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Dinophytes
(Dinoflagellates, Pyrrhophytes)
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Abundant
in
freshwater and marine systems, planktic and benthic, endosymbionts (zooxanthellae)
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Appearance:
2 – 2000 µm in size, two flagella, one of which runs around the cell
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Thecate
forms possess a layer of cellulose plates underneath their cell membrane
(internal plates, in contrast to coccoliths!)
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Phototrophy
and plastids occur in ca. 50% of dinophytes; the other half is obligate
heterotrophic (phagotrophic or parasitic)
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Toxic
species: ca. 60, all phototrophic, coastal marine species /w benthic cysts
(cause red tides)
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Tertiary
plastids of varying origin (cryptophytes,
chlorophytes, diatoms)
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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
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Flagella:
One
flagellum extends free posteriorly from cell (forward movement), the other
wraps transversally around cell in the girdle (rotation)
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Systematics
of dinoflagellates based on number and structure of cellulose plates and
spines
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Cellulose
plates (thecal plates) are individually contained
within membrance-enclosed vesicles
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Valves
are thecal plates of species that possess only two plates to cover the
left and right cell half
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Cell
divided in posterior and anterior half by deep grove = girdle
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Epicone
= anterior half of the theca
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Hypocone
= posterior half of the theca
Dinophyte Motility
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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
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Transversal
flagellum is ribbon-shaped; in addition to
the 9+2 microtubuli it contains a striated, contractile band of the protein
centrin
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Speed
up to 500 µm s-1 = 1.8 m h-1, ±20 m
per day
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Phototaxis
is demonstrated in a number of species
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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
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Eye-spots
are lacking in most phototactic dinoflagellates
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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
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Dinokaryon
is
the unusually large nucleus of dinophytes; chromosomes lack histone proteins
and are permanently condensed
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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;
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Caryokinesis
occurs after mitosis by simple division
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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
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Maximum
division rate: 1 per day
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Timing
of cell division: at night
Sexual Reproduction
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Induction
by nitrogen limitation or asudden change
in temperature
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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.
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Planozygotes
are diploid, flagellate cellswith two longitudinal flagella
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Hypnozygotes
(„sleeping zygotes“): non-flagellate cells ressembling resting cysts
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Meiosis
occurs at zygote germination so that vegetative cellsare haploid in most
species
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Amoeboid
and other stages may be part of the life cycle in dinophytes; Pfiesteria
piscida exhibits ca. 24 different lifecycle 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)
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Resting
cysts can arise from sexual production or
from mitoticdivision; sustain unfavorable conditions for long periods
of time;mostly red pigmented, photosynthetic pigments reduced
Phagotrophy in Dinophytes
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Occurrence
in marine and freshwater species,photosynthetic
and colorless forms
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Function:
obtain organic nutrients (nitrogen)
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Prey
may be other dinophytes, other algae, ciliates, nematodes, invertebrate
larvae, fish
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Feeding
mechanisms:
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Engulfment
of whole cells; mainly naked forms
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Peduncle:
extrusion of plasma forming a feedingtube; peduncle can engulf whole cells
or penetrateprey cell walls and suck in prey plasma; mostthecate dinophytes
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Pallium:
plasma extension forming a feeding veil;prey potoplasma is enzymatically
digested in thepallium, digestion products are transported to cell
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Parasites:
nonmotile dinophytes, may contain pigments;produce motile zoospores, which
find a new host and attach by their peduncle; cells then mature to nonmotileforms
Dinophyte Ecology
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Most
important phytoplankton besides diatoms in
marine waters
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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
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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)
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Storage
capacity for phosphorus is extremely high
Dinophyte Diversity
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