Postelsia palmaeformis is an intertidal brown alga that occurs on rocky shores of the Pacific coast of North America, usually in patches from a few to 100s of individuals. As an autotroph, it produces its own food via photosynthesis. P. palmaeformis is an annual and like many brown algae has two distinct morphologies during its lifecycle – the microscopic gametophyte and the macroscopic sporophyte, which resembles a small palm tree. The sporophyte’s maximum height is variable, but can reach 60-75 cm. The dispersal distance for P. palmaeformis is unusually low for an annual alga – only about 1-3 m. Because of this, the extinction of a patch (via over-harvesting by humans, for example) can be permanent.
The sea palm is an interesting, well-studied, and charismatic organism:
- It’s the only kelp that can stand erect in air – without the support of the water. Yet the strong stipe is also flexible enough to allow the alga to bend with the motion of powerful waves.
- Not only does it live in areas with waves so strong that most other organisms can’t survive, but it actually requires heavy surf and can't establish in areas of calmer water. (Remember this as you explore the intertidal, and be careful – if there’s Postelsia, there will be strong waves.)
- Postelsia has a complex relationship with the mussel, Mytilus californianus, that covers much of its rocky habitat. Bare rock is the best place for sea palms to settle, but if there isn’t any nearby, it will settle on the mussels. The mussels will still eventually crowd it out, unless something removes a clump, leaving room for Postelsia. It’s usually strong waves that suck the mussels from the rocks. And mussels with Postelsia growing on their shells are more likely to be removed. So sea palms can “help” make space for themselves in the mussel-bed. But the mussels may actually help Postelsia survive during parts of its life-cycle, the tiny gametophyte (egg or sperm-making) stage and the young sporophyte (palm-tree-shaped) stage, by providing protection from harsh sun and drying air.
- Postelsia doesn’t disperse very far from its parent. That’s unusual for an annual plant or alga, especially one living in an area with such intense competition for space from other organisms. Annuals don’t tend to be very good at holding onto space, so they rely on being able to send their propagules (parts that can be used for making new individuals) to colonize newly opened spaces.
- People like to eat Postelsia fronds. However, in many places it's illegal to collect it, because of the risk of driving local populations/patches to extinction.
Postelsia palmaeformis, is the only described species in the genus. Like most brown algae, it has two phases in its lifecycle, each with a distinct form. The visible palm-tree-shaped sporophyte is composed of a holdfast, a single stipe, and a canopy of grooved “lanceolate” blades on which spores are produced. The spores attach to the rock (or other hard surface) then germinate and develop into the microscopic gametophyte phase. Male gametophytes produce sperm, which fertilize the eggs made by female gametophytes. Fertilized eggs grow into new sporophytes. (See chapters on Life Cycle and Reproduction)
See labeled illustration above.
SPOROPHYTE (diploid): “Sporangial thalli with relatively small holdfast of stout, branched haptera. Stipe erect, cylindrical and hollow, tapering slightly from base to apex. Apex of stipe with many short, radially disposed, simple branches, each terminating in single blade. Branch and blade splitting longitudinally into 2 blades and branches, usually of equal size, the splitting beginning at junction of branch and blade. Blades sharply pointed, narrowly linear, the margins dentate. Both flattened surfaces of blade with deep, parallel, longitudinal grooves, the grooves of 1 surface alternating with those of other. Sporangia in linear sori, these lining grooves of blade.…Sporangial thalli to 60 cm tall, usually growing in extensive stands; stipes erect and blades pendant when plants are exposed by recession of tide; mature plants golden brown, with 100 or more blades, these to 25 cm long; sporangia first produced in late spring; blades becoming eroded after fruiting; spores released during low tide and remaining in grooves of blades, dripping off the slender tips onto holdfast or nearby rock. Locally abundant in areas exposed to surf, saxicolous, high to midtidal.” (Abbott and Hollenberg 1976, p. 251)
GAMETOPHYTE (haploid): “…dimorphic, dioecious, oogamous, branched uniseriate filaments; growing to a small size and producing gametes, even in the absence of blue light or added iron in the medium, which are 2 conditions found to be necessary for gametogenesis in gametophytes of other members of the order Laminariales.” (Algaebase) See also Lewis 1995.
