2.4.3.2: Brown Algae and Diatoms

Learning Objectives

• Use life history, morphology, and cellular components to identify brown algae.
• Identify the components of a kelp thallus.
• Identify structures and events in the Fucus life cycle and know their ploidy.
• Identify structures and events in the Laminaria life cycle and know their ploidy.
• Use life history, morphology, and cellular components to identify diatoms.
• Classify diatoms based on symmetry and ecology.
• Describe sexual and asexual reproduction in diatoms.

Brown algae (class Phaeophyceae) and diatoms (class Bacillariophyceae) belong to the phylum Ochrophyta. They are the result of a secondary endosymbiosis between a heterokont and a photosynthetic eukaryote. Heterokonts, such as the Oomycota, are united by the presence of a textured, or “hairy,” flagellum and an additional flagellum that lacks hair-like projections (Figure \(\PageIndex{1}\)). Members of this subgroup range in size from single-celled diatoms to the massive and multicellular kelp.

Figure \(\PageIndex{1}\): This stramenopile cell has a single hairy flagellum and a secondary smooth flagellum. This is sometimes referred to as having "heterokont flagella", as this morphology is unique to this group.

Brown algae, diatoms, and oomycetes belong to a single clade, the stramenopiles (aka heterokonts). The photosynthetic stramenopiles share the following characteristics:

• 4-membraned chloroplasts
• a yellow-brown pigment (which gives them their color). It is a carotenoid called fucoxanthin.
• chlorophylls a and c
• most have a diplontic life cycle (as you'll find below, Laminaria has a haplodiplontic life cycle)

Phaeophyceae

This group is commonly called the brown algae and includes rockweeds and kelps. Kelps are some of the fastest growing organisms on the planet! Brown algae are primarily marine and are often found in the intertidal zone. Members of this phylum are used for food in some coastal areas of the world and harvested in the U. S. for fertilizer and as a source of iodine.

Brown algae are brown due to the large amounts of carotenoids they produce, primarily one called fucoxanthin. These organisms are exclusively multicellular and have filamentous, multinucleate cells (much like oomycetes). They can get so large that they require special conductive cells to transport photosynthates from their blades down to the rest of their tissues. These conductive cells are called trumpet hyphae and have sieve plates and resemble sieve tubes found in flowering plants. Brown algae have cellulose cell walls and store carbohydrates in the form of laminarin. The polymer alginate can also be found in the cell walls of brown algae and is used commercially for a variety of purposes, including the high fidelity molds used in dentistry.

Kelp

Much like Saprolegnia, the body of an alga is termed a thallus because it is not differentiated into specialized tissues. The general morphology of a brown alga includes a holdfast, stipe, gas bladder(s), and blade(s) (Figures \(\PageIndex{2-4}\)).

Figure \(\PageIndex{2}\): In the diagram above, there are two kelp thalli. The one on the left side is labeled. At the bottom of the thallus is a network of root-like projections that make up the holdfast. The stem-like structure that travels up from the holdfast is the stipe, which terminates in an inflated gas bladder. There are several leaf-like structures attached to the gas bladder. These are blades. The thallus on the right has all of these components, but in a slightly different arrangement. Can you find them? Artwork by Nikki Harris CC-BY-NC with added labels by Maria Morrow.

Figure \(\PageIndex{3}\): A bull kelp thallus that was washed up on the beach and arranged in a spiral so all parts would be visible in one image. At the center of the spiral is the holdfast. This would be attached to the sea floor. A long stipe connects the holdfast to a gas bladder that is obscured by many thin blades, which all attach to the top of the gas bladder. Photo by Maria Morrow, CC-BY-NC.

Figure \(\PageIndex{4}\): A piece of feather boa kelp with several small gas bladders. The four gas bladders shown all attach to the edge of the stipe. If the full thallus were visible, gas bladders would be attached down the entire length of both sides of the flattened stipe. Do you see any blades present? Photo by Maria Morrow, CC-BY-NC.

Fucus

A model organism for the Phaeophyta life cycle is Fucus (rockweed), which, like its relative Saprolegnia, has a diplontic life cycle. The Fucus thallus has dichotomous branching (forking into two equal branches) and swollen, heart-shaped reproductive tips of the branches. These swollen branch tips are called receptacles (Figure \(\PageIndex{5}\)).

