Xu Yixiao

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Xu
First Name
Yixiao
ORCID
0000-0001-9279-2305

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  • Article
    Influence of environmental variables on Gambierdiscus spp. (Dinophyceae) growth and distribution
    (Public Library of Science, 2016-04-13) Xu, Yixiao ; Richlen, Mindy L. ; Liefer, Justin D. ; Robertson, Alison ; Kulis, David M. ; Smith, Tyler ; Parsons, Michael L. ; Anderson, Donald M.
    Benthic dinoflagellates in the genus Gambierdiscus produce the ciguatoxin precursors responsible for the occurrence of ciguatera toxicity. The prevalence of ciguatera toxins in fish has been linked to the presence and distribution of toxin-producing species in coral reef ecosystems, which is largely determined by the presence of suitable benthic habitat and environmental conditions favorable for growth. Here using single factor experiments, we examined the effects of salinity, irradiance, and temperature on growth of 17 strains of Gambierdiscus representing eight species/phylotypes (G. belizeanus, G. caribaeus, G. carolinianus, G. carpenteri, G. pacificus, G. silvae, Gambierdiscus sp. type 4–5), most of which were established from either Marakei Island, Republic of Kiribati, or St. Thomas, United States Virgin Island (USVI). Comparable to prior studies, growth rates fell within the range of 0–0.48 divisions day-1. In the salinity and temperature studies, Gambierdiscus responded in a near Gaussian, non-linear manner typical for such studies, with optimal and suboptimal growth occurring in the range of salinities of 25 and 45 and 21.0 and 32.5°C. In the irradiance experiment, no mortality was observed; however, growth rates at 55μmol photons · m-2 · s-1 were lower than those at 110–400μmol photons · m-2 · s-1. At the extremes of the environmental conditions tested, growth rates were highly variable, evidenced by large coefficients of variability. However, significant differences in intraspecific growth rates were typically found only at optimal or near-optimal growth conditions. Polynomial regression analyses showed that maximum growth occurred at salinity and temperature levels of 30.1–38.5 and 23.8–29.2°C, respectively. Gambierdiscus growth patterns varied among species, and within individual species: G. belizeanus, G. caribaeus, G. carpenteri, and G. pacificus generally exhibited a wider range of tolerance to environmental conditions, which may explain their broad geographic distribution. In contrast, G. silvae and Gambierdiscus sp. types 4–5 all displayed a comparatively narrow range of tolerance to temperature, salinity, and irradiance.
  • Preprint
    Distribution, abundance and diversity of Gambierdiscus spp. from a ciguatera endemic area in Marakei, Republic of Kiribati
    ( 2014-01) Xu, Yixiao ; Richlen, Mindy L. ; Morton, Steve L. ; Mak, Yim Ling ; Chan, Leo Lai ; Tekiau, Aranteiti ; Anderson, Donald M.
    Ciguatera is a serious seafood poisoning syndrome caused by the consumption of ciguatoxin-contaminated finfish from tropical and subtropical regions. This study examined the community structure of ciguatera-associated dinoflagellates and the distribution pattern, taxonomy and toxicity of Gambierdiscus spp. from a high-risk area of Marakei, Republic of Kiribati. The genera Gambierdiscus, Prorocentrum, Ostreopsis, Amphidinium and Coolia were present, and generally the former three dominated the dinoflagellate assemblage. Among these three, Gambierdiscus was the most abundant dinoflagellate genus observed at three of the four sites sampled, two of which (Sites 1 and 2) were on the northern half of the island and two (Sites 3 and 4) on the southern half. The following patterns of abundance were observed among sites: (1) Average Gambierdiscus spp. abundance at the northern sites exceeded the southern sites by a factor of 19-54; and (2) Gambierdiscus spp. abundance at shallow sites (2-3 m) exceeded deeper sites (10-15 m). The distribution of Gambierdiscus spp. at Marakei corresponded with previously observed patterns of fish toxicity, with fish from southern locations being much less toxic than fish sampled north of the central channel. DNA sequencing identified three Gambierdiscus species (G. carpenteri, G. belizeanus, G. pacificus) and three previously unreported ribotypes (Gambierdiscus sp. type 4, Gambierdiscus sp. type 5, Gambierdiscus sp. type 6) in the samples; Gambierdiscus sp. type 4 may represent a Pacific clade of Gambierdiscus sp. ribotype 1. Toxicity analyses determined that Gambierdiscus sp. type 4 isolates were more toxic than the Gambierdiscus sp. type 5 and G. pacificus isolates, with toxin contents of 2.6-6.0 (mean: 4.3± 1.4), 0.010 and 0.011 fg P-CTX-1 eq cell-1, respectively. Despite low densities of Gambierdiscus spp. observed at Marakei relative to other studies in other parts of the world, the presence of low and moderately toxic populations may be sufficient to render the western coast of Marakei a high-risk area for ciguatera. The long history of toxicity along the western side of Marakei suggests that large-scale oceanographic forcings that regulate the distribution of Gambierdiscus spp. along the western side of Marakei may have remained relatively stable over that time. Chronic as well as acute exposure to ciguatoxins may therefore pose an important human health impact to the residents of Marakei.
  • Preprint
    LSU rDNA based RFLP assays for the routine identification of Gambierdiscus species
    ( 2017-04) Lyu, Yihua ; Richlen, Mindy L. ; Sehein, Taylor R. ; CHINAIN, Mireille ; Adachi, Masao ; Nishimura, Tomohiro ; Xu, Yixiao ; Parsons, Michael L. ; Smith, Tyler B. ; Zheng, Tianling ; Anderson, Donald M.
    Gambierdiscus is a genus of benthic dinoflagellates commonly associated with ciguatera fish poisoning (CFP), which is generally found in tropical or sub-tropical regions around the world. Morphologically similar species within the genus can vary in toxicity; however, species identifications are difficult or sometimes impossible using light microscopy. DNA sequencing of ribosomal RNA genes (rDNA) is thus often used to identify and describe Gambierdiscus species and ribotypes, but the expense and time can be prohibitive for routine culture screening and/or large-scale monitoring programs. This study describes a restriction fragment length polymorphism (RFLP) typing method based on analysis of the large subunit ribosomal RNA gene (rDNA) that can successfully identify at least nine of the described Gambierdiscus species and two Fukuyoa species. The software programs DNAMAN 6.0 and Restriction Enzyme Picker were used to identify a set of restriction enzymes (SpeI, HpyCH4IV, and TaqαI) capable of distinguishing most of the known Gambierdiscus species for which DNA sequences were available. This assay was tested using in silico analysis and cultured isolates, and species identifications of isolates assigned by RFLP typing were confirmed by DNA sequencing. To verify the assay and assess intra-specific heterogeneity in RFLP patterns, identifications of 63 Gambierdiscus isolates comprising ten Gambierdiscus species, one ribotype, and two Fukuyoa species were confirmed using RFLP typing, and this method was subsequently employed in the routine identification of isolates collected from the Caribbean Sea. The RFLP assay presented here reduces the time and cost associated with morphological identification via scanning electron microscopy and/or DNA sequencing, and provides a phylogenetically sensitive method for routine Gambierdiscus species assignment.