Male gametophytes are smaller than females, and their cells are relatively pale. Males produce antheridia (sperm-producing organs), and females produce eggs. (Lewis 1995)
A marine species. It occurs in the upper intertidal of rocky shores with high wave energy.
Disperses as spores, with very short dispersal distances of 1-3 meters (R. T. Paine, personal communication; Kusumo et al. 2006).
Produces its own food via photosynthesis and absorbs nutrients from the water.
Heavy wave action is necessary for the persistence of populations of Postelsia palmaeformis. Sea palms will be overgrown by the mussel Mytilus californianus unless patches of the mussel are periodically removed by strong waves, either directly or via the crushing-force of drift logs. In the absence of bare rock (the best attachment site for Postelsia), sea palms will attach to mussel shells. This increases drag on the mussels, thus increasing the probability that patches of overgrown mussels will be sucked from the rock, leaving space for the next generation of Postelsia to attach and grow. (R. T. Paine, personal communication; Paine 1979)
Mussels might also facilitate the recruitment of new generations of Postelsia by providing protection for gametophytes and young sporophytes (Blanchette 1996).
P. palmaeformis is harvested by humans for food. This can lead to the permanent extinction of a patch. Because of this, all regions where it occurs have regulations that limit or outlaw its collection.
Postelsia competes for space in the rocky intertidal with the mussel Mytilus californianus, which can eventually crowd it out (Paine 1988).
Life History and Behavior
[Refer to life-cycle diagram among images above]
Postelsia has the typical two-phase kelp life-cycle, with a visible diploid sporophyte and a microscopic haploid gametophyte. The grooves on the blades of the sporophyte are lined with sori, in which haploid (1N) spores are produced via meiosis (cell division that results in the halving of the number of chromosomes). Mature spores drip from the hanging blades to the rock below (or possibly mussel valves or barnacle plates) during low tide, i.e., when the alga is exposed to the air. These heterokont spores (with two flagella – one long and mature and the other short and immature) can swim only about 1-3 meters before they attach to the substrate. Within as soon as 1 day after release, a spore can germinate, growing rapidly into a branched filamentous several-celled gametophyte. There is a 1:1 ratio of male to female gametophytes, both of which can be fertile within about 8 days of release. Swimming sperm fertilize the eggs extruded by females. The diploid (2N) zygote over-grows the female gametophyte to become a new palm-tree-shaped sporophyte.
(Dayton 1973, Paine 1988, Lewis 1995, Anderson 2004, Kusumo et al. 2006)
Postelsia palmaeformis is an annual. Sporophytes become visible in February or March, become reproductive during the spring, and post-reproductive individuals are abraded and torn from the substrate in the fall (Dayton 1973, Paine 1988). The next generation of sporophytes probably begins developing within a few weeks of spore release (see Lewis 1995) but are too small to see until late the following winter.
[Refer to life-cycle diagram among images above]
Sea palm sporophytes produce zoospores (motile spores) in structures aligned in the grooves on their blades. Spores have half the number of chromosomes that the parent sporophyte has. After release from the parent, spores grow into tiny gametophytes of only a few-several cells in size. Sperm produced by male gametophytes fertilize eggs extruded by female gametophytes. Most females produce 1 egg, but some produce 2, and a few produce 3. All three can be fertilized to grow into 3 related (at least half-sibling) sporophytes. (Lewis 1995)
There’s no evidence that Postelsia can reproduce vegetatively, though some self-fertilization probably occurs (Lewis 1995; Kusumo et al. 2006).
Evolution and Systematics
Systematics and Taxonomy
Nereocystis luetkeana (bull kelp) is the closest living relative of Postelsia palmaeformis (Lane et al. 2006).
Sea palms survive changing intertidal conditions by adopting various postures, thanks to a unique set of mechanical properties.
