The Science of Seaweeds

Marine macroalgae benefit people culturally, industrially, nutritionally, and ecologically.

Biology Botany

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November-December 2013

Volume 101, Number 6
Page 458

DOI: 10.1511/2013.105.458

Macroalgae are, loosely speaking, those that can be seen with the naked eye. Most of them are classified as benthic, which is to say that they fasten themselves to the seabed. But there is no need to be so formal in talking about benthic macroalgae. We commonly refer to them all the time by the single term seaweeds, and we make use of them far more often than most people realize.

From Seaweeds: Edible, Available, and Sustainable by Ole G. Mouritsen.

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Macroalgae come in a great many varieties. Some of the larger ones have complex structures with special tissues that provide support or transport nutrients and the products of photosynthesis; others are made up of cells that are all virtually identical. The smallest seaweeds are only a few millimeters or centimeters in size, while the largest routinely grow to a length of 30 to 50 meters. Seaweed cells also come in different sizes; in many species they can measure one centimeter or more. These large cells can contain several cell nuclei and organelles in order to ensure that the production of proteins is sufficient to sustain the function of the cell and the rapid growth of the seaweed as a whole.

Macroalgae are classified into three major groups: brown algae (Phaeophyceae), green algae (Chlorophyta), and red algae (Rhodophyta). As all of the groups contain chlorophyll granules, their characteristic colors are derived from other pigments. Many of the brown algae are referred to simply as kelp.

It is estimated that 1,800 different brown macroalgae, 6,200 red macroalgae, and 1,800 green macroalgae are found in the marine environment. Although the red algae are more diverse, the brown ones are the largest. Even though we talk about the three groups of seaweeds as if they were closely related, this is true only to a minor extent. For example, brown algae and red algae belong to two different biological kingdoms and are, in a sense, less related to each other than, for example, a jellyfish is to a bony fish. Green algae and red algae are more closely related to higher plants than brown algae are and, together with diatoms, they evolved earlier than brown algae.

Most species of seaweeds have soft tissues but some are, to a greater or lesser degree, calcified, an example being calcareous red algae. The growth of the calcium layer is precisely controlled by the polysaccharides that are present on their cell walls.

From Seaweeds: Edible, Available, and Sustainable by Ole G. Mouritsen.

Seaweeds, especially the brown algae, are generally made up of three distinctly recognizable parts. At the bottom there is a root-like structure, the holdfast, which, as the name implies, secures the organism to its habitat. It is usually joined by a stipe (or stem) to the leaf-like blades. The seaweed can have one or more blades, and the blades can have different shapes. In some cases, the blades have a distinct midrib. Photosynthesis takes place primarily in the blades and it is, therefore, important that the stipe is long enough to place them sufficiently close to the surface of the water to reach the light. Some species have air-filled bladders, a familiar sight on bladder wrack, which ensure their access to light by holding them upright in the water. These bladders can be up to 15 centimeters in diameter. Because brown algae are so much like plants, they are often confused with them.

Not all seaweeds share these structures. Some smaller species have a tissue that has a less distinctive structure, consisting only of filaments of cells, which may or may not be branched. But almost all varieties have found their place, one way or another, into the human diet.

Originally, seaweeds intended for human consumption were collected along the seashore or picked in the sea. Those that were eaten fresh were harvested locally and consumed in short order. As seaweeds can be dried and, in that form, kept for a long time and transported easily, they were recognized early on as a valuable foodstuff and became a trading commodity. Over time, the demand for seaweeds, for a multiplicity of purposes, grew so great that for many centuries they have been actively cultivated, especially in the Far East.

The Complex Life of a Sea Green

The life history of algae is complicated, and this is what really differentiates them from plants. In fact, macroalgae can pass through life stages so distinct that, in the past, they have been mistaken for separate species. Seaweed reproduction can involve either exclusively sexual or asexual phases, while some species display an alternation of generations that involves both in succession. In the former, the seaweed produces gametes (egg and sperm cells) with a single set of chromosomes and, in the latter, spores containing two sets of chromosomes. Some species can also reproduce asexually by fragmentation—that is, the blades shed small pieces that develop into completely independent organisms.