Figure \(\PageIndex{5}\): The first image shows a Fucus thallus (though a bit of the holdfast remained attached to the rock). The thallus branches dichotomously, making Y shapes each time it branches and forks into two equal pieces. The second image is a closer view of the ends of the branches. The swollen, heart-shaped ends are called receptacles. The bumps on these receptacles are the tops of the conceptacles--vase-like chambers embedded within the receptacle. Photos by Maria Morrow, CC-BY-NC.

The receptacles are covered in small bumps, each with a pore at the center of the bump called an ostiole. The bumps are conceptacles, chambers that house the gametangia (Figure \(\PageIndex{6}\)). Phaeophyta produce oognia, globose gametangia that undergo meiosis to produce eggs, and antheridia, branched gametangia that undergo meiosis to produce sperm (Figure \(\PageIndex{7}\)).

Figure \(\PageIndex{6}\): A cross section through a Fucus receptacle shows inside a conceptacle. The male and female gametangia are housed within the conceptacle chamber. The antheridia are branched structures that look like small trees. These produce sperm with heterokont flagella (not visible in this image). The oogonia are globose structures divided into sections as eggs are produced. The eggs will be fertilized by sperm that swim in through the ostiole, forming a diploid zygote that will be released in the marine water. This zygote will grow by mitosis into a multicellular, diploid thallus. Photo by Maria Morrow, CC-BY-NC.

Figure \(\PageIndex{7}\): A closer view of the antherida and oogonium inside a Fucus conceptacle. There are no eggs distinguishable within the oogonium. However, individual sperm cells can be seen within the antheridia. These sperm cells would each have the typical heterokont flagella. Photo by Melissa Ha, CC-BY-NC.

Fucus Life Cycle

Fucus has a diplontic life cycle (Figure \(\PageIndex{8}\)) where haploid gametes are produced from a diploid thallus. These haploid gametes do not grow, but fuse together to form a zygote. See Figure \(\PageIndex{9}\) for an example of alternation of generations in the Phaeophyta.

Figure \(\PageIndex{8}\): The diplontic life cycle of Fucus. If you start at the bottom of the diagram, there is a diploid thallus. The ends of the thallus branches are receptacles. A cross section through a receptacle shows us the inside of a conceptacle: the opening of the conceptacle is an ostiole. Within the conceptacle, there is a globose oogonium that has been divided into many compartments. These are egg cells and are the result of meiosis within the oogonium. The branched antheridia produce sperm by meiosis, all of which have heterokont flagella. The haploid sperm are released and swim to fertilize an egg, forming a diploid zygote. The zygote grows by mitosis into a diploid thallus, which brings us back to the beginning. Diagram by Nikki Harris, CC-BY-NC with labels added by Maria Morrow.

Laminaria Life Cycle

A variety of algal life cycles is represented by the stramenopiles, but the most complex is alternation of generations, in which both haploid and diploid stages involve multicellularity. Compare this life cycle to that of humans, for instance. Haploid gametes produced by meiosis (sperm and egg) combine in fertilization to generate a diploid zygote that undergoes many rounds of mitosis to produce a multicellular embryo and then a fetus. However, the individual sperm and egg themselves never become multicellular beings. Terrestrial plants also have evolved alternation of generations. In the brown algae genus Laminaria, haploid spores develop into multicellular gametophytes, which produce haploid gametes that combine to produce diploid organisms that then become multicellular organisms with a different structure from the haploid form (Figure \(\PageIndex{9}\)). Certain other organisms, such as the red alga Polysiphonia, perform alternation of generations in which both the haploid and diploid forms look the same.

Figure \(\PageIndex{9}\): Several species of brown algae, such as the Laminaria shown here, have evolved life cycles in which both the haploid (gametophyte) and diploid (sporophyte) forms are multicellular. The gametophyte is different in structure than the sporophyte. The life cycle begins when sporangia undergo meiosis, producing 1n zoospores. The zoospores undergo mitosis, producing multicellular male and female gametophytes. The female gametophyte produces eggs, and the male gametophyte produces sperm. The sperm fertilizes the egg, producing a 2n zygote. The zygote undergoes mitosis, producing a multicellular sporophyte. The mature sporophyte produces sporangia, completing the cycle. A photo inset shows the sporophyte stage, which resembles a plant with long, flat blade-like leaves attached to green stalks via bladder-like connections. Both the blade and stalks are submerged. Sporangia are associated with the leaf-like structures. (credit “laminaria photograph”: modification of work by Claire Fackler, CINMS, NOAA Photo Library)

