Green Algae

Viridiplantae (2)

green plants, is a natural group or clade of around half a million eukaryotes.

They are green because they contain chloroplasts, cell organelles able to produce food by photosynthesis

two lineages of Viridiplantae

Chlorophyta and Streptophyta

Chlorophyta and Streptophyta common ancestor (3)

AGF

Ancestral green flagellate

probably flagellate marine organism, but did not have a true cell wall on surface but was covered with polysaccharide scales

Two common ancestor hypotheses

1. simple small flagellate (1 or 2 flagella) AGF

2. a complex large cell with 4 flagella and ability to phagocytose, capturing cyanobacteria (ancestral phagocytic cells)

AGF resembles (2)

Mamiella

Dolichomastix

Ancestral phagocytic cell resembles (2)

Halosphaera

Cymbomonas

Acritarcha

1.2 billion years old

could be resting stages of prasinophyte algae (not certain)

 Proterocladus (4)

billion year old multicellular fossil primitive cladophoran algae

indirect evidence suggests multinucleated cells

fibres single row, asymmetrically branched, with apical growth

may have formed akinetes

akinetes

thick-walled, dormant resting cells produced by cyanobacteria (often referred to as blue-green algae) and some eukaryotic green algae to survive harsh environmental conditions. They store nutrients and can remain viable for years until favorable growth conditions return.

If Proterocladus is really green algae

divergence of green algae must have occurred much earlier, sometime 1.5 billion years ago

Chlorophyta characteristics (3)

closed mitosis

phycoplast

cell partition formed centripetally (from edge inwards)

Open Mitosis

nuclear envelope completely breaks down so the spindle fibers can interact with the chromosomes in the open cell cytoplasm

Closed Mitosis

The nuclear envelope remains entirely intact; chromosome separation and spindle formation happen inside the closed nucleus

phycoplast

a specialized, planar array of microtubules that organizes and guides cell division (cytokinesis)

It is arranged parallel to the plane of the new cell wall, helping position the cleavage furrow that splits the cell

Prasinodermophyta

once thought to be basal Chlorophyta

now possible third lineage alongside Chlorophyta and Streptophyta

Prasinophyta

paraphyletic group, system of branching lines up to crown UTC clade

core chlorophytes

basal: Chlorodendrophyceae

UTC clade: Ulvophyceae, Trebouxiophyceae, Chlorophyceae

UTC classes monophyletic status (3)

Chlorophyceae: monophyletic

Trebouxiophyceae: probably monophyletic

Ulvophyceae: not monophyletic, order Bryopsidales sites outside other lineages

Chlorophyceae and Trebouxiophyceae

absence of persistent mitotic spindle

Trebouxiophyceae and Ulvophyceae

counterclockwise orientation of flagella

Streptophyta characteristics (3)

open mitosis

phragmoplast

cell partition formed centrifugally (from centre to edge)

phragmoplast

cytoskeletal structure that acts as a scaffold during cell division

Streptophyta basal group (2)

Mesostigmatophyceae

only flagellates within Streptophyta

Mesostigmatophyceae contains (3)

Chlorokybaceae

 Chaetosphaeridiophyceae

Klebsormidiophyceae

ZCC Clade (4)

more derived Streptophyta group

Zygnematophyceae

Charophyceae

Coleochaetophyceae

sister group to terrestrial plants

 Zygnematophyceae

split of green lineage into Chlorophyta and Streptophyta occurred

about 1 billion years ago

very old event compared to groups like diatoms

End of Precambrian

Ediacaran period (650-540 mya)

after great Cyrogenian glaciation

Chlorophyta inhabited almost exclusively phytoplankton of oceans

Streptophyta dominated freshwater habitats, both in plankton and benthos

Paleozoic

490-400 million years ago

complex evolution of green algae

lineages of today arose

Ulvophyceae class macroalgae developed in benthos of seas

Streptophyta furthered diversified in freshwater environments

planktonic representatives of Trebouxiophyceae and Chlorophyceae also began

Mesozoic

360-60 million years ago

marine flagellates no longer dominate phytoplankton

macroscopic ulvophyte algae still present

freshwater environments, rapid evolution where Chlorophyceae and Trebouxiophyceae have created number of lineages that dominate phytoplankton

large radiation of kelp in freshwater acidic habitats

most prasinophyte groups habitat

marine

Chlorophyceae and Trebouxiophyceae are predominantly

freshwater and terrestrial algae

Ulvophyceae primary habitat

marine

Streptophyta habitat

most are freshwater and terrestrial

viridiplantae plastid membranes

two membranes, both of cyanobacterial origin

viridiplantae plastid thylakoids

thylakoids arranged in stacks of various numbers, in some streptophyte algae they are in branches

in land plants differentiated into grana and stroma

pyrenoid

area of ​​concentration of the enzyme Rubisco (ribulose-1,5-bisphosphate carboxylase oxygenase), but some algae do not produce it

can green algae thylakoid pass through pyrenoid

yes

in addition to rubisco, pyrenoid contains

carbonic anhydrase, which converts HCO₃^-ion in plastids for CO₂, which enters the dark phase of photosynthesis

dark phase of photosynthesis (2)

