Seaweeds

Above: Fucus vesiculosus is a common brown seaweed of rocky shores. The holdfast or hapteron firmly fastens the organism to almost any solid surface. The thick midrib of the leaf-like lamina connects to the stalk or stipe which connects directly to the holdfast. The body of the seaweed, or thallus, is strap-like and forks at regular intervals, a growth from called dichotomous branching (branching into two) rarely seen in 'higher' plants. Pairs of gas-filled bladders occur at regular intervals, one each side of the midrib, with a third bladder occurring in the angle of two branches at the branch points.

Click all images for full size versions.

The receptacles are the swollen ends of the frond and each contains many reproductive structures called conceptacles. Each conceptacle is a minute pit or surface cavity and contains sterile hairs and oogonia in female planes, antheridia in male plants. Similar structures, which are sterile, may be fond on the parts of the thallus, these are surface cavities called cryptostomata which contain tufts of sterile hairs. Conceptacles are fertile cryptostomata.

Each growing branch tip has a region of dividing epidermal cells where growth occurs, forming an
apical meristem, which is housed inside an invagination at the tip. This meristem is controlled by the large apical cell which sits in the middle of the meristem. The apical cell arises at the base of a terminal, hair-like filament. The apical cell itself, however, rarely divides, but if it is lost or damaged then growth of the tip ceases. The meristem produces the conceptacles in fertile tips and when conceptacle production is complete the meristem stops producing new cells. (Once the gametes are liberated, then the tissue disintegrates).

Beneath the layer of covering cells, or
epidermis, on the surface of the frond. Benath the epidermis is the meristoderm, consisting of a few cell layers. The epidermis and meristoderm are photosynthetic. Beneath the meristoderm is an outer cortex of tissue. Beneath the cortex is the medulla which fills the interior of the frond. It consists of widely separated columns of cells, forming filaments. The spaces between the filaments are packed with mucilage. Mucilage is especially abundant in the receptacles.

Fucus vesiculosus occurs in the middle intertidal zone (littoral zone) of rocky shores, and so is covered at high tide, but exposed at low tide.

seaweeds on shingle

Fucus vesiculosus (Bladder Wrack) growing amongst Ulva lactuca (Sea Lettuce)on a shingle beach. Note the bladders (air sacs) and the terminal swellings of the reproductive receptacles.

seaweeds on shingle

Ascophyllum nodosum (Knotted Wrack) growing amongst the Fucus vesiculosus also has air bladders.

seaweeds on shingle

Brown Seaweed Structure

The chief example in this section is Laminaria (Kelp). Not all features are present in all brown seaweeds.

seaweed histology

Seaweed stipe

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 - present at least in Laminaria). Mucilage canals - only present in some species, longitudinal ducts carrying mucilage. (See for example Yendo, 1919.)

Notes: 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 and are known to occur in the stipes of Laminaria. 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 and are up to 0.1 micrometre in diameter (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 the sinks (cells where they are used or stored). (See transport in plants for more details of phloem).

Medullary hyphae

Above: hyphae in the medulla of a fertile frond of Fucus (fixed material).

See brown seaweeds for the histology of fresh material.

Reproduction in Seaweeds

Many seaweeds exhibit alternation of generations - alternating between two forms: a haploid gametophyte (containing one set of chromosomes, or n chromosomes) and a diploid sporophyte (containing two sets of chromosomes, one from each parent, or 2n chromosomes). In Fucus, however, there is no gametophyte (or the gametophyte is reduced to gametes). Meiosis in the diploid sporophyte gives rise directly to gametes - the female oospheres (egg cells or ova) and male antherozoa (spermatozoids). The sporophyte is the mature organism.

See the main article on
algae for a description of alternation of generations in green seaweeds.

Fucus is oogamous, that is it produces larger immotile egg cells (ova) and smaller motile spermatozoids (antherozoa). Each egg cell is 75 to 100 micrometres diameter in Fucus. The antherozoa are produced inside antheridia (singular: antheridium) which are borne on branched hairs within the conceptacles. The oospheres, which are produced in groups of 8, are enclosed by three wall layers to form the oogonium which is borne on a single stalk cell.

Structure of the Receptacle and Conceptacles of Fucus.

