*Red Algae are a widespread group of uni-and multicellular
eukaryotes exhibiting a broad variety of morphologies and life
histories.
*Unlike green plants, animals &
even brown algae, red algae have
attained this diversity without
having evolved true tissue
differentiation.
*The molecular and biochemical
mechanisms of their development
remain largely unexplored.
*Red algae were first defined as a taxonomic group based on their
pigmentation.
*Historically, the red algae were considered plants that typically
lacked true roots, shoots, leaves, seeds, or water-conducting tissues.
*Until recently, the relationship between the Red Algae or
Rhodophyta & other Algae or Protists remained inconclusive &
often contradictory.
*Our understanding of algal phylogeny has dramatically increased
with molecular evolutionary methods; the latest research indicates
that the Rhodophyta is a distinct eukaryotic lineage that likely
shares a most common ancestry with the green algae (Van der Auwera et al.
1998, Burger et al., 1999, Moreira et al., 2000).
*Cavalier-Smith (2000) even goes as far as to support a Kingdom
Plantae composed of green plants, red algae & glaucophytes.
*Chloroplast structure and genome analyses support the hypothesis
that green plants (green algae & land plants), red algae and
glaucophytes (e.g. Cyanophora) originated from a single
endosymbiotic event between a cyanobacterium and an eukaryotic
host.
*These results are an important step towards universal acceptance
of a monophyletic origin of plastids.
*There is no significant fossil record of the
evolutionary history of the marine red
algae, except for the order Corallinales
which extends back as far as the Jurassic
(Johansen, 1981).
*The oldest taxonomically resolved
eukaryote on record, ca. 1,200 MY from
arctic Canada, is identified as a
bangiophyte red alga, Bangiomorpha
pubescens, on the basis of diagnostic cell
division patterns in its multicellular
filaments, & marks the onset of a major
protistan radiation near the
Mesoproterozoic/Neoproerozoic boundary
(Butterfield, 2000).
*There are over 10,000 described species of red algae worldwide
*Most are marine and 3% are freshwater.
*Red algae are most common on hard-bottom habitats in marine
environments, as epiphytes on other algae, seagrasses or
mangrove roots, epizooic on animals, epilithic on pebbles & rocks,
or psammophilic in sand.
*They occur at all latitudes from the Arctic to the Antarctic & occupy
the entire range of depths inhabitable by photosynthetic
organisms, from high intertidal regions to subtidal depths as great
as 268 m (San Salvador I, Bahamas is the greatest depth for
known plant life) (Littler et al., 1985).
*Some red algae, the corallines, are important in the formation of
tropical reefs.
*The red color is due to the presence of phycoerythrin which reflects
red light and absorbs blue light.
*Phycoerythin occurs in at least five forms in the red algae which
differ in their absorption spectra, although all have peaks in the
green part of the spectrum (500-570 nm).
*The color varies according to the ratio of phycoerythrin to
phycocyanin & they may appear green or bluish from the
chlorophyll and other masking pigments.
*Because blue light penetrates water to a greater depth than light of
longer wavelengths, these pigments allow red algae to photosynthesize & live at somewhat greater depths than most other algae.
*Such light harvesting pigments are also found in cyanobacteria &
cryptophytes.
*Accessory pigments, such as the phycobilins phycoerythrin,
phycocyanin and allophycocyanin occur attached to proteins,
forming a class of compounds called phycobiliproteins that are
located on the thylakoid surface in granular phycobilisomes, the
principal light-harvesting complexes (Gantt 1990).
*Each chloroplast is surrounded only by its own double-membrane
envelope, and not by an additional layer of endoplasmic reticulum.
*The only chlorophyll present is chl a.
*The thylakoids are singly within the chloroplasts.
*Carotenoids are present with the most important ones beta-carotene,
lutein and zeaxanthin.
*Only a few red algae have chloroplasts that contain pyrenoids in the
center of the chloroplast, but because reserve starch are produced
in the cytoplasm, the exact function is not known.
*The chloroplast DNA is organized into numerous small, nucleoids
scattered throughout the chloroplast.
The most important food reserve is a polysaccharide, floridean starch,
consisting of units of glucose which is similar to glycogen or the
branched amylopectin fraction of green algal & higher plant starch,
but lacks amylose, the unbranched fraction of green algal starch.
