title: Physiological and transcriptional responses to inorganic nutrition in a tropical Pacific strain of Alexandrium minutum: Implications for the saxitoxin genes and toxin production.
abstract: Saxitoxins (STXs) constitute a family of potent sodium channel blocking toxins, causative agents of paralytic shellfish poisoning (PSP), and are produced by several species of marine dinoflagellates and cyanobacteria. Two STX-core genes, sxtA and sxtG, have been well elucidated in Alexandrium but the expression of these genes under various nutritional modes in tropical species remains unclear. This study investigates the physiological responses of a tropical Pacific strain of Alexandrium minutum growing with nitrate or ammonium, and with various nitrogen to phosphorus (N:P) supply ratios. The transcriptional responses of the sxt genes were observed. Likewise, a putative sxtI encoding O-carbamoyltransferase (herein designated as AmsxtI) was recovered from the transcriptomic data, and its expression was investigated. The results revealed that the cellular toxin quota (Qt) was higher in P-depleted, nitrate-grown cultures. With cultures at similar N:P (<16), cells grown with excess ammonium showed a higher Qtthan those grown with nitrate. sxtA1 was not expressed under any culture conditions, suggesting that this gene might not be involved in STX biosynthesis by this strain. Conversely, sxtA4 and sxtG showed positive correlations with Qt, and were up-regulated in P-depleted, nitrate-grown cultures and with excess ambient ammonium. On the other hand, AmsxtI was expressed only when induced by P-depletion, suggesting that this gene may play an important role in P-recycling metabolism, while simultaneously enhancing toxin production.
authors.affiliation: Institute of Biodiversity and Environmental Conservation, Universiti Malaysia Sarawak, Kota Samarahan 94300, Sarawak, Malaysia.
authors.forename: Kieng Soon
authors.initials: KS
authors.lastname: Hii
authors.affiliation: Bachok Marine Research Station, Institute of Ocean and Earth Sciences, University of Malaya, Bachok 16310, Kelantan, Malaysia. Electronic address: ptlim@um.edu.my.
authors.forename: Po Teen
authors.initials: PT
authors.lastname: Lim
authors.affiliation: Institute of Biodiversity and Environmental Conservation, Universiti Malaysia Sarawak, Kota Samarahan 94300, Sarawak, Malaysia.
authors.forename: Nyuk Fong
authors.initials: NF
authors.lastname: Kon
authors.affiliation: The University of Tokyo, Yayoi 1-1-1, Bunkyo-Ku, Tokyo 113-8657, Japan.
authors.forename: Yoshinobu
authors.initials: Y
authors.lastname: Takata
authors.affiliation: Faculty of Science and Technology, Universiti Kebangsaan Malaysia, 43600 Bangi, Selangor, Malaysia.
authors.forename: Gires
authors.initials: G
authors.lastname: Usup
authors.affiliation: Bachok Marine Research Station, Institute of Ocean and Earth Sciences, University of Malaya, Bachok 16310, Kelantan, Malaysia. Electronic address: cpleaw@um.edu.my.
authors.forename: Chui Pin
authors.initials: CP
authors.lastname: Leaw
mesh_terms: D064751:Ammonium Compounds / Q000378:metabolism; D004141:Dinoflagellida / Q000235:genetics / Q000502:physiology*; D009566:Nitrates / Q000378:metabolism; D009584:Nitrogen / Q000378:metabolism; D010758:Phosphorus / Q000378:metabolism; D012530:Saxitoxin / Q000096:biosynthesis* / Q000235:genetics*; D059467:Transcriptome; D014329:Tropical Climate
chemical_list: D064751:Ammonium Compounds; D009566:Nitrates; D010758:Phosphorus; D012530:Saxitoxin; D009584:Nitrogen
keywords: Malaysia; Saxitoxin biosynthesis; sxtA; sxtG; sxtI