Toh Hii Tan | Universiti Malaysia Sarawak (UNIMAS) (original) (raw)
Papers by Toh Hii Tan
A recent (February 2014) mass mortality of fishes was observed in the cage-farming region of the ... more A recent (February 2014) mass mortality of fishes was observed in the cage-farming region of the West Johor Strait of Malaysia, involving over four different species of cultured fishes, numbering 50,000 fish. A field investigation at six stations along the West Johor Strait collected water samples and examined for the presence of harmful species. Dead fishes were collected for necropsy. The phytoplankton composition was dominated by a species of Karlodinium, at a considerably high cell concentration (0.31–2.34 106 cells l1), and constituting 68.8–98.6% of the phytoplankton relative abundance at all stations. Detailed morphological assessment by light and scanning electron microscopy revealed that the species was Karlodinium australe de Salas, Bolch and Hallegraeff. This was supported by molecular evidence of the nuclear encoded large subunit ribosomal gene (LSU rDNA) and the second internal transcribed spacer (ITS2) via single-cell PCR. The sequences of LSU rDNA yielded 3.6–4.0% divergence when compared to the sister taxon, K. armiger; and >6.5% when compared to other Karlodinium species. Fish necropsy showed symptoms similar to those affected by karlotoxin ichthyotoxins. This is the first report of a mass mortality of cage-cultured and wild fishes attributed to the unarmored dinoflagellate K. australe.
Recently, molecular genetics of the Saxitoxin (STX) biosynthesis pathway has become one of the ma... more Recently, molecular genetics of the Saxitoxin (STX) biosynthesis pathway has become one of the major focuses in paralytic shellfish poisoning (PSP) toxin-related studies after the recent discovery of STX biosynthetic genes in toxic cyanobacteria and later in the toxic dinoflagellates. Here we described the two domains of sxtA, SAM-dependent methyltransferase coding gene, sxtA1 and the class II aminotransferase coding gene, sxtA4 from a toxic strain of Alexandrium tamiyavanichii isolated from Samariang, Sarawak. The partial coding sequences of sxtA1 and sxtA4 were 432 bp and 639 bp respectively, with the deduced amino acid sequences revealed polypeptides of 144 and 213 amino acid residues, respectively. The present results showed high sequence similarity and identity (~91% and ~98%, respectively) compared to other PSP toxins-producing dinoflagellates. Indeed, our protein phylogenetic analyses revealed close relationship of both A. tamiyavanichii sxtA1 and sxtA4 to others PSP toxins-producing dinoflagellates, with sxtA from PSP toxins-producing cyanobacteria and putative toxin-related genes forming the sister clades.
Prorocentum lima and P. rhathymum are prorocentrales dinoflagellates that have been known to prod... more Prorocentum lima and P. rhathymum are prorocentrales dinoflagellates that have been known to produce diarrhetic shellfish poisoning (DSP) toxins. Contamination of DSP toxins in commercial edible shellfish is known to cause human gastrointestinal syndromes. Although no fatality case was reported thus far, clinical experiments have proven that the lipophilic toxins are tumor promoters. In the present study, two toxic Prorocentrum species were isolated from the coasts of Malaysian Borneo and subjected to scanning electron microscopy (SEM) for detailed morphological investigation. Prorocentrum lima was isolated from seaweed samples in Kudat, Sabah. SEM observation revealed distinct morphological features of the species, particularly in the periflagellar area. Cells range in 34 µm -38 µm long and 18 µm -24 µm wide (n = 11). Prorocentrum rhathymum was isolated from plankton samples of Semariang, Sarawak. The species was distinguished by its surface pore, distinct cell shape and periflagellar collar which looks like an apical spine. Cell dimension is in the range of 23 µm -29 µm in length and 18 µm -29 µm in width (n =16). These species are potential to cause DSP intoxication or tumor related disease in the country.
A field survey was carried out in Kuching waters to monitor harmful microalgae in Kuching waters.... more A field survey was carried out in Kuching waters to monitor harmful microalgae in Kuching waters. Samples were collected fortnightly from Semariang Batu and Santubong estuaries during high tide. Live samples were used for culture establishment, while preserved samples were processed for morphological observation under epifluorescence microscopy. The occurrence of Alexandrium affine, A. tamutum and A. tamiyavanichii is reported for the first time in the coastal waters of Sarawak, with A. tamutum as a new record in Malaysian waters, which increased the number of Alexandrium species found to eight species. They are A. affine, A. leei, A. minutum, A. peruvianum, A. tamarense, A. tamiyavanichii, A. tamutum and A. taylori. This study has provided further information to the microalgae species inventory of the country.
Harmful algal bloom (HAB) is a natural phenomenon due to the increase of algal cell density in th... more Harmful algal bloom (HAB) is a natural phenomenon due to the increase of algal cell density in the water column that subsequently causes deleterious effects to natural environments as well as mankind. HABs in the country mainly occurred when a particular group of dinofl agellate cells proliferate in the eutrophied semi-enclosed coastal water body. In this study, dinofl agellate species composition in the Tebrau Strait was determined by scanning electron microscope (SEM). Plankton samples were collected by a 20-micron plankton net haul at several locations of the strait. Samples were undergone fi xation, serial dehydration and followed by critical point drying. Samples were then observed under a JEOL analytical SEM. Total of 11 dinofl agellate species were identifi ed, with 7 species known to be associated with HABs events. The occurrence of a fi sh-killing unarmoured dinofl agellate, Karlodinium venefi cum was reported for the fi rst time from Malaysian waters. The presence of this and other potentially harmful dinofl agellate species in the strait should be taken seriously by the respective authorities in future expansion of aquaculture industry in the strait.
