464 CHLOROPLAST E.R.: EVOLUTION OF TWO MEMBRANES

(Fig. 21.32). Nereocystis luetkeana (bull kelp) has a tough whip-like stipe up to 25 m long that terminates in a large air bladder (Fig. 21.38). This air bladder supports secondary stipes and blades that hang down from the surface of the sea. The remarkable aspect of this large seaweed is that it is an annual plant that can grow as much as 6 cm a day. Julescraneia grandicornis is a fossil alga from Miocene diatomite of California that resembles Nereocystis (Parker and Dawson, 1966). Macrocystis (Fig. 21.32) may grow up to 50 m in length and has a life of 5 years, although the individual secondary blades have a life of only 6 months. As in Lessonia there is successive splitting of the primary blade, but in Macrocystis the growth of one of the two segments is arrested. This leads to a long curtain type of thallus. Each of the segments has an air bladder at the base of the secondary stipe, allowing the secondary blade to hang down from the surface of the water.

a l a r i a c e a e

The sporophytes in this family have the sori formed on special sporophylls. Alaria (Fig. 21.39) has a lamina with a wavy margin and a midrib. The short stipe produces thick tongue-shaped sporophylls in the summer, which, after maturation of the sori, are shed during autumn and winter, leaving scars on the stipe. During the winter the blade wears down to the basal meristematic zone, and a new blade is produced the following season.

Fucales

The organisms in this order are parenchymatous with growth from an apical cell. The haploid generation is reduced to the egg and sperm, with the remainder of the life cycle being diploid. The gametes are borne in special cavities, the conceptacles, and gametic union is always oogamous. Conceptacles may be scattered over the surface of the thallus, but more frequently they are limited to the inflated tips of special branches, the receptacles. The Fucales are worldwide in distribution, but those of the Arctic and north temperate seas differ considerably from those of the Antarctic and south temperate waters, Fucus (Fig. 21.40) is a common genus in northern waters, whereas in tropical and subtropical waters Sargassum (Fig.

Fig. 21.39 Alaria esculenta. The old distal part of the blade

is partly eroded away, and the sporophylls are fully

developed. (After Taylor, 1957.)

21.51) is present. In Australian waters, Cystophora is a predominant member of the flora, with the large Durvillea being common in sub-Antarctic waters.

Gene sequencing has revealed that the Fucales diverged early from the remainder of the Phaeophyceae (Draisma et al., 2001).

Morphology and anatomy

The genus Fucus will be used as the representative genus in this order (Fig. 21.40). The thallus is much branched and is supported by a short narrow stalk that is attached to a discoid holdfast. The branching is dichotomous, with each flattened segment having a prominent central midrib surrounded on both sides by a narrower wing. The wings usually bear scattered cryptoblasts, which are basically sterile conceptacles with large numbers of hairs, that facilitate the uptake of nutrients from the seawater (Hurd et al., 1993). At certain times of the year, the tips of the branches

HETEROKONTOPHYTA, PHAEOPHYCEAE	465

Fig. 21.40 The life cycle of Fucus sp. (F. vesiculosus and

F. serratus). (Adapted from Thuret, 1854; Oltmanns, 1889;

Nienburg, 1931; Taylor, 1957.)

466 CHLOROPLAST E.R.: EVOLUTION OF TWO MEMBRANES

are swollen into receptacles that contain the fertile conceptacles. The inflation of the receptacles is due to the production of a large amount of mucilage.

Each branch has an apical cell at its apex (Fig. 21.41). The apical cell divides several times a year, resulting in the formation of a dichotomy or fork, with one arm of the fork being longer than the other. The apical cell in mature Fucaceae is a foursided pyramid with a flattened base. In the other families and in young Fucaceae, the apical cell is a three-sided pyramid. According to Moss (1967), the apical cell itself does not divide (except in the formation of forks), and instead stimulates the cells around it to divide. Thus a meristematic zone extends beneath and around the sides of the apical cell and is called the promeristem. The cells of the promeristem, similar to meristematic cells in other organisms, are very small. At the sides of the promeristem the cells enlarge and divide only transversely, yielding the flattened wings. Mucilage is deposited between the derivatives of the promeristem, causing the files of cells to separate, remaining in contact only where there are pits. The apical cell shows apical dominance, inhibiting the development of laterals beneath it (Moss, 1965, 1970). If the apical cell is destroyed, then the underlying lateral will develop into a new apical cell.

