrical conductivity and potassium content, dehydrogenase activity by degree of conversion of 2,3,5-triphenyl tetrazolium chloride to red triphenylformazan, and functioning of PS II by chlorophyll a fluorescence. In respect to most parameters the tolerant strain was usually less affected by copper than the wild strain and, in some cases. not at all. We demonstrated by inductively coupled plasma atomic emission spectrometry and energy-dispersive X-ray microanalysis that enriched copper in the medium resulted in increased uptake by both wild and tolerant photobiont cells. but extracellular concentrations were significantly higher in the tolerant strain. This suggested that, at least in vitro, extracellular deposition was a mechanism of copper tolerance.