Proteomic response of Euglena gracilis to heavy metal exposure: identification of key proteins involved in heavy metal tolerance and accumulation

Contamination of the environment by heavy metals is an increasing problem globally. While the mechanisms bacteria and yeasts have developed to tolerate these metals are well covered in the published literature, much less is known about algae. In this study, the wild-type Euglena gracilis (Z-strain) and the sugar loving E. gracilis var. saccharophila (B-strain) were exposed to the heavy metals cadmium (Cd), lead (Pb) and mercury (Hg). Minimum inhibitory concentration (MIC) studies indicated that the heavy metal tolerance of both strains was in the order of Pb > Cd > Hg. Based on microwave plasma atomic emission spectroscopy (MP-AES), the maximum Cd accumulation by Z-strain was 8.1 mg Cd per gram of dry weight (DW), which makes it a Cd hyperaccumulator. 

A total of 4493 proteins were quantified by SWATH mass spectrometry to assess the effect of the heavy metals on E. gracilis at the proteome level. In response to Cd, 960 proteins in the Z-strain and 127 in the B-strain changed in relative abundance compared to the untreated control. Proteins of high abundance included the major facilitator superfamily (MFS) transporters, cadmium/zinc-transporting ATPase and heavy metal transporting P1B-ATPase. Also, there was a substantial increase in the abundance of thiol-rich proteins that are paramount in metal chelation and sequestration as well as proteins involved in cellular stress response. A potential mechanism schematic for heavy accumulation in the E. gracilis Z-strain is outlined based on the data collected. The proteomic data presented here contribute to a better understanding of the effects of the exposure of E. gracilis to heavy metals by identifying proteins and thereby genes involved in heavy metal tolerance and accumulation. The information obtained can eventually be utilized for generating highly heavy metal-tolerant Euglena strains for environmental applications.