Dinoflagellates, a major unicellular group (>3000 species), are a significant group of phytoplankton, the intracellular symbiotic microalgae (zooxanthellae) of corals (and many other invertebrates), and major causative agents of harmful algal blooms. Many species are also important in aquaculture, both as disease agents and as feed supplements.
Research in my lab focuses on three inter-related aspects of dinoflagellate cell biology, the architectural organization of liquid crystalline chromosomes, the biogenesis of their cellulosic thecal plates and cell-life cycle transitions. Cellulose, membrane lipids and nucleic acids have to be the three most important biopolymers of lifeforms. Dinoflagellates are famous for production of large diversity of secondary metabolites, including many toxins (e.g. ciguatera toxins) and is an industrial producer of omega-3 PUFAs. Dinoflagellates are rich sources of investigative problems in fundamental and applied cell biology. Recent development in genomics and transgenesis make it strategic to consider the group as a new frontier for molecular biotechnology.
Liquid Crystalline Chromosomes. At high concentrations, aqueous DNAs can form liquid crystalline phases. Biophysical evidences suggested highly anisotropic organization, manifested as strong birefringence when observed under polarizing light (see figure; Chow et al., 2010: Eukaryot. Cell) These dinoflagellates Liquid Crystalline Chromosomes (LCCs) encode some of the largest-known eukaryotic genomes (up to 80 times human genome size) but counter-intuitively had no detectable nucleosomes and have the lowest known chromosomal protein-to-DNA ratios in extant life-forms. Their histone-like proteins, which bear no relationship with core histones (Wong et al., 2003; Eukaryot. Cell) organized DNAs in a concentration-dependent manner, including looping of DNAs (Chan et al., 2007, NAR). Architectural organization of LCCs (Wong 2019, doi:10.3390/microorganisms7020027) will be of general interests in understanding how highly condensed genome-sized DNAs can be organized, replicated and transcribed.
Cellulosic Thecal Plates. Cellulose is the most abundant biopolymer on earth. Thecate dinoflagellates are well known in their ability to produce intricate cellulosic thecal plates (CTPs), which are intracellular and three-dimensional in well-defined shapes, contrast with extracellular and two-dimensional nature of plant cell wall. CTPs are deposited in precision (See figure), widely used as taxonomic characters, and have the hardness of wood (Lau et al., 2007 Nanosci. And Nanotechnol.). We are interested in the mechanism leading to biodeposition of CTPs and its potential biotechnological application. Knockdown of a dinoflagellate cellulose synthase led to severe malformation of CTP and impediment of life-cycle transition (Chan et al.,2019; Front. Microbiol.).
Cell size and life-cycle/cell-cycle transitions. Cell size is not only an intriguing philosophical problem, but have both applied and biological applications, affecting buoyancy (or sinking), cell harvesting and prey-predator relationship. Dinoflagellate chromosomes and cell wall remodel during life-cycle and cell-cycle transitions, which are critical in the dynamics of red-tides and in coral-zooxanthallae symbiotic relationship. Both ecdysal cyst formation and cellular growth involved calcium transient and depletion of cortical calcium stores led to cell-wall remodelling (Tsim et al., 1997; J.Cell Sci.; Lam et al., 2005; Yeung et al., 2006 Cell Calcium). Cellular cellulose, lipid and size increased stepwise with cell cycle progression, reflecting coordination with growth at late G1 (Kwok and Wong, 2003 (Plant Physiol.), 2005(Plant Cell Physiol.)); it was coincident with growth-induced cyclic ADP-ribose transient as the switch between binary versus multiple fission (see figure, a 32 cell stage; Lam et al., 2009, Cell Calcium); it was one of the few cases in which the link between a growth signal at G1 was biochemicaly linked to cell division. Activities of a walled-bound cellulase was coupled to and was required for cell cycle progression (Kwok and Wong 2010, Plant Cell).