"Postelsia palmaeformis--the scientific name makes the same allusion as the common name, 'sea palm'--lives in the lower intertidal zones of rocky, wave-swept shore on the west coast of North America. The plant, shown in figure 21.4, never reaches a meter in height, so it's not exactly a big tree. But, like trees, it has three parts, an attachment (holdfast), a column (stipe), and photosynthetic laminae (fronds). It stands against gravity, getting higher in dense stands, and it bends in response to lateral force, for it, waves. Unlike any tree, though, it responds to lateral force by bending until almost prostrate and then springing back upright. Concomitant with that untreelike behavior are a set of most untreelike material properties, set out in a lovely paper by Holbrook, Denny, and Koehl (1991). Table 21.1 lists these, along with typical values for wood.
"Postelsia's 'trunk' certainly looks wimpy next to wood. It gives rather than standing tall in the face of fierce force, getting from a high second moment of area just enough flexural stiffness to stand erect at all. Only in work of extension does it play in the same league as wood, although it gets its high value by quite a different tactic--high stretchiness instead of high strength. Regaining its erect posture depends on reinvesting that work of extension, which requires a high resilience, which it has. But, as Holbrook, Denny, and Koehl emphasized, the energy storage underlying that resilience undermines its toughness. Postelsia is notably brittle and sensitive to scratches--its stipe may be soft, but cracks propagate all too readily. Before dismissing this alga as just a lowly tree wannabe, bear in mind that it has extraordinarily high photosynthetic productivity and that it invests far more, proportionately, in photosynthetic organs than any ordinary tree. Its fronds represent no less than 38 percent of its energy content or 35 percent of its weight (Lawrence and McClintock 1988)--compare this with the trivial weight and burning yield of the annual leaf-fall of a tree that yields a cord or so (perhaps 2 metric tons) of dry firewood." (Vogel 2003:434-435)
"Sea-palms Postelsia palmaeformis Ruprecht are annual brown algae that grow on wave-swept rocky shores, often forming dense stands. Unlike most macroalgae, Postelsia stands upright in air--like trees. The stipe flexibility that permits Postelsia to withstand waves is provided by the low elastic modulus
(5-10 MPa) of stipe tissue; in spite of the weakness (low breaking stress, ~ 1 MPa) of this tissue, a large amount of energy ( ~ 100 kJ/m 3) is required to break a stipe because they can be extended by 20-25 % before breaking. Although made of such easily deformed tissue, Postelsia can stand upright in air due to the width (high second moment of area) and resilience of their stipes, but the brittleness (low work of fracture, 400-900 J/m 2) that accompanies this resilience renders them susceptible to breakage if they sustain deep scratches. Although wave-induced stresses experienced by individuals in aggregations are not lower than those experienced by isolated sea-palms, photon flux densities of photosynthetically active radiation within these dense groves are less than 10% of those above Postelsia canopies. A number of morphological features differ between canopy, understory, and isolated individuals. Canopy plants in dense aggregations are taller than isolated individuals and may exceed limiting proportions for elastic stability. Postelsia shows photosynthetic characteristics of "shade-adapted" plants, understory individuals being especially effective at using low light. Despite this, blade growth rates of understory plants are lower than those of either canopy or isolated individuals." (Holbrook et al. 1991:39)
Watch Video (closer view at end)
Learn more about this functional adaptation.
Molecular Biology and Genetics
In laboratory studies, Lewis (1995) found a diploid number of 26-34 chromosomes in sporophytes (the macroscopic “palm-tree” stage) and a haploid number of 14-17 chromosomes in sporangia (the location of meiosis to produce spores) and in gametophytes (the microscopic stage that produces eggs and sperm).
Barcode data: Postelsia palmaeformis
Statistics of barcoding coverage: Postelsia palmaeformis
Public Records: 3
Specimens with Barcodes: 6
Species With Barcodes: 1
Over-harvest by humans. Though this is regulated throughout its range.
The fronds (aka blades) are edible by humans and have become increasingly popular as additions to soups or salads. Blades can regenerate if removed during the spring and summer growing season, but the alga cannot regenerate if cut through the stipe. Blades produce the spore phase of the lifecycle, so if too many are removed before spores are released, a local population can be endangered or exterminated (Report to Fish & Game Commission 2004).
Recent research suggests that some of the harvesting practices claimed as “sustainable” may not be (Thompson et al. in press). The authors recommend the following limits on harvest: allow the trimming of fronds only, never the stipe, and allow only one harvest per year, before reproduction begins.
