Division Phaeophyta: Brown Algae

The brown algae (Table ) are almost exclusively marine; only a few fresh-water species are known. They prefer cold water that is very agitated and aerated. They can most easily be found on rocky coasts growing in the littoral zone, the region between low tide and high tide, also called the intertidal zone, where they are periodically exposed to air and full sunlight. The upper sublittoral zone also contains numerous brown algae if it is rocky and offers stable surfaces for attachment. Over f 500 species are known, grouped into about 250 genera.

The brown algae are the most complex algae anatomically and morphologically, some considerably more complex than mosses and liverworts. Although very distinct from true plants biochemically and ecologically, the two groups have remarkable parallels in the types of bodies and life cycles that have evolved.

The differences between brown algae and green organisms (both green algae and plants) are clear-cut: Brown algae have chlorophyll a and c and large amounts of a variety of xanthophyll pigments such as fucoxanthin, violaxanthin, and diatoxanthin. Carotenes are also present. Their suite of pigments permits the brown algae to carry out photosynthesis at numerous levels in the ocean. Sunlight differs not only in intensity but also in quality at different depths: White light with a full spectrum occurs at the surface, but primarily just blue-green light reaches depths of 50 meters or more.

The storage product of brown algae is laminarin (a polymer of glucose), mannitol, or fats, but not starch. Some brown algae store large amounts of laminarin, up to 34% of their body weight. Most algae, especially the marine species, live in such a stable environment with regard to light, temperature, and nutrients that photosynthesis and growth occur more or less continuously, so large reserves are unnecessary. But many brown algae; especially the kelps (order LaminHelveticaes) that grow near the surface, have large bodies made up of three parts: holdfasts and stipes (stalks) that may be perennial, and photosynthetic blades that are annual (Figs. 1 and 2). When the blades become moribund and decompose, the holdfasts and stalks must subsist on stored nutrients until the new blade can be formed and begin photosynthesis in the spring.

Cell walls of brown algae contain cellulose and alginic acid, an unusual polymer of D-mannuronic acid and L-guluronic acid not found in other algae. The alginic acid component of the wall is gummy or slimy and causes the filaments of cells to adhere into a compact body. It may have another, unknown function, however, because as much as 24% of the body dry weight can be alginic acid in Ascophyllum

.

FIGURE 1:In many kelps, the blade is annual but the stipe and holdfast are perennial. A new blade is formed each year; if the old blade has not been completely destroyed by wave action, it is sloughed off.

FIGURE 2:Many kelps become extremely large, but their bulk is due to great length, not great width. The stipe and air bladder are the thickest parts, up to several centimeters, but the blade is always thin. Long, narrow, elastically flexible bodies are well adapted for life in a tidal region. Think of the damage that wind causes to land plants. Water is more dense and massive, so it is virtually impossible to build a large body that could resist such constant pressure from currents. The flexible bodies of brown algae, like those of willows, bend without breaking and so remain undamaged by the fluid flowing around them. (Gregory Ochocki/Photo Researchers).

A remarkable feature of brown algae is that they are all multicellular; no unicellular species is known to exist, and individuals of many species of kelps become huge and complex. Plants of Nereocystis that measure up to 45 m—taller than most trees—are not uncommon, but they are not bulky, being less than 5 cm thick. Any plant that becomes as long as half a football field must have specialized regions to its body. The holdfasts of a kelp are located in deep, dark, poorly aerated water, whereas the blades exist in brightly lighted and well-aerated shallow waters. To keep from being damaged by wave actions, the bodies must be firm, elastic, and thick—too thick for diffusion alone to mediate the exchanged of gases, nutrients, and wastes. Some species have an epidermis-like outer covering, a paren- chymatous middle tissue that resembles cortex, and a cylinder of trumpet cells that re­semble phloem cells so much that many biologists do call them sieve tube members (Fig 3). Trumpet cells carry out long-distance transport of carbohydrates through the body They are elongate, large holes occur in a sieve-like arrangement in the end walls, and the holes are lined with callose. If radioactive carbon dioxide is given to the blade, a short time later radioactive photosynthates can be detected in the trumpet cells of the stipe. The flow rate can be as high as 65 to 78 cm/hr.

FIGURE 3: (a) A longitudinal section through the stipe of the kelp Macrocystis. In the center are numerous trumpet cells (X 290) (b) A young trumpet cell, showing the enlarged end walls where holes have developed (X 500). (Courtesy of M. L. Shih, J. -Y. Floch, and L. M. Srivastava, Simon Fraser University).

