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| Seaweeds |
Above: Fucus vesiculosus is a common brown seaweed of rocky shores. The holdfast or hapteron firmly fastens
the body of the organism (called the thallus, plural thalli) to the substratum (such as rocks, piers, etc.). The thallus
consists of a short stalk, or stipe, which contains into the leaf-like frond as the midrib which is flanked on two sides
by the broad and flat blade or lamina.
It may be easy for people to belittle the humble seaweed and perhaps some would point to their economic value in
order to justify their study. However, seaweeds are important subjects of study for several intrinsic reasons, including
the fact that they shed so much light on plant evolution, they also exhibit beautiful ways in which organisms can build
bodies and adapt in ways that exploit the laws of physics to their advantage, they are accessible and make good
subjects for the study of organism processes and of course their intrinsic beauty makes them very worthy of study.
Seaweeds were once classified as plants, but really they are protoplants (protophytes) and are now grouped in the
kingdom Protoctista (protoctistans) along with the protozoa (protoanimals) and protofungi. They are also algae - a
diverse group that includes both certain bacteria (now in kingdom Monera or Prokaryota) and eukaryotic algae
(algae whose cells possess a membrane-bound nucleus). The principle reason they are not classified as plants is
because the embryonic development of seaweeds is very different from that in plants, whilst seaweeds develop from
germinating spores, however, they are clearly plant-like and the ancestors of all land plants probably resembled
certain seaweeds.
The brown algae, Phaeophyta, produce the most complex and most plant-like of algae - the brown seaweeds. In
particular, forms like Fucus (above) and kelp (belongs to the laminariales ) are structurally and physiologically the
most advanced. Giant kelp forms large underwater forests in which individual kelps can grow up to 40-60 m in length!
Many people probably think of seaweeds as simple creatures, but their superficial simplicity hides a wealth of
complexity. The diagrams below shows a section through a brown seaweed stipe or midrib. These diagrams have
been based upon several species, including kelps and Fucus which share the same general construction, but with
more cell layers in the larger kelps.
the body of the organism (called the thallus, plural thalli) to the substratum (such as rocks, piers, etc.). The thallus
consists of a short stalk, or stipe, which contains into the leaf-like frond as the midrib which is flanked on two sides
by the broad and flat blade or lamina.
It may be easy for people to belittle the humble seaweed and perhaps some would point to their economic value in
order to justify their study. However, seaweeds are important subjects of study for several intrinsic reasons, including
the fact that they shed so much light on plant evolution, they also exhibit beautiful ways in which organisms can build
bodies and adapt in ways that exploit the laws of physics to their advantage, they are accessible and make good
subjects for the study of organism processes and of course their intrinsic beauty makes them very worthy of study.
Seaweeds were once classified as plants, but really they are protoplants (protophytes) and are now grouped in the
kingdom Protoctista (protoctistans) along with the protozoa (protoanimals) and protofungi. They are also algae - a
diverse group that includes both certain bacteria (now in kingdom Monera or Prokaryota) and eukaryotic algae
(algae whose cells possess a membrane-bound nucleus). The principle reason they are not classified as plants is
because the embryonic development of seaweeds is very different from that in plants, whilst seaweeds develop from
germinating spores, however, they are clearly plant-like and the ancestors of all land plants probably resembled
certain seaweeds.
The brown algae, Phaeophyta, produce the most complex and most plant-like of algae - the brown seaweeds. In
particular, forms like Fucus (above) and kelp (belongs to the laminariales ) are structurally and physiologically the
most advanced. Giant kelp forms large underwater forests in which individual kelps can grow up to 40-60 m in length!
Many people probably think of seaweeds as simple creatures, but their superficial simplicity hides a wealth of
complexity. The diagrams below shows a section through a brown seaweed stipe or midrib. These diagrams have
been based upon several species, including kelps and Fucus which share the same general construction, but with
more cell layers in the larger kelps.
Reproduction in Seaweeds
When we look at reproduction in seaweeds, then we realise that they are very different from true plants (like mosses,
ferns, conifers and flowering plants). Fucus is one of the more familiar genera of brown seaweeds found on rocky
shores and are commonly known as wracks. Fucus vesiculosus, or bladder-wrack, is famous for its (popable) air
bladders; Fucus spiralis or spiral-wrack has spiral fronds that twist more on drying out and Fucus serratus, or
saw-wrack, has a toothed margin (the margin of F. vesiculosus, shown at the top of the page, is smooth or 'entire').