Receptacle and conceptacle

Note that in dioecious species (those with separate male and female fronds) separate male and female connceptacles are borne on different thalli, whereas in monoecious species either separate female or male conceptacles are borne on one thallus, possibly in different receptacles, or the conceptacles are mixed, containing both oogonia and antheridia.

In Fucus vesiculosus, each oogonium contains 8 egg cells when ripe, enclosed by 3 wall layers. When the time is right, the outer wall layer ruptures, releasing the packet of 8 egg cells surrounded  y the two inner wall layers. These packets are ejected from the conceptacle by mucilage secretion. Once outside, in the water, the second layer (now outermost) splits apart as the innermost layer swells and splits, liberating the egg cells. Each antheridium contain a packet of 64 spermatozoids enclosed by two wall layers. The outer layer splits and the packet, surrounded by the remaining wall layer, is again ejected from the conceptacle by mucilage secretion. Once in the water, the inner layer splits open and the spermatozoids are liberated.

The spermatozoids swim towards the egg cells by chemotaxis towards a chemical (pheromone) secreted by the egg cell: fucoserratene in F. vesiculosus. Many spermatozoids may surround each egg cell, but as soon as one penetrates the egg, the others disperse and the egg secretes a cell wall.

In Fucus vesiculosus the spermatozoids are orange-coloured, due to the presence of carotenoids which make up the stigma, part of the light-sensor. This gives male receptacles an orange colour when cut open. Female receptacles, however, are green-brown due to the presence of chloroplasts in the egg cells.

Each antherozoid is a tiny pear-shaped cell with two flagella. The anterior flagellum is 'tinselated' or
pantonematic, meaning it has side-branches like tinsel, and is used to pull the cell along. The posterior flagellum is a smooth 'whiplash' or acronematic type of flagellum, which pushes the cell along by undulating movements. The antherozoid also has a light-sensor to help guide the cell. Each spermatozoid has a protruding proboscis at its front end, which is supported  by curved microtubules (microtubules are protein cylinders inside the cell and are part of the cytoskeleton).

When the oogonia are ripe, they are forced out through the opening of the conceptacle, the
ostiole, by mucilage secretion and the outermost enclosing walls splits open to release the egg cells, still packaged as a group of 8 enclosed in the two inner walls of the oogonium. This occurs during low tide, when the algae are exposed to air, as the air helps the conceptacles dry and shrink. The antherozoids are inclosed within the inner antheridium wall and are similarly extruded as a packet from the antheridium by mucilage secretion. The oospheres are still enclosed within the two inner walls of the oogonium. When the tide returns it washes away the extruded mucilage with the enclosed gametes. The outer wall around the oospheres gelatinises and the inner splits upon hydration, releasing the oospheres. The antheridial packet also splits open when hydrated, as the inner antheridial wall gelatinises to release the antherozoids which swim to the egg cells, attracted by the pheromone fucoserratene secreted by the eggs. One spermatozoid only fertilises the egg cell, which then encloses itself in a wall to prevent other spermatozoids gaining access. This is a diploid zygote.

The zygote proceeds to generate a new seaweed by mitosis (cell division). The zygote adheres to a solid surface within a few hours and germinates after about 12 hours as the rhizoid (bottom) pole elongates. The point of spermatozoid entry initially determines the side of the egg cell which becomes the rhizoid pole. Most zygotes settle within 2 meters of the parent plant, but some drift considerably further.

The plane of the first division, however, is determined by the direction of the light source. The zygote responds to a gradient of blue or UV light and divide first perpendicular to the light, after about 24 hours. This division is unequal, the smaller cell, furthest from the light,is the
rhizoid progenitor cell (RPC) which will give rise, by repeated cell division and differentiation, into the holdfast, whilst the larger cell nearest the light is the thallus progenitor cell (TPC) which has most of the chloroplasts and will give rise to the rest of the embryo and mature frond. The RPC inherits more of the Golgi and mitochondria (see cell structure). The initial egg cell has no polarity in its organelles, which are evenly dispersed throughout its cytoplasm, it is the zygote which acquires the polarity firstly by fertilisation (point of spermatozoid entry) and secondly by the direction of the light. The RPC gives rise early on to branching filaments, the TPC to a globular thallus. If the zygotes develop in darkness, then the orientation of the first cell division plane is random.

