Effects of Cu(II) and Zn(II) on growth and cell morphology of thraustochytrids isolated from fallen mangrove leaves in Taiwan

Three Schizochytrium species isolated from three mangrove sites in Taiwan, Erjen River in Kaohsiung (isolate EJ56), Yantian Ecological Park in Tainan (isolate YT90), and Chunan mangrove in Miaoli (isolate CN75), were screened for their tolerance to heavy metals. Two common heavy metal pollutants present in Taiwan rivers were used in this study: copper [Cu(II)] and zinc [Zn(II)]. Growth at varying concentrations of these heavy metals was assessed, as well as their effects on cell morphology. There was a significant reduction in growth of isolate YT90 exposed to minimal concentrations (0.5-8 ppm) of Cu(II) and Zn(II), and isolate CN75 exposed to Cu(II). Isolate EJ56 had growth comparable to the control at 0.5-8 ppm of Cu(II) and Zn(II). Tolerance of the test isolates varied between each of the heavy metals tested. YT90 was most tolerant to Cu(II), and CN75 to Zn(II). Light and scanning electron micrographs of Schizochytrium cells exposed to high levels of heavy metals (> 32 ppm) showed formation of holes in the cell wall, as well as shrinkage and general deformation of the cell structure. At the highest heavy metal concentrations tested (256-384 ppm), cell lysis occurred, resulting in leakage of cellular content. Hence, high concentrations of heavy metal pollutants in mangrove habitats will result in growth retardation of thraustochytrids and, eventually, cell death.