Fucus serratus and F. vesiculosus are both dioecious, whilst F. serratus is monoecious and self-fertilising.
(Dioecious meaning that the sexes are separate, such that there exist male and female thalli and monoecious means
the organism is hermaphroditic). In all cases, the tips of mature fertile fronds become swollen with mucilage and
develop into receptacles that are covered with tiny pores, each pore connecting to a chamber called a
conceptacle. The conceptacles are the reproductive organs of Fucus and when ripe and swollen give the
receptacle a warty appearance. These structures are illustrated below:
When we look at reproduction in seaweeds, then we realise that they are very different from true plants (like mosses,
ferns, conifers and flowering plants). Fucus is one of the more familiar genera of brown seaweeds found on rocky
shores and are commonly known as wracks. Fucus vesiculosus, or bladder-wrack, is famous for its (popable) air
bladders; Fucus spiralis or spiral-wrack has spiral fronds that twist more on drying out and Fucus serratus, or
saw-wrack, has a toothed margin (the margin of F. vesiculosus, shown at the top of the page, is smooth or 'entire').
Fucus serratus and F. vesiculosus are both dioecious, whilst F. serratus is monoecious and self-fertilising.
(Dioecious meaning that the sexes are separate, such that there exist male and female thalli and monoecious means
the organism is hermaphroditic). In all cases, the tips of mature fertile fronds become swollen with mucilage and
develop into receptacles that are covered with tiny pores, each pore connecting to a chamber called a
conceptacle. The conceptacles are the reproductive organs of Fucus and when ripe and swollen give the
receptacle a warty appearance. These structures are illustrated below:
Note that in dioecious species separate male and female connceptacles are borne on different thalli, whereas in
monoecious species, the same conceptacles contain both male and female reproductive structures. However, this is
not hard and fast - different species of Fucus, even dioecious and monoecious ones can cross-fertilise and
produce hybrids. sometimes unisexual conceptacles will be found on the receptacle of a monoecious species, which
may then have both mixed and unisexual conceptacles.
Some Terminology:
The structures or cells that generate gametes are called gametangia (the female oogonia (sing. oogonium) and
the male antheridia (sing. antheridium)).
A gametophore is a structure that bears the gametangia - the single stalk cell that bears the female gametangia
(oogonia) and the branched multicellular hairs that bear the male gametangia (antheridia). The suffix 'phore' refers
to a bearing structure, so a gametophores bears gametangia and a sporophore bears spores or a spore-containing
capsule (as in mosses).
The thallus is diploid (meaning that, like adult humans, it has two sets of chromosomes, 2n, one maternal and the
other paternal, where n = number of chromosomes in a complete set) and the gametes are produced by meiosis (a
reduction cell-division - each daughter cell becomes a haploid gamete with just n chromosomes).
Paraphysis (plural paraphyses) - refers both to the unbranched sterile hairs found in female conceptacles and the
branched hairs (sterile parts of the antheridiophores) that bear the antheridia, though more usually to the former.
Mechanism of reproduction:
The packets of 8 oospheres are enclosed in a three-layered (non-cellular) wall. When ripe, the outer wall ruptures
and the packet of 8 oospheres is released, still surrounded by two wall-layers - it is pushed out by mucus secreted
by the conceptacle, and then one after the other, the two remaining wall-layers are shed, liberating the 8 egg cells
(ova).
The male antheridia form on colourless branched hairs. Each antheridium is enclosed by a double wall-layer (non-
cellular). When ripe, one wall layer breaks down and packets of spermatozoa surrounded by the inner wall-layer of
the antheridium are expelled from the conceptacle by secretion of mucilage. Once released, the inner wall-layer
breaks down releasing the spermatozoids. The spermatozoids are pear-shaped cells with two unequal flagella - one
a so-called tinsel-flagellum pleuronematic (having one or more rows of hairs, called mastigonemes, projecting
laterally from it) or specifically pantonematic (having two rows of lateral hairs / mastigonemes) like tinsel which
projects forwards and pulls the cell along, and one smooth hairless-flagellum (an acronematic flagellum).
