Sami Zhioua

Introduction

Cyanobacteria are phototrophic organisms that are found in waterbodies, but also in soil crusts and many surfaces, all over the planet. They adapted morphologically and physiologically to their different niches, resulting, among others, in specific structures such as heterocysts (nitrogen-fixing cells) or akinetes (spore-like cell). In addition to these adaptations, cyanobacteria have acquired or evolved different secondary metabolites that help them defend themselves against grazers or environmental stress. Some of these metabolites are called cyanotoxins and come in two broad functional classes: hepatoxic cyanotoxins such as microcystins or nodularins, and neurotoxic cyanotoxins such as anatoxin-a or saxitoxin. In vertebrates, the ingestion of these compounds can lead to health issues and even death.
A number of deaths of domestic animals have been linked to the ingestion of mats formed by the benthic cyanobacterium Microcoleus anatoxicus. This species is a filamentous cyanobacterium which does not form heterocysts or akinetes, and it is found attached on substrates in the bed of the river. Under optimal growth conditions, M. anatoxicus produces high amounts of oxygen which gets trapped under the mats, creating a positive bouncy and resulting in the mat being pulled to the surface. These floating mats contain anatoxin-a and dihydroanatoxin-a, both alkaloid neurotoxins that can kill a dog within 15-20 minutes. Moreover, M. anatoxicus produces geosmin, a volatile organic compound, whose musty smell may attract dogs.
During the last decade, toxic benthic cyanobacterial mats produced by M. anatoxicus and other cyanotoxin-producing species have become a pressing concern in many temperate countries. Because benthic cyanobacteria are less studied and not systematically monitored by governments, our knowledge about them is severely lacking. My research therefore focuses on deepening our understanding of the dynamics of benthic communities in different ecosystems and environments. My study systems include the Areuse river (Neuchâtel, Switzerland), where dogs have perished due to M. anatoxicus, but also the Lake of Neuchâtel, the Lake of La Gruyère (Fribourg, Switzerland), and watersheds in the Engadin (Graubünden, Switzerland). Overall, I am interested in how environmental parameters shape the community composition and toxicity of benthic mats, as well as in the impact of cyanotoxins on the ecosystem as a whole.

Publications

Junier P, Cailleau G, Fatton M, Udriet P, Hashmi I, Bregnard D, Corona-Ramirez A, Francesco ED, Kuhn T, Mangia N, Zhioua S, Hunkeler D, Bindschedler S, Sieber S, Gonzalez D. A cohesive Microcoleus strain cluster causes benthic cyanotoxic blooms in rivers worldwide. Water Res X. 2024 Sep 4;24:100252. doi: 10.1016/j.wroa.2024.100252. PMID: 39308956; PMCID: PMC11416633.