*The low-molecular mass carbohydrate floridoside has an
osmoregulatory function.
*The cell walls of red algae consist of cellulosic fibers embedded in a
matrix of non-fibrillar materials, the phycocolloids.
*The most abundant of these polysaccharides are referred to either as
agars and carrageenans, and are of economic importance.
*Agar is used as a nutrient medium for growing bacteria and fungi &
also in the food and drug industries.
*Carrageenan is used as a substitute for gelatin, or as food in Japan
& the Philippines.
*The red algae share a suite of characters that do not occur
together in any other eukaryote.
*They are unique by a complete lack of flagellated stages including
absence of centrioles, flagellar basal bodies, or other 9+2 structures.
*Mitotic spindle radiates from “nuclear associated organelles” which
are often ring-shaped; centrioles are lacking
*Presence of polar rings instead of centrioles at the poles of the
mitotic spindle is another reflection of the complete absence of
flagella & related structures, and is a fundamental difference
between the Rhodophyta and other groups of algae.
*During mitosis, the telophase spindle and the nuclear envelope,
although perforated by holes, persist and mitosis is therefore closed.
*Cell division is by an
ingrowing furrow of the
plasmalemma, which is
filled with cell wall
polysaccharides
*cleavage is incomplete,
leading to the formation of an
open protoplasmic connection
between the daughter cells,
which becomes closed by a
proteinaceous stopper,
the pit plug (Pueschel, 1990).
A fundamental characteristic shared by all multicellular
florideophycean red algae is that the plant body is composed
entirely of branched filaments.
*Cells within filaments
are linked by pit plugs,
making it possible to
follow each filament
cell by cell as seen with
light microscopy.
*Cells become differentiated in shape & cytological properties
Depending on location within filament
*Reveals how cells from different lineages become connected
Pit plugs are a highly characteristic
feature of the Rhodophyta & various
different kinds can be distinguished on
the basis of their ultrastructure,
providing important characters for
distinguishing among the orders
(Pueschel, 1989).
*Ultrastructural ordinal criteria based on features of pit plugs was
started with Pueschel and Cole (1982) with the justification of 2 new
orders (Batrachospermales and Hildenbrandiales) and the
confirmation of four previously proposed orders (Bonnemaisoniales,
Corallinales, Gelidiales and Palmariales).
*Four aspects of Pueschel’s hypothesis are consistent with results of
molecular systematic studes:
-naked pit pugs represent the ancestral type
-outer cap layers are homologous structures
-domed outer caps are ancestral to plate-like outer caps
-cap membranes are a derived feature within the two-cap-layer
lineage.
Multicellular red algae are composed entirely of a filamentous
organization which may result in complex peudoparenchymatous
thalli as a result of predominant apical
growth.
UNIAXIAL GROWTH
*Three major patterns of mitosis and cytokinesis exist that relate to
mode of vegetative growth & level of thallus organization.
*Distinguishing characters include:
- location & orientation of the mitotic apparatus within the dividing
cell
- arrangement of the chromosomes during metaphase & anaphase
- path of migration of the 2 sets of daughter chromosomes during
anaphase
-timing of septum initiation.
*One characterizes the Bangiophycidae, a second type the lower
Florideophycidae, usually regarded to be primitive, a third type the
higher Florideophycidae, usually considered to be advanced.
UNIAXIAL
MULTIAXIAL
*In Asia, red algae are important sources of food with a high vitamin
& protein content, such as nori.
*Many red algae metabolize polyunsaturated fatty acids to oxidized
products resembling the eicosanoid hormones from mammals.
*Because of their biological properties, seaweed-derived oxylipins
have potential utility as pharmaceutical and research biochemicals
(Gerwick et al., 1993).
*Some species reproduce by vegetative fragmentation or spore
formation, but most undergo a complex life cycle involving an
alternation of generations.
*Reproduction is typically oogamous.
*It was only after culture methods were introduced (von Stosch
1965) that it was finally verified that in most red algae there is a
fundamental linkage of the sexual system and a life history
consisting of three phases.