Summary of Characteristics for Brown Algae

• Morphology: Multicellular thallus
• Cell wall composition: Cellulose and calcium alginate
• Chloroplasts: 4 membranes, pigments are chlorophyll a, chlorophyll c, and fucoxanthin
• Storage carbohydrate: Laminarin
• Life cycle: Primarily diplontic (alternation of generations in some species)
• Ecology: Marine

Bacillariophyceae

Diatoms are another photosynthetic lineage of photosynthetic heterokonts that was derived from the secondary endosymbiotic event. Diatoms are an incredibly diverse group of unicellular organisms containing anywhere from 20,000 to 2 million species. These organisms are unicellular and surrounded by a frustule, a silica shell made from two distinct valves that enclose the plasma membrane. Frustules are amazingly intricate, covered with small pores in an arrangement specially adapted for capturing sunlight (Figure \(\PageIndex{11}\)). Some diatoms exhibit a slit in their silica shell, called a raphe. By expelling a stream of mucopolysaccharides from the raphe, the diatom can attach to surfaces or propel itself in one direction.

Like the brown algae, they have golden chloroplasts with 4-membranes (Figure \(\PageIndex{10}\)). Diatoms store carbohydrates in the form of chrysolaminarin. The silica frustules of diatoms found in sediments (diatomaceous earth) are used for myriad commercial purposes, including toothpaste additives (as an abrasive), filters, and insulation.

Figure \(\PageIndex{10}\): A diatom filled with golden organelles (chloroplasts). Photo by Vicente Franch Meneu, CC-BY-NC.

Figure \(\PageIndex{11}\): An Isthmia nervosa frustule showing the intricate pattern of pores. There appear to be multiple layers with a different pattern of pores to each. Scientists study these pore configurations when designing solar panels in an effort to mimic the sunlight harvesting abilities of diatoms. Photos by Lama Mark Webber, CC-BY-NC.

Morphology

We are still trying to figure out how to determine what a diatom "species" is and, so far, they have been classified based on the morphology of the frustule. Using this classification, historically there were two major groups of diatoms: centric (have radial symmetry, see Figure \(\PageIndex{12}\)) and pennate (have bilateral symmetry, see Figure \(\PageIndex{13}\)). These classifications have improved and increased in complexity, so here we will cover just the broad strokes. For a more in-depth look at current diatom morphological classification and fantastic images, check out this website.

Figure \(\PageIndex{12}\): These images show centric diatoms. You can draw several lines of symmetry through each of these organisms. Centric is still a morphological description used for diatom genera. First: Triceratium, photo by Ryan Watson, CC-BY. Second: Arachnoidiscus ehrenbergii found on Ulva, photo by Randall, CC0.

Figure \(\PageIndex{13}\): These images show a variety of "pennate" diatoms. This has historically classified many varieties of bilaterally symmetric diatoms which have since been further divided into better-described groups (some even asymmetrical). When considered three-dimensionally, only one line of symmetry can be drawn through this type of diatom. First: Gomphonema acuminatum, scale bar = 10 µm. Photo by Enviroethan, CC-BY-NC. Second: Surirella undulata, photo by Lila_137, CC-BY-NC.

Figure \(\PageIndex{14}\): Many diatoms live in colonies, where unicellular diatoms adhere together to make a more complex structure. This may make it easier to float in the water column (raft-formation) or make it more difficult to be engulfed by predators The examples above show colonies of diatoms in a variety of shapes. First: Asterionella formosa, a bilaterally symmetric diatom, forming a star-shaped colony. Photo by Mindy Morales, CC-BY-NC. Second: Chaetoceros debilis, a centric diatom forming a spiral-shaped colony. Photo by Sarka Martinez, CC-BY-NC.

