Toxicity of graphene oxide against algae and cyanobacteria: Nanoblade-morphology-induced mechanical injury and self-protection mechanism

Graphene oxide (GO) is the most extensively studied two-dimensional material and has many potential applications in biomedicine, biotechnologies, and environmental technologies. However, its toxicological effects on aquatic organisms have not been properly investigated. Here, we compare the toxicity of differently oxidized graphene oxide systems towards the green alga Raphidocelis subcapitata and the cyanobacterium Synechococcus elongatus. The cyanobacterium exhibited higher GO sensitivity and more rapid growth inhibition than the alga, in keeping with the established antibacterial properties of GO. The toxic effects of GO included shading/aggregation of GOs and nutrient depletion; however a detailed mechanistic study revealed that GO acted against R. subcapitata via an additional, new mechanism. Remarkably, lightly oxidized GO samples induced significantly greater membrane integrity damage than more heavily oxidized GO samples. Flow cytometry and microscopy experiments revealed that lightly oxidized GO can act as a nano-blade that causes mechanical damage to algal cells, probably because of the comparatively low coverage of oxygen-bearing functionalities at the edges of such GO sheets. The degree of oxidation of GO samples thus affects their ecotoxicity. Interestingly, longer incubations activated stress-induced defense reactions involving extracellular protein and carbohydrate biosynthesis in both algae and cyanobacteria. (C) 2019 Published by Elsevier Ltd.