Harvest of Postelsia palmaeformis is illegal throughout most of its range – Oregon, Washington, and British Columbia. Recreational harvest is prohibited in California, but the California Fish and Game Commission issues licenses to commercial seaweed harvesters who pay a royalty to the state per wet-ton collected (Report to Fish & Game Commission 2004).
Relevance to Humans and Ecosystems
Humans eat the fronds of Postelsia, but unless proper harvesting techniques are used, this can lead to the extinction of a patch (Thompson et al. in press; Report to Fish & Game Commission 2004).
Postelsia palmaeformis, also known as the sea palm (not to be confused with the southern sea palm) or palm seaweed, is a species of kelp and classified within brown algae. The sea palm is found along the western coast of North America, on rocky shores with constant waves. It is one of the few algae that can survive and remain erect out of the water; in fact, it spends most of its life cycle exposed to the air. It is an annual, and edible, though harvesting of the alga is discouraged.
The sea palm was known by the natives of California by the name of Kakgunu-chale before any Europeans entered the region. Postelsia was first scientifically described by Franz Josef Ruprecht (1814–1870) in 1852 from a specimen found near Bodega Bay in California. Ruprecht, an Austro-Hungarian who became curator of botany at the Academy of Sciences in St. Petersburg in 1839, studied seaweed specimens collected by botanist Ilya Vosnesensky, and published a paper describing one seagrass and five seaweeds, one of which was Postelsia. The sea palm has been used by several textbooks, such as the Campbell–Reece Biology textbook, as an example of multicellular protists, as well as an example of the class Phaeophyceae.
The generic name, Postelsia honors Alexander Philipov Postels, an Estonian-born geologist and artist who worked with Ruprecht, while the specific name, palmaeformis, describes the alga's superficial similarity in appearance to true palms.
Fossils from Monte Bolca, a lagerstätte near Verona, were originally named Zoophycos caput-medusae and previously thought to be trace fossils, but were later found to be plants instead and given the name Algarum by French zoologist Henri Milne-Edwards in 1866. The type specimen collected by Italian paleobotanist Abramo Bartolommeo Massalongo before 1855 is at the Natural History Museum of Verona and was preserved in a lithographic limestone upper and lower slab.
When Italian botanist Achille Forti (1878–1937) worked on the specimens in 1926, they were reinterpreted as close relatives of Postelsia, now known to be a brown algae, which had lived in the coastal waters of the Eocene sea. Forti renamed the species Postelsia caput-medusae putting it in the genus Postelsia, which now has only one species, Postelsia palmaeformis. The appearance of the plant fossil is a holdfast on the bottom, with a stem-like stipe between there and the fronds which are about 5 centimetres (2.0 in) to 10 cm (3.9 in). In life, the fronds would have hung vertically when the tide was in but flop over the stipe when exposed by low tide.
Other specimens from this deposit collected and described by Massalongo in 1855 were actually trace fossils; only this one specimen was assigned to Postelsia.
Postelsia has two distinct morphologies: one for its diploid, monoicous sporophyte stage, which is the dominant portion of the life cycle, and one for its smaller, haploid, dioecious gametophyte stage. Like all seaweeds, the sporophyte stage of Postelsia consists of a thallus, which is made up of a stem-like stipe topped with possibly over 100 leaf-like blades, and rests on a root-like holdfast. The holdfast anchors the organism to the rocks it lives on. The sea palm has no vascular system; the stipe is only for support of the organism and holds the fronds up over other organisms so they can receive more light. The stipe is merely a firm, hollow tube, able to withstand the open air of low tide conditions as well as the crashing waves of high tide. The blades are grooved, with the sporangia held within these grooves. The gametophyte stage is microscopic, consisting of only a few cells. The gametophytes produce sperm and eggs to create new sporophytes.
Like all phaeophytes, sea palms use the pigments chlorophyll a, chlorophyll c, fucoxanthin, and carotenes in photosynthesis. Their cell walls are composed of alginate. They use laminarin and mannitol for storage.