A recent (February 2014) mass mortality of fishes was observed in the cage-farming region of the ... more A recent (February 2014) mass mortality of fishes was observed in the cage-farming region of the West Johor Strait of Malaysia, involving over four different species of cultured fishes, numbering 50,000 fish. A field investigation at six stations along the West Johor Strait collected water samples and examined for the presence of harmful species. Dead fishes were collected for necropsy. The phytoplankton composition was dominated by a species of Karlodinium, at a considerably high cell concentration (0.31–2.34 106 cells l1), and constituting 68.8–98.6% of the phytoplankton relative abundance at all stations. Detailed morphological assessment by light and scanning electron microscopy revealed that the species was Karlodinium australe de Salas, Bolch and Hallegraeff. This was supported by molecular evidence of the nuclear encoded large subunit ribosomal gene (LSU rDNA) and the second internal transcribed spacer (ITS2) via single-cell PCR. The sequences of LSU rDNA yielded 3.6–4.0% divergence when compared to the sister taxon, K. armiger; and >6.5% when compared to other Karlodinium species. Fish necropsy showed symptoms similar to those affected by karlotoxin ichthyotoxins. This is the first report of a mass mortality of cage-cultured and wild fishes attributed to the unarmored dinoflagellate K. australe.
Recently, molecular genetics of the Saxitoxin (STX) biosynthesis pathway has become one of the ma... more Recently, molecular genetics of the Saxitoxin (STX) biosynthesis pathway has become one of the major focuses in paralytic shellfish poisoning (PSP) toxin-related studies after the recent discovery of STX biosynthetic genes in toxic cyanobacteria and later in the toxic dinoflagellates. Here we described the two domains of sxtA, SAM-dependent methyltransferase coding gene, sxtA1 and the class II aminotransferase coding gene, sxtA4 from a toxic strain of Alexandrium tamiyavanichii isolated from Samariang, Sarawak. The partial coding sequences of sxtA1 and sxtA4 were 432 bp and 639 bp respectively, with the deduced amino acid sequences revealed polypeptides of 144 and 213 amino acid residues, respectively. The present results showed high sequence similarity and identity (~91% and ~98%, respectively) compared to other PSP toxins-producing dinoflagellates. Indeed, our protein phylogenetic analyses revealed close relationship of both A. tamiyavanichii sxtA1 and sxtA4 to others PSP toxins-producing dinoflagellates, with sxtA from PSP toxins-producing cyanobacteria and putative toxin-related genes forming the sister clades.
Prorocentum lima and P. rhathymum are prorocentrales dinoflagellates that have been known to prod... more Prorocentum lima and P. rhathymum are prorocentrales dinoflagellates that have been known to produce diarrhetic shellfish poisoning (DSP) toxins. Contamination of DSP toxins in commercial edible shellfish is known to cause human gastrointestinal syndromes. Although no fatality case was reported thus far, clinical experiments have proven that the lipophilic toxins are tumor promoters. In the present study, two toxic Prorocentrum species were isolated from the coasts of Malaysian Borneo and subjected to scanning electron microscopy (SEM) for detailed morphological investigation. Prorocentrum lima was isolated from seaweed samples in Kudat, Sabah. SEM observation revealed distinct morphological features of the species, particularly in the periflagellar area. Cells range in 34 µm -38 µm long and 18 µm -24 µm wide (n = 11). Prorocentrum rhathymum was isolated from plankton samples of Semariang, Sarawak. The species was distinguished by its surface pore, distinct cell shape and periflagellar collar which looks like an apical spine. Cell dimension is in the range of 23 µm -29 µm in length and 18 µm -29 µm in width (n =16). These species are potential to cause DSP intoxication or tumor related disease in the country.
A field survey was carried out in Kuching waters to monitor harmful microalgae in Kuching waters.... more A field survey was carried out in Kuching waters to monitor harmful microalgae in Kuching waters. Samples were collected fortnightly from Semariang Batu and Santubong estuaries during high tide. Live samples were used for culture establishment, while preserved samples were processed for morphological observation under epifluorescence microscopy. The occurrence of Alexandrium affine, A. tamutum and A. tamiyavanichii is reported for the first time in the coastal waters of Sarawak, with A. tamutum as a new record in Malaysian waters, which increased the number of Alexandrium species found to eight species. They are A. affine, A. leei, A. minutum, A. peruvianum, A. tamarense, A. tamiyavanichii, A. tamutum and A. taylori. This study has provided further information to the microalgae species inventory of the country.
Harmful algal bloom (HAB) is a natural phenomenon due to the increase of algal cell density in th... more Harmful algal bloom (HAB) is a natural phenomenon due to the increase of algal cell density in the water column that subsequently causes deleterious effects to natural environments as well as mankind. HABs in the country mainly occurred when a particular group of dinofl agellate cells proliferate in the eutrophied semi-enclosed coastal water body. In this study, dinofl agellate species composition in the Tebrau Strait was determined by scanning electron microscope (SEM). Plankton samples were collected by a 20-micron plankton net haul at several locations of the strait. Samples were undergone fi xation, serial dehydration and followed by critical point drying. Samples were then observed under a JEOL analytical SEM. Total of 11 dinofl agellate species were identifi ed, with 7 species known to be associated with HABs events. The occurrence of a fi sh-killing unarmoured dinofl agellate, Karlodinium venefi cum was reported for the fi rst time from Malaysian waters. The presence of this and other potentially harmful dinofl agellate species in the strait should be taken seriously by the respective authorities in future expansion of aquaculture industry in the strait.