Calvin cycle

in stroma of chloroplast

green algae photosynthates (2)

stored in the form starch (α-1,4-glucan), in the plastid, often around the pyrenoid –

either in the form of grains or forming a homogeneous starch envelope

green algae chloroplast DNA

in the form of nucleoids, which are scattered in the plastid and attached to the thylakoids.

if cell has one chloroplast

division of the plastid and the nucleus must be precisely synchronized

green algae chlorophyll

a and b, absorbing red and blue light

green light is partially absorbed by

primary carotenoids (carotenes and xanthophylls) in photosystems

main carotenes (4)

beta-carotene

xanthophyll

zeaxanthin

neoxanthin

orange xanthophylls (2)

important for marine organisms to capture light in deeper sea

eg. prasinoxanthin

secondary carotenoids

eg. astaxanthin

stored in cytoplasm as droplets

absorb excess sunlight to avoid photoinhibition

monad

a type of unicellular microalgae that possesses whip-like appendages called flagella to move independently through aquatic environments.

usual amount of flagella

2,4 (eg Pyramimonas) or more

rarely one or three (Prasinophyta)

example of high number of flagella

Oedogonium zoospore

Chlorophyceae

forty pairs of flagella fused into wreath

flagella usual appearance

same length, cylindrical and similar structure

may be covered on surface with polysaccharide scales (Prasinophyta) or fine hairs (Chlamydomonas)

flagella is covered by

cytoplasmic membrane

axoneme

highly conserved, microtubule-based cytoskeletal core of eukaryotic flagella and motile cilia.

It acts as the structural scaffold and motility engine that pushes or pulls cells through fluid and moves substances across cell surfaces

cross section axoneme (3)

9 + 2 structure

Outer Ring: Nine fused pairs of microtubules, called doublets, encircle the perimeter

Central Core: Two individual (singlet) microtubules sit in the exact center, surrounded by a central sheath

one of the peripheral pairs of flagella

always has dynein arms, which, while consuming ATP, move along the neighboring pair, causing the flagellum to bend

transition zone Chlorophyceae

interface between the axoneme and the basal body

on cross section there is star shaped central ring from which rays emanate toward peripheral pairs

in longitudinal section: structure forms letter H

basal body

in cross section a circumferential ring of nine interconnected microtubule triplets and in the middle without a central pair. At the distal end of the basal body is a structure described ascarriage wheel("court-wheel") = central circle and radiating "courts" leading to peripheral triplets.

ensure regrowth of flagella after loss

microtubular roots

grow from triplets of basal bodies

anchor flagella to cell

made of 2/4/more fused microtubules

at certain distance from basal body untwist and become part of cytoskeleton

basal bodies can be connected to each other by

contractile, cross-shaped connecting fibres

Rhizoplast

striated contractile fiber that runs from the basal body to the surface of the nucleus, where the rhizoplast branches. It probably serves to manipulate the nucleus at the beginning of mitosis.

 striated connecting fibers and the rhizoplast are made of

protein centrin

Chlamydomonas flagella

at angle of 90 degrees

connected by upper and lower striated filaments

anchored to cell by microtubular roots, which extend into cell becoming part of cytoskeleton

connection between basal bodies and nucleus in cells

Flagella replication

basal bodies replicated at beginning of cell division

one of basal bodies is taken by mother, other by daughter

flagellum itself is formed in the following cell cycle

centrioles (3)

often found at the poles of the mitotic spindle.

The structure contains a circumferential ring of nine microtubule triplets (similar to the basal body).

