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Dinophytes (Dinoflagellates, Pyrrhophytes)
  • Abundant in freshwater and marine systems, planktic and benthic, endosymbionts (zooxanthellae)
  • Appearance: 2 – 2000 µm in size, two flagella, one of which runs around the cell
  • Thecate forms possess a layer of cellulose plates underneath their cell membrane (internal plates, in contrast to coccoliths!) 
  • Phototrophy and plastids occur in ca. 50% of dinophytes; the other half is obligate heterotrophic (phagotrophic or parasitic)
  • Toxic species: ca. 60, all phototrophic, coastal marine species /w benthic cysts (cause red tides)
  • Tertiary plastids of varying origin (cryptophytes, chlorophytes, diatoms)
  • Pigments: Chl.a, Chl.c, b-carotene, peridinin (characteristic accessory pigment for dinophytes), gyroxanthin (used to track toxic blooms of Gymniodinium breve)
Thecate Dinophyte Structure
  • Flagella: One flagellum extends free posteriorly from cell (forward movement), the other wraps transversally around cell in the girdle (rotation) 
  • Systematics of dinoflagellates based on number and structure of cellulose plates and spines
  • Cellulose plates (thecal plates) are individually contained within membrance-enclosed vesicles
  • Valves are thecal plates of species that possess only two plates to cover the left and right cell half
  • Cell divided in posterior and anterior half by deep grove = girdle
  • Epicone = anterior half of the theca
  • Hypocone = posterior half of the theca

Dinophyte Motility
  • Type of motion: The longitudinal flagellum propelles the cell forward; the transversal flagellum run within the girdle and is attached to the cell wall except at its end; by undulating motions of the transversal flagellum the cell rotates around its axis while swimming
  • Transversal flagellum is ribbon-shaped; in addition to the 9+2 microtubuli it contains a striated, contractile band of the protein centrin

  • Speed up to 500 µm s-1 = 1.8 m h-1, ±20 m per day
  • Phototaxis is demonstrated in a number of species
  • Light sensing seems to occur at the base of the longitudinal flagellum, probably by a protein-bonded carotenoid; flavin-based light detectors as in other flagellates are lacking and signal processing is poorly understood
  • Eye-spots are lacking in most phototactic dinoflagellates
  • Warnowia: possess a complex eye-spot (ocellus) with lense (hyalosome) and cup-shaped retinoid; mechanism is not understood but might provide visual impressions to „see“ prey

  • Warnowia sp. with ocellus
Asexual Reproduction
  • Dinokaryon is the unusually large nucleus of dinophytes; chromosomes lack histone proteins and are permanently condensed
  • Mitosis occurs within the intact nuclear envelope, but the microtubular spindle is entirely extranuclear; bundles of microtubuli pass through tunnels in the nuclear envelope to attach to chromosomes; 
  • Caryokinesis occurs after mitosis by simple division
  • Cytokinesis occurs either after shedding of theca or by splitting; daughter cells resynthesize the missing parts or whole of the theca; cells are non-motile in cytokinesis with theca shedding

  • Maximum division rate: 1 per day
  • Timing of cell division: at night
Sexual Reproduction
  • Induction by nitrogen limitation or a sudden change in temperature
  • Gametes look like vegetative cells, fusion at night in phototrophic species; isogamy and anisogamy occur

    Male (small) gamete attached to large (female) gamete of Ceratium sp.

  • Planozygotes are diploid, flagellate cells with two longitudinal flagella
  • Hypnozygotes („sleeping zygotes“): non-flagellate cells ressembling resting cysts
  • Meiosis occurs at zygote germination so that vegetative cells are haploid in most species
  • Amoeboid  and other stages may be part of the life cycle in dinophytes; Pfiesteria piscida exhibits ca. 24 different life cycle stages

    Life cycle of the fish-killing dinoflagellate Pfiesteria piscida from North Carolina's estuaries (upper) and electron microscope views of different life stages (lower)
  • Resting cysts can arise from sexual production or from mitotic division; sustain unfavorable conditions for long periods of time; mostly red pigmented, photosynthetic pigments reduced
Phagotrophy in Dinophytes
  • Occurrence in marine and freshwater species, photosynthetic and colorless forms
  • Function: obtain organic nutrients (nitrogen)
  • Prey may be other dinophytes, other algae, ciliates, nematodes, invertebrate larvae, fish
  • Feeding mechanisms:
    • Engulfment of whole cells; mainly naked forms
    • Peduncle: extrusion of plasma forming a feeding tube; peduncle can engulf whole cells or penetrate prey cell walls and suck in prey plasma; most thecate dinophytes
    • Pallium: plasma extension forming a feeding veil; prey potoplasma is enzymatically digested in the pallium, digestion products are transported to cell
  • Parasites: nonmotile dinophytes, may contain pigments; produce motile zoospores, which find a new host and attach by their peduncle; cells then mature to nonmotile forms
Dinophyte Ecology
  • Most important phytoplankton besides diatoms in marine waters
  • Extremely vulnerable to turbulence, so dinophytes predominate during calm weather; storms can destruct large numbers of cells
  • Large size results in low surface-area to volume ratio, which contrains nutrient uptake; dinophytes are >10 time less efficient in carbon production per unit biomass then nanopyhtoplankton, growth rates are lower
  • Phagotrophy and osmotrophy complement nutrient uptake (organic nitrogen and phosphorus, microorganisms) as well as photosynthetic carbon fixation
  • Tropcial waters exhibit dinophytes with long spines, outgrows, and „wings“ at greater depths (subsurface chlorophyll maximum); such outgrows are thought to increase surface are for better nutrient uptake, provide sinking resistance, and defeat grazing except for large zooplankton
  • Vertical migration over substantial depths ranges are prominent in numerous dinophytes: descent at night for nutrient uptake below thermocline, ascend in the morning for photosynthesis (enabled by phototactic response and fast swimming speeds)
  • Storage capacity for phosphorus is extremely high
Dinophyte Diversity