Descriptive and mechanistic toxicity of conazole fungicides using the model test alga Dunaliella tertiolecta (chlorophyceae)

Abstract

Conazole fungicides are commonly used to prevent fungal growth on turf grass and agricultural crops. As many of these sites are adjacent to coastal waterways and estuaries, there exists the potential for nontarget effects of runoff on marine organisms. This study reports 96 h EC(50) values for four selected conazole fungicides (triadimefon = 5.98 mg/L; triadimenol = 5.51 mg/L; propiconazole = 2.33 mg/L; hexaconazole = 0.91 mg/L) to the model test alga Dunaliella tertiolecta. We further investigated possible mechanisms of toxicity by examining sublethal effects of exposure on cell morphology, osmoregulatory function, and lipid composition. These mechanistic studies revealed that conazole exposure does not inhibit synthesis of the cell's glycerol osmolyte, but does result in an overall increase in cellular volume and total lipid content. Both fungi and chlorophytes rely on ergosterol to maintain membrane structure and fluidity, and we provide evidence that the sterol-inhibiting conazoles may interfere with ergosterol biosynthesis in the cell membrane of Dunaliella. These findings suggest that green algae may be especially susceptible to nontarget effects of sterol-inhibiting fungicides in marine systems.

Enhanced microalgae growth and lipid production: a study of cytostatic inhibitors and glycerol assimilation

“…Whilst published research results were unable to be replicated with Dunalliela tertiolecta (Baird and DeLorenzo, 2010) the key findings from this study reaffirmed that conazole treatment is cytostatic to microalgae and yielded increases in lipid production in Scenedesmus sp..…”

“…sodium oxamate). The second builds on the published observation that conazole fungicides halt cell replication and can lead to lipid accumulation (Baird and DeLorenzo, 2010). The third approach takes the argument that glycerol, a low value biodiesel waste product, can be used as a carbon source for heterotrophic growth, thereby making biodiesel production more efficient while sourcing a feedstock for microalgal photo/heterotrophic growth.…”

“…Conazole fungicides have been reported by Baird et al (2010) to inhibit cellular replication, and to produce an increase in both cell size and the lipid content of the microalgae Dunaliella tertiolecta (Baird and DeLorenzo, 2010). The four conazoles they tested (i.e.…”

“…Ergosterol is the major sterol component found in the plasma membranes of fungi and is also commonly synthesised in Chlorophytes such as Chlamydomonas reinhardtii and D. tertiolecta (Baird and DeLorenzo, 2010, Brumfield et al, 2010. It reportedly has a similar role to cholesterol in vertebrates, being required for the maintenance of cell membrane structure, permeability and fluidity (Nes, 1973).…”

“…In triazoles, the nitrogen heterocyclic rings function as sterol 14 α-demethylase (EC 1.14.13.70) inhibitors by binding as the sixth ligand of the heme iron and occupying the active site with the side chain subsequently interacting with the polypeptide structure (Figure 45a) (Jefcoate et al, 1969, Yoshida and Aoyama, 1987, Benvenist, 1986. In fungi, this prevents the demethylation of ergosterol precursors (lanosterol) which leads to compromised membrane function and a buildup of sterol intermediates and inhibits cellular replication (Baird and DeLorenzo, 2010, Eicher and Hauptmann, 2003, White et al, 1998, Georgopapadakou and Walsh, 1996, Barrett-Bee and Dixon, 1995, Bossche et al, 1992, Lepesheva and Waterman, 2007. Of the fungicides tested by Baird et al (2010); triadimenol, propiconazole (Figure 45b), and hexaconazole ( Figure 45c) were of notable interest with over 600% increase in total lipid over the control (Table 6).…”