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Introduction
to the Green Algae
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Appearance:
mostly bright green, because chlorophyll is little masked by accessory
pigments; some species cause reddish discoloration of water and snow
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Organisation:
from unicells, filaments, coenobia to large macroalgal thalli, some siphonaleous
forms (multinucleate)
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Distribution:
freshwater, marine, soils
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Calcium
carbonate precipitation (e.g. Halimeda)
produces substantial carbonate sand in tropical shallow regions
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Use of
green algae: production of organic compounds
(Haematococcus, Dunaliella), human food supplement (Chlorella,
Ulva),
bioassays for water pollution (Selenastrum), genetic research (Acetabularia,
Chlamydomonas, Volvox), electrophysical research (Chara)
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Endosymbionts
in freshwater animals (Chlorella) and lichens
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Evolutionary
origin: monophyletic
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Phylogeny
of green algae is not related to morphological groups, i.e. similarly appearing
species (e.g. coccoid) are not closely related and differently appearing
species are closely related
Upper
row: Charophyceans
Lower
row: chlorophycean, ulvophycean, ulvophycean
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Three
major lineages in the chlorophytes:
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Charophyceans:
named after the stonewort Chara, multicellular, multilayered thalli,
but also filamentous forms (Zygnema, Spyrogyra); closest
related to higher plants
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Ulvophyceans,
Trebouxiophyceans, Chlorophyceans: referred
to as UTC clade;
contain multicellular, multilayered thalli (Ulvophyceans) as well as filaments
and unicellular forms
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Prasinophyceans:
unicellular group
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Colorless,
phagotrophic flagellates related to the chlorophytes
(as evolutionary early ancestors) are unknown
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Phototrophy
is common to almost all green algae
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Photoheterotroph
species supplement photosynthesis by osmotrophic uptake of dissolved organic
carbon (mixotrophy), but only when light is present and photosynthesis
becomes nutrient limited
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Flagella
occur in multiples of two and lack tripartite hairs, mostly equal length
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Plastids
possess two membranes (primary plastids); chlorophyll a, b in thyllakoids
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Storage
product is starch inside the plastids (often in pyrenoid), which is unique
to chlorophytes; stains with iodine (best way to identify algae as chlorophytes)
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Eye-spots,
if present, are located inside the plastid, not outside such as in euglenophytes,
dinophytes
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Plastid
forms are diverse, but uniform within one
given genus (taxonomic marker)
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Centrioles
are generally present in chlorophytes to organize the division spindle
of microtubuli
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Closed
mitosis: nuclear envelope remains intact (Ulvophyceans)
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Metacentric
mitosis: centrioles are located near the metaphase
plate of chromosomes rather than at the spindle poles
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Open
Mitosis: nuclear envelope breaks down early
in mitosis (Charaphyceans and in higher plants)
closed,
metacentric, open mitosis
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Phycoplast:
chlorophyceans with well-developed cell wall produce a set of microtubuli
(phycoplast) parallel to the plance of cell division; separates daughter
nuclei
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Plasmodesmata
(plasma connection between cells) develop from the phycoplast
Phragmoplasts
are found in the higher Charo- phyceae; different from phycoplasts in that
phragmoplasts contain double-microtubuli
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