Analyzing life-history traits and lipid storage using CARS microscopy for assessing effects of copper on the fitness of Caenorhabditis elegans

Lipid storage provides energy for cell survival, growth, and reproduction and is closely related to the organismal response to stress imposed by toxic chemicals. However, the effects of toxicants on energy storage as it impacts certain life-history traits have rarely been investigated. Here, we used the nematode Caenorhabditis elegans as a test species for a chronic exposure to copper (Cu) at EC20 (0.50 mg Cu/l). Effects on the fatty acid distribution in C. elegans body were determined using coherent anti-Stokes Raman spectroscopy (CARS) to link population fitness responses with individual ecophysiological responses. Cu inhibited nematode reproductive capacity and offspring growth in addition to shortening the lifespan of exposed individuals. In adult nematodes, Cu exposure led to significant reduction of lipid storage compared to the Cu-free control: Under Cu, lipids filled only 0.5% of the nematode body volume vs. 7.5% in control nematodes, lipid droplets were on average 74% smaller and the number of tiny lipids (0–10 µm²) was increased. These results suggest that (1) Cu has an important effect on the life-history traits of nematodes; (2) the quantification of lipid storage can provide important information on the response of organisms to toxic stress; and (3) CARS microscopy is a promising tool for non-invasive quantitative and qualitative analyses of lipids as a measure of nematode fitness.