The second meristem in the apical area of the thallus is the meristoderm or outer row of cells

Fig. 21.41 Upper portion of a mature juvenile thallus of

Fucus, showing the apical depression, apical cell (a), remnants of terminal hairs (h), the promeristem (p), and the medulla (m). (After Oltmanns, 1889.)

derived from the promeristem. This is a closely packed layer of brick-shaped cells that divides anticlinally at first and then periclinally to produce new tissues to the inside.

The anatomy of the Fucales is similar to that of the Laminariales, with a mucilaginous cuticle covering the epidermal layer of cells (Figs. 21.41, 21.42). Inside this is the cortex with the medulla in the center. Hyphae are produced by the inner cortical cells, but there are no trumpet hyphae present. There is an orientation of organelles within the epidermal cells. They have an outer layer of alginic acid vesicles with a basal nucleus and chloroplasts (Fulcher and McCully, 1969; Rawlence, 1973). The cap of alginic acid vesicles may shield the chloroplasts and nucleus from intense illumination, especially at low tide when the plants are usually exposed. The organelles of the cortical cells are arranged just the reverse of the epidermal cells, having an outer layer of chloroplasts. The chloroplasts of the medullary and hyphal cells are much reduced.

Like the Laminariales, the Fucales are able to translocate organic materials (Floc’h and Penot, 1972). Mannitol (Fig. 1.7) is the form of photosynthate translocated. The growing apex acts as a sink, with the mannitol translocated to the growing apex from the blades of the alga (Diouris, 1989). Structurally, the Fucales have a system of conducting elements very similar to the Laminariales. In the Fucales, the sieve elements run from the apical meristem to the base of the plant. The medullary elements have sieve plates (1 m thick) at their ends. The pores in the sieve plates enable a continuous system of cytoplasm for the translocation of materials both longitudinally and transversely through the cross connections. The pores in the sieve plates are 0.1 m2 or less, which makes them smaller than the pores reported in the Laminariales (Alaria has pores ranging in size from 0.1 to 0.3 m2) (Moss, 1983).

Gas vesicles (air bladders) (Fig. 21.40) originate not far from the apex as a result of growth of the surface layers of cells accompanied by an increase in the thickness of the cortex. This leads to the rupture of the medulla, the remnants of which are commonly around the edge of the hollow. The air bladders are apparently filled with gases similar to those in the atmosphere.

HETEROKONTOPHYTA, PHAEOPHYCEAE	467

Fig. 21.42 Fucus virsoides. Transverse section of a mature thallus. A mucilaginous cuticle covers the outer epidermal layer in the cortex with the medulla in the center. (From Mariani et al., 1985.)

Life cycle

Fucus will again be used as an example of a typical member of the Fucales (Fig. 21.40). The gametes are borne in conceptacles (Fig. 21.43) that are similar to the cryptoblasts except that the colorless hairs are restricted to a small area near the aperture. The wall of the conceptacle is lined with flat cells that bear branched paraphyses with few chloroplasts. The conceptacles originate from an initial that is a superficial cell of the thallus. The initial divides into an outer tongue cell and an inner basal cell. The tongue cell either degenerates or does not contribute to the development of the conceptacle. The basal cell then divides to form the floor of the conceptacle. At the same time, the cells surrounding the original initial grow and divide so that the derivatives of the initial cell become open to the outside. The mature conceptacle consequently has cells lining the floor derived from the conceptular initial and cells lining the walls derived from the cells surrounding the original initial.

The gametes are released into the seawater during daytime and at times when there is little water motion, reducing the amount of gamete dilution, and ensuring high rates of fertilization (usually about 95%) (Pearson and Brawley, 1998; Ladah et al., 2003). Tide-pool populations of fucoids release their gametes during low tide

when the tide pools are isolated and calm. Intertidal populations release their gametes at slack high tide when the water is calmest.

