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MG is very toxic to aquatic life: it's toxic to several species belonging to different trophic levels, like fishes, invertebrates, algae and microorganisms. In general mortality results up to low concentrations; the severity of effects depends on the exposure time and on the species.

Effects observed in fish include effects on skin and on the behaviour similar to those observed in several teleostean species exposure to various pesticides (Srivastava et al. 1993); significant modifications in the activity of LDH's (Lanari et al. 1996). Hepatopancreas, posterior kidney and spleen result the most affected organs during chronic MG exposure (El-Neweshy, 2011). Hematologic effects comprise erythrocyte and leukocyte mean levels decrease (Saglam, 2003; see 6.4).

Furthermore MG result toxic for reproduction because affects the reproductive success and reduces the birth rate (Adeymo, 2011)

MG shows a greater sensitivity at basic pH (pH=9.5) and this is probably due to conversion to cationic form (carbinol)(Bills et al. 1977).The absence of positive charge may facilitate the absorption, whichoccurs almost exclusively through the gills (Plakas et al. 1995; see 6.5).The toxicity of MG is not affected by variation in water hardness (Bills et al. 1993) and by variation of temperature.

Studies on biotransformation and kinetics show that MG is fairly quickly taken up by the fish (within 12 hours), and transformed to Leucomalachite Green (LG), which then is only slowly excreted/degraded from the fish (Kuiper, 2004; see 6.5). MG is characterized by rapid and pH-dependent uptake during waterborne exposure, wide distribution and concentration in the tissues; MG results persistent in tissue (Plakas, 1995; see 6.5).

Schuetze (2008) confirms that LG is the dominating residue in tissue and that the ratio measured for LG and MG varied between 5:1 and 7:1.

The risk for aquatic environmental is very high, so MG should not be used without appropriated risk management measures to avoid any release of the substance directly in aquatic environment or in the municipal waste water.