Life cycle and growth
Like most brown algae, Postelsia goes through alternation of generations, and is an annual species. The diploid sporophyte produces, through meiosis, haploid spores, which drip down through the grooves in the blades onto the substrate, which may be mussels, barnacles, or bare rock. These spores develop, through mitosis, into small, multicellular haploid gametophytes, male and female. The male and female gametphytes create sperm and eggs, respectively. The sperm of the male reaches the female egg and fertilizes, resulting in a diploid zygote, which develops into a new sporophyte.
Postelsia are green in color as juveniles, and change to a golden brown as they age, reaching a height of 50–75 cm (20–30 in).
As a Postelsia alga grows, its stipe thickens in the same manner as a tree's trunk. The cells beneath the epidermis, called the meristoderm, divide rapidly to form rings of growth, again, like a tree. However, the greater flexibility of Postelsia 's stipe over that of a woody tree makes for some distinct differences. Postelsia must be thicker than a tree of equal height in order to support itself. However, the stipe is very much more suited to the coastal habitat, as it allows the seaweed to bend with the constant wave action. Such an environment would cause the inflexible, woody tree to break.
The blades of the new sporophyte grow from one or two initial blades by splitting. A tear forms in the middle of the blade at its base, which then continues along the entire length of the blade until it is split in two.
Sea palms are found on the rocky shores of western North America, from as far north as Vancouver Island, to the southern central coast of California. They live in the middle to upper intertidal zones in very wavy areas. High wave action may increase nutrient availability and moves the blades of the thallus, allowing more sunlight to reach the organism so that it can photosynthesize. In addition, the constant wave action removes competitors, such as the California mussel. Recent studies have shown that Postelsia grows in greater numbers when such competition exists. A control group with no competition produced fewer offspring than an experimental group with mussels; from this it is thought that the mussels provide protection for the developing gametophytes. Alternatively, it is thought that the mussels may prevent the growth of competing algae such as Corallina or Halosaccion, allowing Postelsia to grow freely after wave action removes the mussels.
When Postelsia release their spores, they tend to fall within a few meters of the parent sporophyte for two reasons. The first is that though spores are flagellated and can swim, they are often released at low tide and are deposited directly to the substrate below. Secondly, Postelsia gametophytes need to be close to each other in order for fertilization to occur. As such, sea palms tend to live very close to each other in large aggregations. Some juvenile sporophytes will grow on competing organisms, like mussels or barnacles, and rip them from the rocks when the waves come, gripping them with holdfasts of incredible strength.
Two other, smaller brown algae, of the family Ectocarpaceae, Ectocarpus commensalis and Pylaiella gardneri, as well as the two red algae Microcladia borealis and Porphyra gardneri, are epiphytic on Postelsia. Pylaiella gardneri is an obligate epiphyte to Postelsia. As with all epiphytes, these algae are not harmful to Postelsia, and merely use the larger alga as a substrate to grow upon.
The blades (and less often, the stipes) of Postelsia are sometimes used in certain dishes, usually in California. Postelsia is a protected species, however, and harvesting it is illegal throughout much of its range, as clipping the blades too low, below the meristem, prevents reproduction. Postelsia can regenerate blades cut above the meristem, but removing the blades can limit a sporophyte's ability to produce spores and contribute to subsequent populations. Postelsia has also been in danger of overharvesting at some points. It is illegal to harvest Postelsia in British Columbia, Washington and Oregon. In California, Postelsia is a partially protected species: recreational harvesting is illegal, but commercial harvesting is legal. Between 2000 and 2001, an estimated 2 to 3 tons of Postelsia were harvested in California. The blades are eaten raw or are dried, and dried blades sell for up to US$45 per pound. Commercial harvesters of Postelsia must purchase a $100 license, pay a royalty to the State of California ($24 per wet ton of algae harvested), and submit a monthly harvest log.
An experiment done to try to prove or disprove the claims of Postelsia harvesters that their gathering methods are sustainable yielded results stating that recovery from collection depended greatly on the season of collection.
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- Thompson, Sarah Ann, Karina J. Nielsen, Carol A. Blanchette, Brennan Brockbank, Heather R. Knoll. "Effects of commercial collection on growth and reproductive output of Postelsia palmaeformis", Sonoma State University, University of California, Santa Barbara, 13 July 2007.
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