Diatoms　and　dinoflagellates　dominate　coastal　marine　phytoplankton　communities　as　major　players　of　marine　biogeochemical　cycles　and　their　seasonal　succession　often　leads　to　harmful　algal　blooms　(HABs).　What　regulates　their　respective　dominances　and　the　development　of　the　HABs　remains　elusive.　Here　we　conducted　time-sequential　metatranscriptomic　profiling　on　a　natural　assemblage　that　evolved　from　diatom　dominance　to　a　dinoflagellate　bloom　to　interrogate　the　underlying　major　metabolic　and　ecological　drivers.　Data　reveals　similarity　between　diatoms　and　dinoflagellates　in　exhibiting　high　capacities　of　energy　production,　nutrient　acquisition,　and　stress　protection　in　their　respective　dominance　stages.　The　diatom-to-dinoflagellate　succession　coincided　with　an　increase　in　turbidity　and　sharp　declines　in　silicate　and　phosphate　availability,　concomitant　with　the　transcriptomic　shift　from　expression　of　silicate　uptake　and　urea　utilization　genes　in　diatoms　to　that　of　genes　for　light　harvesting,　diversified　phosphorus　acquisition　and　autophagy-based　internal　nutrient　recycling　in　dinoflagellates.　Furthermore,　the　diatom-dominant　community　featured　strong　potential　to　carbohydrate　metabolism　and　a　strikingly　high　expression　of　trypsin　potentially　promoting　frustule　building.　In　contrast,　the　dinoflagellate　bloom　featured　elevated　expression　of　xanthorhodopsin,　and　antimicrobial　defensin　genes,　indicating　potential　importance　of　energy　harnessing　and　microbial　defense　in　bloom　development.　This　study　sheds　light　on　mechanisms　potentially　governing　diatom-　and　dinoflagellate-dominance　and　regulating　bloom　development　in　the　natural　environment　and　raises　new　questions　to　be　addressed　in　future　studies.　©　2019　Zhang,　Lin,　Shi,　Lin,　Luo,　Li　and　Lin.