Molecular mechanism of glucose-6-phosphate utilization in the dinoflagellate Karenia mikimotoi Chao Zhang a,b , Hao Luo c , Liangmin Huang a , Senjie Lin c,d, * a South China Sea Institute of Oceanology, Chinese Academy of Sciences, Guangzhou, Guangdong, China b Institute of Genetic Engineering, Southern Medical University, Guangzhou, China, Guangdong Province Key Laboratory of Biochip, School of Basic Medical Science, Southern Medical University, Guangzhou, China c Key State Laboratory of Marine Environmental Science and Marine Biodiversity and Global Change Research Center, Xiamen University, Xiamen, Fujian, China d Department of Marine Sciences, University of Connecticut, Groton, CT 06340, USA A R T I C L E I N F O Article history: Received 18 June 2017 Received in revised form 19 June 2017 Accepted 19 June 2017 Available online xxx Keywords: Karenia mikimotoi Glucose-6-phosphate Suppression subtraction hybridization Alkaline phosphatase DOP utilization A B S T R A C T Phosphorus (P) is an essential nutrient for marine phytoplankton as for other living organisms, and the preferred form, dissolved inorganic phosphate (DIP), is often quickly depleted in the sunlit layer of the ocean. Phytoplankton have developed mechanisms to utilize organic forms of P (DOP). Hydrolysis of DOP to release DIP by alkaline phosphatase is believed to be the most common mechanism of DOP utilization. Little effort has been made, however, to understand other potential molecular mechanisms of utilizing different types of DOP. This study investigated the bioavailability of glucose-6-phosphate (G6P) and its underlying molecular mechanism in the dinoflagellate Karenia mikimotoi. Suppression Subtraction Hybridization (SSH) was used to identify genes up- and down-regulated during G6P utilization compared to DIP condition. The results showed that G6P supported the growth and yield of K. mikimotoi as efficiently as DIP. Neither DIP release nor AP activity was detected in the cultures grown in G6P medium, however, suggesting direct uptake of G6P. SSH analysis and RT-qPCR results showed evidence of metabolic modifications, particularly that mitochondrial ATP synthase f1 gamma subunit and thioredoxin reductase were up-regulated while diphosphatase and pyrophosphatase were down- regulated in the G6P cultures. All the results indicate that K. mikimotoi has developed a mechanism other than alkaline phosphatase to utilize G6P. © 2017 Elsevier B.V. All rights reserved. 1. Introduction Phosphorus (P) is an essential nutrient element for living organisms because it is required in nucleic acids (DNA, RNA), phospholipids (membrane constituents), inositol triphosphate (signaling molecule), reduced nicotinamide adenine dinucleotide (NADH) and its phosphorylated form (NADPH) (both reducing equivalents), and adenosine triphosphate (ATP; energy currency). Besides, many enzymes critical in major metabolic pathways and cell division cycle progression are regulated by phosphorylation and dephosphorylation (Li et al., 2016). In the sunlit layer of the ocean (euphotic zone), P is rapidly consumed for photosynthesis and resupply (mainly from terrestrial input or upwelling) is slow. As a result, the preferred form of P, dissolved inorganic phosphorus (DIP), often occur in growth-limiting concentrations in many parts of the ocean, including oligotrophic oceans and some coastal waters (Karl, 2014; Lin et al., 2016; Smith, 1984; Sylvan et al., 2006; Thingstad et al., 2005; Wu et al., 2000). Phytoplankton have evolved adaptive mechanisms to cope with DIP paucity (Lin et al., 2016), e.g. enhancing ability to take up low-abundance DIP by means of high affinity phosphate transporters (Orchard et al., 2009), scavenging P from dissolved organic phosphorus via the action of alkaline phosphatase and other hydrolytic enzymes (Benitez-Nelson and Buesseler, 1999; Reynolds et al., 2014), and decreasing phosphorus demand by substituting phospholipids with sulfolipids and nitrolipids (Van Mooy et al., 2009, 2006) or accelerating phospholipid turnover to provide short term P supply (Martin et al., 2011). A diversity of enzyme systems, such as nuclease, nucleotidase, and alkaline phosphatase, has been shown to enable phytoplank- ton to utilize P from various forms of P (Lin et al., 2016). Among these, utilization of phosphorus esters facilitated by AP is believed * Corresponding author at: Department of Marine Sciences, University of Connecticut, Groton, CT 06340, USA. E-mail addresses: senjie.lin@uconn.edu, senjie.lin@gmail.com (S. Lin). http://dx.doi.org/10.1016/j.hal.2017.06.006 1568-9883/© 2017 Elsevier B.V. All rights reserved. Harmful Algae 67 (2017) 74–84 Contents lists available at ScienceDirect Harmful Algae journal home page : www.elsevier.com/locat e/hal