Fertilisation: The liberated spermatozoids swim towards the liberated ova, attracted by a pheromone chemical
secreted by the ova. In F. vesiculosus and some other Fucus species, this pheromone is a chemical called
fucoserratene (octa-1,3,5-triene or 1,3,5-octatriene - an unsaturated alkene hydrocarbon comprising a chain of 8
carbon atoms with 3 C=C double bonds). One spermatozoid only will be allowed to fuse with the egg, fertilising it to
produce a diploid cell called a zygote. Many sperm may surround each egg, all competing to fertilise it, but as soon
as one penetrates the egg, the others disperse and the egg secretes a wall around itself.
Development:
monoecious species, the same conceptacles contain both male and female reproductive structures. However, this is
not hard and fast - different species of Fucus, even dioecious and monoecious ones can cross-fertilise and
produce hybrids. sometimes unisexual conceptacles will be found on the receptacle of a monoecious species, which
may then have both mixed and unisexual conceptacles.
Some Terminology:
The structures or cells that generate gametes are called gametangia (the female oogonia (sing. oogonium) and
the male antheridia (sing. antheridium)).
A gametophore is a structure that bears the gametangia - the single stalk cell that bears the female gametangia
(oogonia) and the branched multicellular hairs that bear the male gametangia (antheridia). The suffix 'phore' refers
to a bearing structure, so a gametophores bears gametangia and a sporophore bears spores or a spore-containing
capsule (as in mosses).
The thallus is diploid (meaning that, like adult humans, it has two sets of chromosomes, 2n, one maternal and the
other paternal, where n = number of chromosomes in a complete set) and the gametes are produced by meiosis (a
reduction cell-division - each daughter cell becomes a haploid gamete with just n chromosomes).
Paraphysis (plural paraphyses) - refers both to the unbranched sterile hairs found in female conceptacles and the
branched hairs (sterile parts of the antheridiophores) that bear the antheridia, though more usually to the former.
Mechanism of reproduction:
The packets of 8 oospheres are enclosed in a three-layered (non-cellular) wall. When ripe, the outer wall ruptures
and the packet of 8 oospheres is released, still surrounded by two wall-layers - it is pushed out by mucus secreted
by the conceptacle, and then one after the other, the two remaining wall-layers are shed, liberating the 8 egg cells
(ova).
The male antheridia form on colourless branched hairs. Each antheridium is enclosed by a double wall-layer (non-
cellular). When ripe, one wall layer breaks down and packets of spermatozoa surrounded by the inner wall-layer of
the antheridium are expelled from the conceptacle by secretion of mucilage. Once released, the inner wall-layer
breaks down releasing the spermatozoids. The spermatozoids are pear-shaped cells with two unequal flagella - one
a so-called tinsel-flagellum pleuronematic (having one or more rows of hairs, called mastigonemes, projecting
laterally from it) or specifically pantonematic (having two rows of lateral hairs / mastigonemes) like tinsel which
projects forwards and pulls the cell along, and one smooth hairless-flagellum (an acronematic flagellum).
Fertilisation: The liberated spermatozoids swim towards the liberated ova, attracted by a pheromone chemical
secreted by the ova. In F. vesiculosus and some other Fucus species, this pheromone is a chemical called
fucoserratene (octa-1,3,5-triene or 1,3,5-octatriene - an unsaturated alkene hydrocarbon comprising a chain of 8
carbon atoms with 3 C=C double bonds). One spermatozoid only will be allowed to fuse with the egg, fertilising it to
produce a diploid cell called a zygote. Many sperm may surround each egg, all competing to fertilise it, but as soon
as one penetrates the egg, the others disperse and the egg secretes a wall around itself.
Development:
- The released unfertilised egg of Fucus is about 70 - 100 micrometres in diameter and spherical. The
organelles are distributed evenly within the egg which has no obvious polarity and the nucleus is located in
the centre. It contains small chloroplasts (chromoplasts) which are actively engaged in photosynthesis when
there is sufficient light.
- After a few hours, the zygote adheres to the substratum. After about 12h (at 15 degrees C) one end (pole) of
the zygote protrudes as a papilla (wart-like protuberance or bump). This is the growing rhizoid tip which will
eventually form the holdfast.