The plants are either monoecious or dioecious, and in the monoecious forms the antheridia and oogonia can be in the same or different conceptacles. The antheridia are usually formed on paraphyses. Antheridial parent cells are distinguished by dense cytoplasm with few vacuoles, a large central vacuole, and chloroplasts with only a few thylakoids (Berkaloff and Rousseau, 1979). Following meiosis, which occurs during the first two divisions of the primary nucleus, the nuclei undergo four mitoses. Mature antheridia (Fig. 21.43(c), (d)) thus contain 64 nuclei, each of which becomes incorporated into a spermatozoid. Cells in the receptacle release potassium and chloride ions into the mucilage of the conceptacle. This results in swelling of the mucilage that carries the antheridia (or oogonia in the female conceptacle) out of the conceptacle into the seawater (Speransky et al., 2001). The wall of the antheridium is composed of two layers. At liberation, the outer wall ruptures, releasing the inner wall containing the spermatozoids and mucilage. This packet passes out of the conceptacle and into the sea, where the inner wall gelatinizes at one or both ends, releasing the spermatozoids.

468 CHLOROPLAST E.R.: EVOLUTION OF TWO MEMBRANES

a

c

Fig. 21.43 Fucus vesiculosus. (a) Whole plant showing receptacle (arrow). (b) Cross section of a female receptacle showing three conceptacles. (c) Cross section of a male conceptacle. (d) Branched antheridial filaments bearing antheridia. (b), (c), (d) are scanning electron micrographs of frozen tissue. (From Speransky et al., 1999.)

b

d

HETEROKONTOPHYTA, PHAEOPHYCEAE	469

The spermatozoids are spherical at first and then uncoil to give the elongated biflagellate form (Fig. 21.44). There is an eyespot consisting of a single layer of pigment globules inside a reduced chloroplast. The basal portion of the posterior flagellum is closely applied to the plasmalemma in the area of the eyespot. The anterior portion of the cell contains 13 microtubules that make up the proboscis, a structure that may function in the detection of the female sex attractant. The proboscis microtubules pass from the area of the basal bodies, extending themselves in one plane in front of the spermatozoid, and then pass beneath the plasmalemma to the posterior portion of the cell (Manton and Clarke, 1950, 1951, 1956).

The oogonia are usually borne on a stalk cell that is embedded in the wall of the conceptacle (Figs. 21.40, 21.43 (b)). The oogonial cell undergoes three nuclear divisions, yielding eight haploid

nuclei. The cytoplasm then cleaves into eight eggs. The wall of the oogonium is composed of three layers, the thin outer layer or exochite, the thick middle layer or mesochite, and the thin inner layer or endochite. When the oogonium is mature, the exochite ruptures, releasing the packet of eggs, still surrounded by the other two wall layers, into the sea. In the sea, the mesochite ruptures apically, slips backward, and exposes the eggs within the endochite. The endochite rapidly dissolves, releasing the eggs.

The sperm are attracted to the eggs by a species-non-specific pheromone, fucoserraten (Fig. 21.8), released by the eggs (Müller and Jaenicke, 1973). The species-specific recognition between eggs and sperm is based on specific oligosaccharides on the eggs and sperm. The oligosaccharide side chains of the egg-surface glycoproteins contain fucosyl, mannosyl, and/or glucosyl residues (Wright et al., 1995a). The surface of

Fig. 21.44 Spermatozoid of

Fucus. (a) Whole spermatozoid. (b) Semidiagrammatic drawing showing the anterior flagellum (af), eyespot (e), mitochondria (m), proboscis (p), and the posterior flagellum (pf). (c) Section through a spermatozoid illustrating the close appression of the posterior flagellum (pf) to the eyespot area

(e) of the reduced chloroplast.

(n) Nucleus. (After Manton and

Clarke, 1956.)

470 CHLOROPLAST E.R.: EVOLUTION OF TWO MEMBRANES

the egg is not homogeneous, but instead is organized into different domains, each containing different glycoproteins (Stafford et al., 1992). Likewise, the sperm contain glycoproteins organized into domains on the anterior flagellum plasma membrane, the mastigonemes of the anterior flagellum, and the sperm body (Jones et al., 1988). On reaching the egg surface, sperm exhibit a characteristic behavior involving movement over the plasma membrane of the egg and a probing or “searching” of the egg membrane with the anterior flagellum (Brawley, 1991). The glycoproteins on the sperm eventually bind to the complementary glycoproteins on the egg. This results in two “blocks” to further sperm penetration (Wright et al., 1995b):

1A “fast block” within seconds (Fig. 21.45) caused by depolarization of the plasma

membrane due to Na and Ca2 influx. Excess sperm detach from the egg following depolarization.