Are these cells sieve tube members? Although trumpet cells and sieve tube members are extremely similar structurally and metabolically, they are not at all homologous. They did not evolve from a common ancestral cell, nor did trumpet cells evolve from sieve tube members or vice versa. Trumpet cells and sieve tube members are outstanding examples of convergent evolution.

One of the simplest of the brown algae is Ectocarpus (Fig. 4). It has an alternation of isomorphic generations, both generations consisting of uniseriate branched filaments that arise from a prostrate branched system attached to rocks, shells, or other, larger algae. In the diploid sporophytes, some of the terminal cells of small lateral branches enlarge greatly and become unilocular sporangia. The nuclei divide repeatedly, then 32 or 64 zoospores are released. The first nuclear division is meiotic, so the spores are haploid. After swimming temporarily, zoospores settle down and grow into gametophytes that are almost identical to the sporophytes. The primary difference is that at the ends of the branches are multicellular gametangta, not unicellular sporangia as in sporophytes. Because they are multicellular, they are called plurilocular gametangia. The gametes are anisogamous: Some settle and attract others by secreting a sex hormone ectocarpene. After fertilization, the zygote grows into a new sporophyte.

FIGURE 4:An individual of the brown alga Ectocarpus, a species that has an alternation of isomorphic generations. From this photo, it is not possible to tell if this is a gametophyte or a sporophyte (X 8). (Visuals Unlimited/Cabisco).

A more complex brown alga is Fucus, which is common on rocks in the intertidal zone (Figs. 5). The diploid individuals are exposed at low tide, and their bodies can be seen to be large (up to 2 m), dichotomously branched, and attached to the rock by holdfasts. The bodies are complex histologically with epidermis, cortex, and a central region. The ends of the branches are called receptacles and are swollen with large deposits of hydrophilic compounds. Scattered over the surface of the receptacles are minute openings that lead to small cavities, conceptacles; some conceptacle cells undergo meiosis, producing either large eggs or small sperms. At low tide, individuals are exposed to air, and the conceptacles contract, squeezing out gametes. When the tide comes in, gametes are washed free and fertilization occurs in the water. The fertilized eggs settle to the bottom and grow into new diploid individuals. No free-living haploid generation occurs; Fucus is monobiontic.

FIGURE 5:Life cycle of Fucus; details are given in text.

The large kelps, such as Nereocystis, have complex bodies and complex life cycles. The large, visible individuals are diploid sporophytes and generally consist of a holdfast, a sip with an enlarged air bladder (pneumatocyst) that provides flotation, and several leaf-like blades on each air bladder. The kelps all have bodies composed of true parenchyma, rather than filaments that adhere to each other (Fig. 6). All body parts have an outer meristoderm that is both meristematic and photo synthetic, a cortex of parenchyma, and a central region of elongate cells and trumpet cells.

Holdfasts, stipes, and air bladders are often perennial, and one specimen of Pterygo -phora is known to be 17 years old. These portions grow in circumference each year through the activity of the meristoderm, which adds new layers to the surface. Trumpet cells, like sieve tube members, function only temporarily, then are replaced by new ones that differentiate from cortex cells.

FIGURE 6:The bodies of kelps can be quite complex. (a) The outer layer is a meristoderm. (b) Cortex with photosynthetic cells and mucilage ducts, each of which is surrounded by secretory cells (c). (d) The central region of the stipe or the blade midrib is composed of trumpet cells, which have pores in their end walls (e and f). (g) During reproduction, surface cells elongate greatly, becoming sporangia in which meiosis will occur.

The junction between an air bladder and a blade is an intercalary meristem capable of prolonged growth, which produces blades several meters long. Growth can be extremely rapid, up to 6 cm/day. Blades are photosynthetic and vegetative for a period after their formation by the intercalary meristem, but then portions become fertile and produce haploid zoospores by meiosis (Fig. 6g). Under inductive conditions, sporangia form either in patches or over the blade's whole surface. In Macrocystis, some blades remain vegetative and others are specialized and produce sporangia. Macrocystis releases as many as 76,000 spores/min/cm² of reproductive blade surface; the mean rate over long periods is 5000 spores/min/cm². Spores grow into tiny, filamentous gametophytes that somewhat resemble small plants of Ectocarpus—an alternation of heteromorphic generations. Game- tophytes produce oogametes, and after fertilization, a new sporophyte is formed.