Secondary growth results in thickening of the frond as a layer of mersitematic cells give rise to outer meristoderm cells and inner cortex cells. Cells in the medulla do not divide. The mature seaweed is perennial, living for up to 5 years in the case of
Fucus.

Kelps - Seaweed Giants

Kelps are brown seaweeds belonging to the order Laminariales. They occur at 50 to 120 meters depth where light levels are only about 0.6% that of sunlight, forming extensive kelp forests. 'Kelp' is a general term that includes such genera as Laminaria, Macrocystis and Nereocystis. These do show clear alternation of generations, alternating between a haploid gametophyte stage and a diploid sporophyte stage. The gametophyte forms a microthallus, consisting of microscopic, branched filaments one cell wide (that is the filaments consist of single chains of cells). The sporophyte forms a macrothallus which is foliose (leaf-like) and up to 10 metres in length, or as much as 80 metres in Macrocystis and Nereocystis which may grow as fast as half a metre a day!

Kelps are long-lived, for example
Laminaria hyperborea may live for up to 20 years, and exhibit substantial secondary growth. The meristoderm continues to thicken the stipe, though in older parts a layer of cortex takes over this function and annual cortical growth rings may be produced as growth alternately accelerates and
decelerates with changing seasons. The first-formed cortex is the primary cortex, and the second ring of cortex is secondary cortex. In  
Laminaria hyperborea, for example, there is also a meristem between the stalk and blade, which is active in spring, producing a new blade when day length and temperature reach certain critical values. The seaweed also has an internal biological clock to measure the time of the seasons. This new blade consumes food reserves stored in the old blade during the previous spring and summer, and the old blade remains attached to the new blade whilst the new blade is growing.

In
Macrocystis the meristoderm is photosynthetically active and linked to the cortex and hence to the medulla by a network of protoplasmic connections via plasmodesmata (tiny channels through the cell wall of adjacent cells which contain cytoplasm and link the cytoplasms of neighbouring cells together). Organic compounds produced by photosynthesis (photoassimilates), mostly the carbohydrate mannitol and also amino acids and hexose monophosphates (phosphorylated sugars) are then transported to other parts of the medulla, via its trumpet hyphae, which specialise in long-distance transport of photoassimilates. Transport is quite rapid in the large Macrocystis, with photoassimilates traveling upwards and downwards throughout the plant at about 70 cm/h and this probably occurs mostly in the trumpet hyphae.

In
Laminaria saccharina, sporangia form in the autumn, when days are short and temperatures around 10-15C. these produce haploid spores by meiosis, which are unicellular gametophytes.  These spores germinate when light levels are adequate to form fertile gametophytes within a week or two. They germinate into short, branched filaments of cells, the microthallus. Blue light triggers the development of gamete-producing gametangia: unicellular antheridia, each of which produces a single spermatozoid, and unicellular oogonia, each with one egg cell. The oogonia are released into the water a few minutes after dusk and secrete a pheromone called lamoxirene, which stimulates release of the spermatozoids and attracts the spermatozoids towards the oogonium (a process called chemotaxis). Fertilization is therefore external in this species. The zygotes develop into new sporophytes in late winter or early spring.

Seaweed growth rings

Above:annual growth rings in the stipe of Laminaria.

Fucus life-cycle

Male conceptacle
Above: section through a male conceptacle of Fucus, showing the many antheridia borne on branched hairs. trumpet hyphae can be seen in the medulla.

Male conceptacle

Below: close-up of the antheridia.

Antheridia Antheridia

Updated: 18/12/2014 (the original file was corrupted and has been reconstructed, though some of the
original information may have been lost); one or two corrections were also made.

6 Mar 2021.

Selected References

Moss, B.L. 1983. Sieve elements in the Fucales. New Phytol. 93: 433-437.

Yendo, K. 1919. A monograph of the genus
Alaria. J. of the College of Sci., Tokyo Imperial Uni. 43: 1-145.

Round, F.E. 1965. The Biology of the algae. Edward Arnold (Pub.) Ltd. [For
Fucus development]

Van den Hoek, C., G.G. Mann and H.M. Jahns, 1995. Algae - An introduction to phycology. Cambridge University Press. (For structure of the algal thallus and general biology).

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