2A “slow block” results from the formation of a cell wall around the zygote (Fig. 21.46) by the release of cellulose, phenolics, sulfated fucans, vanadate peroxidase, and alginates from cortical vesicles to form the adhesive glycocalyx (Vreeland et al., 1998; Schoenwaelder and Clayton, 1999).

The male nucleus migrates to the female nucleus along associated microtubules. As it migrates, the nuclear envelope of the male nucleus breaks up. The egg nucleus becomes convoluted along the surface nearest the advancing male nucleus. Immediately prior to nuclear fusion, many egg mitochondria accumulate in the vicinity of the male nucleus (Brawley et al., 1976).

The spherical zygote germinates by forming a primary rhizoid from one side, while the rest of the zygote gives rise to the embryo. There are a number of stages in the determination of which part of the zygote will develop into the rhizoid and which will develop into the embryo (Fig. 21.47) (Belanger and Quatrano, 2000; Bisgrove and Kropf, 2004):

1Apolar – In the early stages of development, the fucoid zygotes are apolar and appear to have no inherent order. However, staining of the cells shows the accumulation of F-actin under the plasma membrane in the area of the zygote where the sperm entered.

2Axis induction – A potential polar axis is generated within the zygote. The axis is determined by a number of environmental stimuli, including the direction of incident light, the position of neighboring zygotes, water currents, chemical or ionic gradients, and electrical fields. If none of these stimuli occurs, the axis will be where the sperm originally entered the zygote. During axis induction, the polarity is labile and can be re-

oriented by the subsequent exposure to a stimulus in a different direction. F-actin microfilaments are produced de novo in the area where the rhizoid will develop (e.g., the shaded area of the zygote in light is responsible for axis induction).

3Axis fixation – The F-actin microfilaments guide Golgi vesicles to the plasma membrane where the vesicles release components of a cell wall that is different in composition from the glycocalyx. At this point the axis is fixed and is no longer susceptible to re-orientation by the environment.

Fig. 21.45 The fast block after fertilization of a Fucus egg by a sperm. The potential of the plasma membrane of the egg drops as Na

flows into the zygote after fertilization.

HETEROKONTOPHYTA, PHAEOPHYCEAE	471

Fig. 21.46 Scanning electron micrographs of the egg (left) and zygote (right) of the fucoid Acrocarpia paniculata. After fertilization, the zygote extrudes droplets of material that produce the glycocalyx. (From Schoenwaelder and Clayton, 1998.)

Fig. 21.47 Diagram showing the key events in establishing a polar axis and site of rhizoid germination in a Fucus embryo.

472 CHLOROPLAST E.R.: EVOLUTION OF TWO MEMBRANES

4Rhizoid germination – The spherical symmetry of the zygote is broken by the emergence of a polar bulge representing the emergence of the rhizoid. The direction of the rhizoid germination defines the direction of the polar axis. Unequal cytokinesis follows, perpendicular to the direction of rhizoid germination, yielding an embryo consisting of two freshly differentiated cells, a large rounded thallus cell, which is the precursor of the frond and receives most of the chloroplasts, and a smaller rhizoid cell, which generates the stipe and holdfast.

The embryo divides to form a minute cylindrical plant with at least one apical cell with trichothallic growth, producing a hair above (which may function in the uptake of nutrients (Steen, 2003)) and the thallus below. Eventually the apical cell ceases production of the hair and becomes the three-sided apical cell, which later becomes four-sided. It is only after the initiation of the apical cell that the thallus begins to assume its mature flattened shape.