From Seaweeds: Edible, Available, and Sustainable by Ole G. Mouritsen.

Asexual reproduction allows for fast propagation of the species but carries with it an inherent danger of limited genetic variation. Sexual reproduction ensures better genetic variation, but it leaves the species that depend on this method of reproduction with an enormous match-making problem, as the egg and sperm cells need to find each other in water that is often turbulent.

The life history of algae is complicated, and this is what really differentiates them from plants.

Some species solve the match-making problem by equipping the reproductive cells with light-sensitive eyespots or with flagella so that they can swim. Others make use of chemical substances, known as pheromones or sex attractants. These are secreted and released by egg cells and serve to attract the sperm. Some species (for example, the large seaweed masses in the Sargasso Sea) secrete enormous quantities of slime, which ensures that the egg and sperm cells stick close to each other and do not go astray.

From Seaweeds: Edible, Available, and Sustainable by Ole G. Mouritsen.

The red alga Porphyra has an especially complicated life cycle, with a fascinating aspect that merits further discussion because of the interesting history associated with its discovery. It relates directly to the cultivation of Porphyra for the production of nori, which is especially widely used in Japanese cuisine—most familiarly, as for the wrapping for maki rolls (See the recipe in the caption for the nori roll image below.).

The blades used in nori production grow while the seaweed is in the generation that reproduces sexually, although the organism itself can actually develop asexually from spores. The blades produce egg cells and sperm cells. The egg cells remain on the blades, where they are fertilized by the sperm cells. The fertilized eggs can then form a new type of spores, which are released. These spores germinate into a calcium-boring filament stage that can grow in the shells of dead bivalves, such as oysters and clams, in the process developing spots that give the organism a pinkish sheen.

Until the 1940s it was thought that this sexual stage was actually an entirely separate species of alga, given the name Conchocelis rosea. Without an understanding of the true life cycle, it was not possible to grow Porphyra effectively in aquaculture. No one knew where the spores for the fully grown Porphyra originated. This was the main reason for the recurring problems experienced by the Japanese seaweed fishers in their attempts to cultivate Porphyra in a predictable manner.

It was an English alga researcher, Dr. Kathleen Mary Drew-Baker, who discovered the secret of the sexual segment of the Porphyra life cycle. Drew-Baker was unaware of the difficulties of the seaweed fishers. Instead, she was preoccupied with shedding light on the mystery of why the species of laver (Porphyra umbilicalis) that grew around the coast of England seemed to disappear during the summer, reappearing again only toward the end of autumn. She tried without success to germinate spores that she had collected.

Finally, after nine years’ worth of effort to grow the alga in light- and temperature-controlled tanks, she discovered that the spores would germinate if they were allowed to settle on a sterilized oyster shell. They would even grow on an eggshell. A few months later, the resulting small, roseate sprouts produced their own spores that, in turn, could germinate and develop into the well-known large purple laver.

Drew-Baker published her results in 1949. Shortly thereafter the Japanese phycologist Sokichi Segawa repeated her experiments using local varieties of Porphyra and found that they behaved in the same way as the English species. The mystery was solved and the results were quickly put to use in Japan. Drew-Baker died at a relatively young age in 1957, apparently unaware that her curiosity and seminal research had laid the foundations for the development of the most valuable aquaculture industry in the world.

Lighting the Ocean Garden

As in green plants, photosynthesis enables seaweeds to convert sunlight into chemical energy, which is then bound by the formation of the sugar glucose. Glucose is the building block for the seaweeds’ carbohydrates and, at the same time, an energy source for the production of other organic substances that the seaweeds need in order to grow and to carry out life processes. The photosynthetic process uses up carbon dioxide, which is thereby removed from the water. In addition, phosphorous, a variety of minerals, and especially nitrogen are required. Oxygen is formed as a by-product, dissolved in the water, and then released into the atmosphere. This by-product is of fundamental importance for those organisms that must, like humans, have oxygen to be able to breathe. Photosynthesis can even, to a certain extent, be carried out when seaweeds are exposed to air and partially dehydrated.