- The first cell division of the fertilised egg or zygote is unequal and gives rise to one larger cell and one
smaller cell. The smaller cell receives most of the Golgi apparatus and most of the mitochondria and is the
rhizoid progenitor cell (RPC) which will give rise to the rhizoid system which forms the holdfast. The larger cell
is the thallus progenitor cell (TPC) and contains most of the chloroplasts and will give rise to the thallus
(stipe, midrib and blade).
- A key to understanding development is polarity - how does a developing organism know which end is which
and how do cells know what type of cell they are supposed to differentiate into (i.e. how do they know where
they are situated within the organism). Seaweeds are useful model organisms to study these fundamental
developmental processes which are of importance to all multicellular organisms. They are readily available
and the eggs are large and apolar (the organelles are evenly distributed so that no one end or pole of the
egg cell is different from its opposite pole).
- When the zygote divides for the first time, the organism becomes polar - it has distinct poles, one comprising
the smaller RPC and the other the larger TPC. It is of fundamental importance to understand what has made
the originally non-polar egg acquire an axis and two distinct poles.
- Initially, the point of entry of the sperm gives the zygote polarity, with this point becoming part of the rhizoid
pole. However, additional factors can change the polarity of the zygote, which is labile for about 10h at which
time the polarity is fixed.
- A directional beam of blue light (or the blue component of white light) or ultraviolet light striking the zygote
causes it to acquire polarity with the illuminated side becoming the TPC and the darker side the RPC as the
plane of the first cell division occurs at right-angles to the direction of the incoming light. If kept in darkness,
then the orientation of this plane of division is random. This makes sense, since the thallus must grow
towards the light.
- The stimuli of sperm entry and light striking the zygote cause electrical gradients within the zygote, as
electrically charged ions move across the cell membrane, entering or leaving the cell. (The cell membrane in
general is an electrical capacitor). These electrical waves established within the cell then give the cell polarity
by activating the cytoskeleton which repositions the organelles at their respective poles.
- The rhizoid progenitor cell divides predominantly in one plane initially, forming branching filaments that form
the early holdfast, whilst the TPC divides in several planes to produce a small globular thallus.
- As the young thallus grows it increases in length by growing at the tip of the frond which occasionally divides,
with each branch continuing growth at its tip. The tip or apex contains a large 4-sided pyramidal apical cell.
This cell causes surrounding tissues (some of which may be directly derived from the apical cell by cell
division) to form a meristem. A meristem is a growing zone of cell division (mitosis) in plants and protoplants.
This meristem produces new cells.
- Older parts of the thallus, away from the apical meristem continue growth, so-called secondary growth, as the
thallus increases in thickness. The meristoderm undergoes cell division, as do zones within the cortex of the
stipe, the latter adding annual growth rings of secondary cortex. Cells in the medulla and inner cortex of all
parts undergo division to produce new sieve hyphae which grow down the midrib and stipe. Some secondary
growth is also due to enlargement of existing cells.
- The older parts of the blade towards the holdfast are eventually abraded away by wave-action, leaving the
much thickened midrib here as the stipe.
- When mature, fertile frond tips (fruiting tips) become receptacles and develop conceptacles. The
conceptacles evidently develop from hair-pits which also form on sterile parts of the frond. Hair pits are
invaginations of the epidermis lined by hairs whose prime function appears to be secretion of mucilage. On
fertile tips, some of these hair-pits mature into conceptacles and the life-cycle repeats.
Cuticle – a thin layer of mucilage slime (mucilage is the algal/plant equivalent of mucus in animals and both contain
glycoproteins, though mucilage also contains polysaccharide); meristoderm – includes the cubical or columnar
epidermis where this is meristematic and 2 or 3 layers of large hypodermal cells that are meristematic and
photosynthetic and contain chromoplasts; outer cortex – radial rows of typically parenchymatous cells, may be
meristematic; inner cortex – larger cells that are cylindrical and arranged in columns; medullary sheath – not always
present as a distinct layer, e.g. occurs in parts of the stipe and in the midrib of Alaria, modified inner cortex comprised
of compact cylindrical and narrow-lumened/thick-walled cells; perimedulla – large, thick-walled cylindrical cells
irregularly interspersed among compact narrow fibres (thick-walled cells with small lumens and cylindrically arranged),
give off radial branches that lead into the medulla and give rise to the hyphae of the medulla; medulla – loosely
packed septate fibres (hyphae – multicellular filaments embedded in mucus) that cross the medulla transversely then
move longitudinally up and down the axis as longitudinal cylinders with enlarged ends where cells join together at
porous sieve plates (forming so-called trumpet hyphae - inset). Mucilage canals – only present in some species,
longitudinal ducts carrying mucilage.