The Fucales show a periodicity in the formation of receptacles and conceptacles. In Fucus and Ascophyllum (Fig. 21.49) in the North Atlantic, receptacle initiation is a short-day phenomenon, with receptacles being initiated in a 8 : 16 and 12 : 12 light–dark photoperiod and inhibited under a 16 : 8 and continuous light photoperiod (Bird and McLachlan, 1976; Terry and Moss, 1980). White light in the dark period inhibits the short-day response. In the field, conceptacle development commences during September and October, and continues during the following spring until gametes are mature and are liberated during April and May. Following gamete discharge, the conceptacle-bearing receptacles and the rest of the lateral shoot are shed. Halidrys (Fig. 21.50) (Moss and Sheader, 1973) has a different periodicity, with vegetative growth in the spring and summer followed by initiation of the conceptacles. The gametes are then shed during the winter. Even though this is the darkest period of the year, the gametes are able to germinate and secure themselves to a substrate with their rhizoids. These germlings are then able to sustain themselves during the periods of little

light and subsequently grow normally as soon as there is sufficient daylight. Many of the Fucales show a temperature tolerance similar to that of the Laminariales, with 20 °C being the highest temperature at which eggs of Halidrys will germinate.

Ecology

Both the geographical distribution and the location on the shore of a member of the Fucales depend on the ability of the fertilized egg to settle and germinate under the environmental conditions present (Chapman, 1995). Embryos of Pelvetia fastigata (Fig. 21.48) will almost all survive if they settle under adult Pelvetia thalli. Those that settle on exposed rock will almost all die. Within red-algal tufts, most of the younger embryos survive, with survival declining with the increasing age of the settling embryos (Brawley and Johnson, 1991). Sedimentation on top of the embryos reduces embryo survival, particularly if hydrogen sulfide is present (Chapman and Fletcher, 2002; Bergstrom et al., 2003). In addition, the newly attached zygotes also fall prey to browsing molluscs – in particular, the limpets and Littorina species or periwinkles. The mollusc pressure on fucoid development is one

Fig. 21.48 Pelvetia fastigata. (r) Receptacle. (After

Smith, 1969.)

HETEROKONTOPHYTA, PHAEOPHYCEAE	473

of the key factors in determining the number of plants present.

Lodge (1948) carried out an experiment on the Isle of Man to determine the rate of recolonization of a shore with primarily fucoid algae. A strip of shore 5 m wide was cleared of all macroscopic biological life. In the first spring after the clearance, green algae (Enteromorpha, Urospora, and Chaetomorpha) covered the shore, along with diatoms. Fucus germlings then developed beneath the green algae, starting first at the high-tide mark and then establishing themselves toward the low-water mark. The main Fucus species during the first year was F. vesiculosis, whereas during the second year F. serratus became prominent. After the fucoids became established, the green algae gradually disappeared, and a sparse undergrowth of red algae appeared (Dumontia, Laurencia). After two years the Fucus plants dominated the shore, covering a more diverse undergrowth. By this time the limpets had started to recolonize the shore, slowing down colonization by algae. Seven years after the beginning of the experiment, the shore had returned to its original condition.

The littoral Fucales are adapted to the difficult conditions in which they live, being able to withstand freezing temperatures and summer temperatures up to 34 to 36 °C (Malm and Kautsky, 2003). The zonation of the different fucoid species is due partly to their ability to photosynthesize better when exposed to air (Madsen and Maberly, 1990) and partly to withstand desiccation during both germination and growth. The plants that live higher up in the littoral zone have thicker walls, more fucoidin, and a higher water content, and reach their dry weight on evaporation later than those lower in the littoral zone. The proportion of polysaccharides also reflects the fucoid position in the littoral zone. Fucus spiralis and Pelvetia canaliculata, which grow highest in the littoral zone, contain the highest amount of fucoidin, 18% to 24% of the dry weight. Fucus serratus, which grows near the low-tide mark, has much less fucoidin, about 13% on a dry-weight basis (Black, 1954b).