During mitosis, each of the two poles of the spindle is occupied by a pair of centrioles, whose axes form a right angle

basal bodies and centrioles

In flagellates, basal bodies serve as microtubule organizers (MTOC)at the spindle poles during nuclear division, non-flagellated cells have centrioles

flagellar apparatus

evolutionarily conservative

particularly relative position of basal bodies and microtubular roots when viewed from cell apex in direction of posterior end

Flagellar apparatus Chlorophyta

characterised by cruciform arrangement of microtubular roots

180 degree rotational symmetry

two microtubular roots grow from each basal body, one of which contains two fused microtubules

other consists of 4 or more microtubules

Flagellar apparatus Pyramimonas (Prasinophyceae) (4)

4 basal bodies arranged in parallel

microtubular roots alternate in number 4-2-4-2

rhizoplast connected to nucleus

without 180 degree symmetry

3 basic arrangement types basal bodies and their microtubular roots

DO- orientation (directly opposed)

CCW orientation (counter clockwise)

CW orientation (clockwise)

DO orientation (2)

basal bodies placed against each other

eg: Pediastrum zoospores

CCW orientation (3)

basal bodies turned counterclockwise relative to each other

likely ancestral state

Eg: zoospores/gametes of many Ulvophyceae and Trebouxiophyceae

CW orientation

basal bodies turned clockwise relative to each other

Eg: Chlamydomonas

Chaetophorales (Chlorophyceae) basal body orientation

upper part of flagellae has CCW orientation

lower part CW

zoospores of primitive Streptophytes flagellae (5)

anterior end, from one side, two flagellae emerge from the cell

one pair of basal bodies carries a flat microtubular suspension composed of at least 60 microtubules (multilayered structure MLS)

runs along surface of cell towards its posterior end

second basal body is equipped with single pair of microtubules

basal bodies interconnected by transversely striated fibres

Chlorophyceae range DNA content

0.01 pg per cell (Ostreococcus) up to 5.8 pg DNA/cell

Streptophytes genome size

generally larger than Chlorophytes

amount of DNA proportional to size of cells

metacentric mitosis (5)

closed mitosis

nuclear envelope is preserved in all phases

centrioles are located in the equatorial plane of nucleus

chromosomes are aligned in metaphase

described in Trebouxiophyceae but all species

Phragmoplast cytokinesis

microtubular system of two antiparallel rows of microtubules that form ring growing centrifugally

acts as transport rails from Golgi apparatus with material for new cell wall construction, fuse together to form new cell plate that grows centrifugally

after fusion of partition with mother cell wall, phragmoplast disappears

Phragmoplast cytokinesis usually follows

open mitosis

fully developed phragmoplast system occurs in

bryophytes and vascular plants

ZCC clade algae

Cytokinesis with bilateral cell constriction

lacks phycoplast/phragmoplast

cleavage furrow grows in equatorial plane of cell from periphery to the centre of cell (centripetally)

Cytokinesis with phycoplast (3)

microtubular system appears after completion of nuclear division in plane perpendicular to the original axis of mitosis

indicate course of dividing line, which grows between the daughter protoplasts and separates them

eg: in Chlorophyceae phycoplast formed after closed mitosis

Trebouxiophyceae: cell division

follows metacentric mitosis,

the phycoplast and cleavage furrow are established unilaterally, in the equatorial plane, which is marked by position of centrioles

life cycle of green algae is mostly

haplontic

Haplontic

 most of the life cycle the alga is in the haploid stage and the diploid stage is only the zygote, which undergoes reduction division before germination

Haplo-diplontic

 found in the class Ulvophyceae, it evolved there several times independently.

mitospores

asexual spores produced by mitotic division

Types of mitospores

Zoospores

Aplanospores

hemiplanospores

autospores

thallus fragments

zoospores (3)

flagellated, freely moving cells that arise in zoosporangia. They often have a stigma.

After a short motile phase, they shed their flagella, the cell becomes rounded and, in the case of naked zoospores, they secrete a cell wall to the surface.

Zoospores of sessile species attach to the surface by the anterior end of the cell.

aplanospores

they look like zoospores, but they do not have a flagellum

hemiaplanospores

 non-motile cells with some remnants of a flagellated cell, for example, they have a stigma and pulsating vacuoles, but they do not have a flagellum

autospores

miniature copies of vegetative cells inside the parent individual

sexual reproduction process

fusion of gametes to produce diploid zygotes

algae zygote

dormant stage for many algae to overcome adverse conditions

algae gametes

formed either by transformation of vegetative cells or develop in specialised gametangia

may be equipped with flagella, not only for movement but recognition of compatible cells

Monoecious species

male and female gametes can be formed on same individual

Unisexual/dioecies species

male and female gametes on different individuals

sexual process is controlled by

pheromones, direct movement of sperm

types of sexual processes (3)

isogamy

ansiogamy

oogamy