It has been argued that selection has favored the evolution of a
triphasic life history in red algae as a compensation for an inefficient
fertilization in the absence of motile gametes (Searles, 1968).
*One phase, the free-living haploid gametophyte, is the sexual female
& male individual which produces the gametes.
As a result of fertilization of the female egg cell by an unflagellated
male gamete carried by water currents to the elongated tip
(trichogyne) of a carpogonium, a diploid carposporophyte develops
directly in situ, parasitically, on the female gametophyte.
*The carposporophyte forms carpospores which germinate into a
diploid tetrasporophyte.
*The tetrasporophyte forms tetrasporangia in which meiosis occurs,
with each tetraspore germinating into a haploid female or male
gametophyte.
*When gametophyte & tetrasporophyte
individuals are morphologically
similar = isomorphic alternation of
generations, in contrast to a
heteromorphic alternation of
generations in which gametophytes
alternate with small free-living
tetrasporophytes, often a crust which
doesn't bear any morphological
resemblance to the gametophyte.
*When individuals lack carposporophytes but possess wart-like
tetrasporophytes that are parasitic (tetrasporoblasts or
carpotetrasporophytes) on the female gametophytes, the life history
is biphasic with meiosis occurring in the tetrasporangia in the
tetrasporoblasts.
*A fourth type is the direct type of life history involving only female
gametophytes that apomictally produce carposporophytes.
The reproductive cells are naked and extruded from the gametangia
as a result of the formation of copious amounts of mucilage.
*Once an interaction has taken place between a carposporophytic
cell and a vegetative cell, the fruiting body is called a cystocarp
rather than a carposporophyte.
*The great diversity of cystocarp
types ranging from simple to very
complex has traditionally formed
the basis for the classification of
red algae (Kylin, 1956)
morphological approach
differentiation of morphological
structure is described as function
of filament ontogeny and
cytological modification, and
structures seen in different taxa
are compared for similarities and
differences at each
state of development
Superficial morphological similarities may mask significant
developmental differences that result in taxonomic confusion.
*Special staining & clearing techniques makes it possible to
interpret the organization of complex structures & the rather
obscure & ephemeral events of the sexual cycle.
*Without exceptions to date, molecular phylogenetic analyses seem
to highlight the evolution of the earliest stages in the development of
the female apparatus and associated cells.
*The molecular-based phylogenies each provide an independent
test of classification to the one based on morphological or
ultrastructural evidence.
*Besides elucidating relationships, phylogenetic hypotheses inferred
from gene sequence data provide the critical framework for studies
of morphological character evolution and life history evolution.
Major groups
*The Rhodophyta contains the monophyletic class Rhodophyceae &
usually two subclasses, the paraphyletic Bangiophycidae &
monophyletic Florideophycidae.
*The name Bangiophycidae continues to be used to identify the early
diverging algae from which the Florideophycidae have evolved.
*The classification at the ordinal level is in a constant flux, &
recent DNA sequencing studies have raised serious questions
concerning the correctness of present systems of classification
among the Rhodophyta & have led to proposals of new &
recircumscribed orders.
*The Bangiophycidae are the ancestral pool from which the more
morphologically complex taxa in the Florideophycidae have arisen, &
they are the sources for the independent origins of the plastids,
through secondary endosymbioses, for the Cryptophyta, Haptophyta
& Heterokonta as inferred from DNA sequence comparisons
(Oliveira & Bhattacharya 2000).
*They range from unicells to multicellular filaments (uniseriate or
multiseriate, branched or unbranched) or sheet-like thalli
(monostromatic or distromatic).
*Some taxa are parasitic on other red algal hosts.
*Morphological similarities of many parasites & their hosts have led
to the speculation that some groups of red algal parasites may have
developed directly from their hosts.
*These parasites, termed adelphoparasites may evolve
monophyletically from one host and radiate secondarily to other
hosts or, these parasites may arise polyphyletically, each arising
from its own host (Goff et al., 1996, 1997).
Cells typically have a single axial stellate plastid with a large
pyrenoid, though some taxa contain a cup-shaped plastid or a
complex interconnected plastid lacking a pyrenoid (Gantt et al. 1986,
Broadwater & Scott 1994).
Pit connections are rare and sexual reproduction has not been
described in many members of this subclass.