Generally the life-span of most shore fucoids is about 2 to 3 years (Boney, 1966). The only exception to this being Ascophyllum (Fig. 21.49), which

Fig. 21.49 Ascophyllum nodosum. Small portion of the distal end of a plant in late summer condition. (ab) Air bladder;

(l) lateral; (r) receptacle. (After Taylor, 1957.)

has an average age from 12 to 15 years. The average age of the Ascophyllum plants will vary according to their position in the littoral zone. On the Welsh coast, plants from the top of the littoral zone were found to be 4 to 5 years old, whereas those from the bottom of the zone were 5 to 15 years old (David, 1943). Although being long-lived, Ascophyllum produces a paucity of sporelings and it takes a couple of decades for recolonization of a denuded area. This has resulted in severely depleted populations in areas where the alga is commercially exploited (Bacon and Vadas, 1991).

Morphology of fucoid plants will vary with environment. Moss and Sheader (1973) showed that in Halidrys siliquosa (Fig. 21.50) the germlings at 10 °C in total darkness produced long rhizoids and a short thallus. If the same plants were grown at a high light intensity (5936 lux), the thalli were long and unbranched, whereas the rhizoids remained short but were pigmented. If the thalli were grown at 20 °C under high light intensity, branched thalli were obtained. Vesiculation will also vary with the environment. Fucus vesiculosus growing in areas subjected to very severe wave action will lack vesicles, whereas those growing in calmer areas have gas

474 CHLOROPLAST E.R.: EVOLUTION OF TWO MEMBRANES

Fig. 21.50 Halidrys siliquosa. (ab) Air bladder;

(r) receptacle.

vesicles. A minimum branch length seems necessary before vesicle formation can begin, and vesiculation is postponed to the following year if growth has not attained this minimum length (Boney, 1966).

Although the members of the order are normally lithophytes, they are also widely represented by unattached growth-forms lacking holdfasts and propagating mostly vegetatively. These free-living forms arise by vegetative growth of detached branches of the normal attached form that have been transported to a sheltered habitat, or by the development of zygotes in a quiet environment into unattached plants. These unattached plants are referred to as ecads, a term for a plant whose morphology has been altered by growth in an unusual environment. Most of these ecads are found in sheltered habitats such as bays and salt marshes.

Fucoids are basically salt-water algae. Attempts to colonize habitats of lower salinity fail, probably because the “fast block” after fertilization (see section on reproduction of Fucus) relies on depolarization of the egg membrane by Na (e.g., NaCl) flowing in from seawater (Serrão et al., 1999). Low salinity causes loss of the fast block and more than one sperm fertilizing the egg. This commonly results in lost embryos because polyspermy is almost always lethal.

In discussing salt marsh ecads, Boney (1966) states that the fucoids show the following characteristics: (1) vegetative propagation as the main means of propagation; (2) absence of a holdfast; (3) dwarf habit; (4) spiral twisting of the thallus; and (5) profuse branching. Many of the salt marsh forms grow embedded (but not attached by holdfasts) in a muddy substratum, and some are entwined around the stem bases of the dominant angiosperms. The protection afforded by the canopy of angiosperms enables the fucoids to survive at high levels in the marshes.

Ascophyllum nodosum ecad mackaii is an unattached form common in Scotland (Gibb, 1957; Moss, 1971). Normally if a plant of Ascophyllum becomes detached from rocks in the intertidal zone, it is cast up by the waves and soon disintegrates. In the unusually calm waters at the head of some Scottish lochs, the detached thalli are gently covered and uncovered as the tide advances and recedes, but they never dry out and disintegrate. The external forms of the attached plant and of the ecad are in complete contrast. The attached plant is flattened in one plane and generally dichotomizes once a year in the spring after differentiation of an air bladder. During the summer a series of lateral nodes are developed, from which receptacles are produced as laterals the following year. This yearly cycle of differentiation is completely lacking in the marsh form of the ecad. Here there are no air bladders and no regular dichotomy to mark off one year’s growth from another. Instead, apical branching is frequent and in all planes, giving rise to the characteristic cushion form of the ecad. Also there are no lateral nodes and thus no lateral meristems to give rise to lateral branches. Instead, receptacles are sometimes differentiated behind the apices of any branch, apparently at random.

The form of the ecad is caused by the destruction of the apical meristem and the lateral nodes of the ecad. The branches are eventually regenerated from wound-healing tissue to give rise to the reduced ecad form. Growth of the ecad is slow, and large tufts on the shore may have taken several years to grow. The ecad often still has a prominent apical cell, but there is little meristematic activity when compared to the