Ecology

In addition to morphology, diatoms can also be classified by where they occur. Free-floating diatoms are planktonic. Diatoms attached to other organisms (like giant kelp) are epiphytic (Figure \(\PageIndex{15}\)). Epiphytic diatoms can be found in aquatic ecosystems on algae and aquatic angiosperms like eelgrass, as well as terrestrial ecosystems, living in the damp crevices of tree bark. Benthic diatoms tend to dwell toward the bottom of a body of water. In general, these three categorizations refer to aquatic ecosystems. However, diatoms can be found just about anywhere there is water in terrestrial ecosystems. The community composition of diatoms varies depending on location. Because of this, diatoms have been used in forensic investigations to determine where someone drowned (depending on the diatom species present) and how long ago they drown (based on how far the diatoms had migrated into their tissues).

Diatoms are major producers in aquatic environments; that is, they are responsible for as much as 40% of the photosynthesis that occurs in fresh water and in the oceans. They serve as the main base of the food chains in these habitats, supplying calories to heterotrophic protists and small animals. These, in turn, feed larger animals. During periods of nutrient availability, diatom populations bloom to numbers greater than can be consumed by aquatic organisms. The excess diatoms die and sink to the sea floor where they are not easily reached by saprotrophs that feed on dead organisms. As a result, the carbon dioxide that the diatoms had consumed and incorporated into their cells during photosynthesis is not returned to the atmosphere. In general, this process by which carbon is transported deep into the ocean is described as the biological pump, because carbon is “pumped” to the ocean depths where it is inaccessible to the atmosphere as carbon dioxide. The biological carbon pump is a crucial component of the carbon cycle that maintains lower atmospheric carbon dioxide levels.

Figure \(\PageIndex{15}\): Epiphytic diatoms. These diatoms were photographed from a prepared slide of the red alga Polysiphonia. It is a fan-like colony of pennate diatoms that have attached to the surface of the red alga specimen used to make the slide. When the slide was made, it went through a staining bath. This turned the many golden chloroplasts within the diatoms blue. You can see the chloroplasts within the diatoms because the silica frustules are transparent, like glass. Photo by Maria Morrow, CC-BY-NC .

Reproduction

Diatoms primarily reproduce asexually by binary fission, similar to prokaryotes. During binary fission, the two valves of the frustule are separated and each new cell forms a new valve inside the old one. However, the new valve is always smaller. If diatoms only reproduce in this way, it results in a continual decrease in average size. When some minimal size is reached, this can trigger sexual reproduction. When diatoms sexually reproduce, they have a diplontic life cycle and produce a very large auxospore (Figure \(\PageIndex{16}\)).

Figure \(\PageIndex{16}\): When diatoms sexually reproduce, they make a large structure called an auxospore. In this picture, the auxospore is a lightbulb-shaped cell located at the end of the colony of epiphytic diatoms. There are many golden chloroplasts visible each diatom. Photo by Maria Morrow, CC-BY-NC .

Diversity

Video \(\PageIndex{1}\): This video shows some of the incredible diversity of diatom shapes and the amazing art Klaus Kemp makes with them. Sourced from YouTube.

Summary of Characteristics for Diatoms

• Morphology: Unicellular
• Cell wall composition: Silica frustule
• Chloroplasts: 4 membranes, pigments are chlorophyll a, chlorophyll c, and fucoxanthin
• Storage carbohydrate: Chrysolaminarin
• Life cycle: Diplontic
• Ecology: Everywhere! Marine, freshwater, and terrestrial.

Summary

Though brown algae and diatoms seem to have very little in common morphologically, they are descended from a common ancestor. Both of these groups have a diplontic life cycle during some stage of which a cell will have heterokont flagella. They have 4-membraned chloroplasts that contain the pigments chlorophyll a, chlorophyll c, and fucoxanthin. This latter pigment gives the chloroplasts in these groups a golden color. This is about where the similarities end.

Brown algae are exclusively multicellular and found in marine habitats, most typically in the intertidal zone. Their cell walls contain cellulose and they store their carbohydrates as laminarin.

Diatoms are exclusively unicellular and found in almost every habitat where there is water. Their single cell is surrounded by a silica frustule composed of two distinct valves. They store their carbohydrates as chrysolaminarin.

Attribution

Curated and authored by Maria Morrow, CC-BY-NC, using the following sources:

• 19.1.2 Protists from Biology by John. W. Kimball (licensed CC-BY)
• 23.3 Groups of Protists from Biology 2e by OpenStax (licensed CC-BY). Access for free at openstax.org.

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