Algal bioaccumulation and toxicity of platinum are increased in the presence of humic acids

Rationale. There is a growing interest for platinum in ecotoxicology, mainly because of its use in automobile exhaust catalysts. When it reaches aquatic ecosystems, platinum can interact with ligands such as natural organic matter. According to the Biotic Ligand Model, the formation of such complexes should reduce metal bioavailability. As a consequence, toxicity should decrease in the presence of organic matter. Methodology. This study focused on the uptake of platinum by two microalgae species (Chlorella fusca and Chlamydomonas reinhardtii) and its subsequent inhibitory effects on growth (96 h). Cells were exposed to platinum (5-300 mu g L-1) at three concentrations (0, 10 and 20 mg C L-1) of standard Suwannee River humic acid (SRHA). Platinum bound to humic acid was determined experimentally using partial ultrafiltration to relate metal uptake and toxicity to speciation. Results. Unexpectedly, results show that platinum toxicity, expressed as ultrafiltrable Pt (not bound to humic acid) and total Pt concentrations, is enhanced in the presence of humic acid for both algae. For C. fusca, the half maximal effective concentration (EC50) values decreased from 93 to 37 and 35 mu g L-1 of ultrafiltrable Pt in the presence of 10 and 20 mg C L-1 SRHA and from 89 to 36 and 0.31 mu g L-1 for C. reinhardtii. Discussion. In contradiction with the Biotic Ligand Model, the results show that the presence of SRHA can significantly and importantly increase platinum uptake and toxicity as determined in two unicellular green algae, C. reinhardtii and C. fusca. The present work raises the issue of the impact of platinum on microalgae under realistic environmental conditions (ubiquitous presence of organic matter), primary producers being of great ecological importance.

Automated summary

This study investigates the uptake of platinum by two microalgae species and its inhibitory effects on growth. Surprisingly, the presence of organic matter enhances platinum toxicity in the algae. This raises concerns about the impact of platinum on microalgae under realistic environmental conditions.