Note: T.S. = transverse section (essentially a cross-section) as opposed to L.S. = longitudinal or lengthwise section.
The stipe is similar to the midrib.
The trumpet hyphae have a very important role. Like plants, seaweeds are photosynthetic, and only those parts filled
with chloroplasts exposed to the light photosynthesise - the meristoderm principally of the frond blade. If cells
elsewhere are to survive, then these cells must transport photoassimilates (the organic products of photosynthesis,
mainly amino acids and sugars like mannitol) surplus to their own needs to other parts of the thallus. The meristoderm
cells are connected to cortical cells via plasmodesmata and these cortical cells take what they need and pass the rest
on to the perimedulla and medulla where they are transported over longer distances in the hyphae. The cross-walls
between cells making up the trumpet hyphae have numerous large pores in them, connecting adjacent hyphal cells
together. These pores are larger than plasmodesmata (so the cross-walls are sieve plates) and allow the
photoassimilates to travel with greater ease - the trumpet hyphae are the algal equivalent of the sieve-tubes found in
higher plants except that they lack accompanying cylinders of companion cells. Through these hyphae, the
photoassimilates move from the sources (cells where they are made) to teh sinks (cells where they are used or
stored). (See transport in plants for more details of phloem).
glycoproteins, though mucilage also contains polysaccharide); meristoderm – includes the cubical or columnar
epidermis where this is meristematic and 2 or 3 layers of large hypodermal cells that are meristematic and
photosynthetic and contain chromoplasts; outer cortex – radial rows of typically parenchymatous cells, may be
meristematic; inner cortex – larger cells that are cylindrical and arranged in columns; medullary sheath – not always
present as a distinct layer, e.g. occurs in parts of the stipe and in the midrib of Alaria, modified inner cortex comprised
of compact cylindrical and narrow-lumened/thick-walled cells; perimedulla – large, thick-walled cylindrical cells
irregularly interspersed among compact narrow fibres (thick-walled cells with small lumens and cylindrically arranged),
give off radial branches that lead into the medulla and give rise to the hyphae of the medulla; medulla – loosely
packed septate fibres (hyphae – multicellular filaments embedded in mucus) that cross the medulla transversely then
move longitudinally up and down the axis as longitudinal cylinders with enlarged ends where cells join together at
porous sieve plates (forming so-called trumpet hyphae - inset). Mucilage canals – only present in some species,
longitudinal ducts carrying mucilage.
Note: T.S. = transverse section (essentially a cross-section) as opposed to L.S. = longitudinal or lengthwise section.
The stipe is similar to the midrib.
The trumpet hyphae have a very important role. Like plants, seaweeds are photosynthetic, and only those parts filled
with chloroplasts exposed to the light photosynthesise - the meristoderm principally of the frond blade. If cells
elsewhere are to survive, then these cells must transport photoassimilates (the organic products of photosynthesis,
mainly amino acids and sugars like mannitol) surplus to their own needs to other parts of the thallus. The meristoderm
cells are connected to cortical cells via plasmodesmata and these cortical cells take what they need and pass the rest
on to the perimedulla and medulla where they are transported over longer distances in the hyphae. The cross-walls
between cells making up the trumpet hyphae have numerous large pores in them, connecting adjacent hyphal cells
together. These pores are larger than plasmodesmata (so the cross-walls are sieve plates) and allow the
photoassimilates to travel with greater ease - the trumpet hyphae are the algal equivalent of the sieve-tubes found in
higher plants except that they lack accompanying cylinders of companion cells. Through these hyphae, the
photoassimilates move from the sources (cells where they are made) to teh sinks (cells where they are used or
stored). (See transport in plants for more details of phloem).
Structure of the Receptacle and Conceptacles of Fucus.
Above: hyphae in the medulla of a fertile frond of Fucus.
Above and left: section through a male
conceptacle of Fucus.
Below: antheridia of Fucus.
Click images to enlarge.
conceptacle of Fucus.
Below: antheridia of Fucus.
Click images to enlarge.