From Seaweeds: Edible, Available, and Sustainable by Ole G. Mouritsen.

During the night, when the light level is low, photosynthesis stops and the seaweeds begin to take in oxygen, burn glucose, and give off carbon dioxide. Under normal conditions, photosynthesis is the dominant process, allowing the seaweeds to build up their carbohydrate content. To the extent that they have access to light in the water, seaweeds actually utilize sunlight more efficiently than terrestrial plants.

Marine algae are a much better source of iron than foods such as spinach and egg yolks.

The red macroalgae normally grow at the greatest depths, typically as far as 30 meters down, the green macroalgae thrive in shallow water, and the brown algae in between. This distribution of species according to the depth of the water is somewhat imprecise, however; a given species can be found at a location where there are optimal conditions with respect to substrate, nutritional elements, temperature, and light.

In exceptionally clear water, one can find seaweeds growing as far as 250 meters below the surface of the sea. It is said that the record is held by a calcareous red alga that was found at a depth of 268 meters, where only 0.0005 percent of the sunlight penetrates. Even though the waters at that depth may appear pitch-dark to human eyes, there is still sufficient light to allow the alga to photosynthesize. In turbid waters, seaweeds grow only in the top, well-lit layers of water, if at all.

Formerly it was thought that seaweed species had adapted to their habitat by having pigments that were sensitive to the different wavelengths of the light spectrum. In this way they could take advantage of precisely that part of the spectrum that penetrated to the depths at which they lived. For example, the blue and violet wavelengths reach greater depths. The red algae that live in these waters must contain pigments that absorb blue and violet light and, as a consequence, appear to have the complementary color red. Experiments have since shown that this otherwise elegant relationship does not always hold true. Seaweed species that live at the ocean’s surface may also contain pigments that protect them from the sun’s ultraviolet light.

Given that all the substances that seaweeds need in order to survive are dissolved in the water, macroalgae, unlike plants, have no need of roots, stems, or real leaves. Nutrients and gases are exchanged directly across the surface of the seaweed by diffusion and active transport. In some species there is no meaningful differentiation, and each cell draws its supply of nutrients from the surrounding water. On the other hand, specialized cell types and tissues that assist in the distribution of nutrition within the organism can be found in a number of brown macroalgae.

Access to nitrogen is an important limiting factor in seaweed growth, particularly for green algae. The increasing runoff into the oceans of fertilizer-related nitrogen from fields and streams has created favorable conditions for the growth of algae, especially during the summer when it is warm and the days are long.

From Seaweeds: Edible, Available, and Sustainable by Ole G. Mouritsen.

Different species of seaweeds avail themselves of a variety of strategies in order to grow. In sea lettuce (Ulva lactuca), the cells all undergo division more or less randomly throughout the organism. Other species, among them several types of brown algae, have a growth zone at the end of the stipe and at the bottom of the blade; this is where an existing blade grows and new blades are formed. The oldest blades are outermost, eventually wearing down and falling off as the seaweed ages. As a result, the stipe can be several years old, while the blades are annuals. This growth mechanism allows the seaweed to protect itself from becoming overgrown by smaller algae, called epiphytes, which fasten on to it.

On certain seaweed species, the epiphytes are found overwhelmingly on the stipes, which can become covered with them, while the blades retain a smooth surface as long as they are young and still growing. Finally, some types of seaweeds, such as bladder wrack (Fucus vesiculosus) and the majority of the red algae, grow at the extremities of the blades.

The overall effect of seaweeds on the global ecosystem is enormous. It is estimated that all algae, including the phytoplankton, are jointly responsible for producing 90 percent of the oxygen in the atmosphere and up to 80 percent of the organic matter on Earth. We can compare their output with that of plants by looking at the amount of organic carbon generated per square meter on an annual basis. Macroalgae can produce between 2 and 14 kilograms, whereas terrestrial plants, such as trees and grasses in temperate climates, and microalgae can generate only about 1 kilogram. The vast productive capacity of macroalgae can possibly be best illustrated by the fact that the largest brown algae can grow up to half a meter a day. That amounts to a couple of centimeters an hour!

Underwater Dinner Harvest

Seaweeds are made up of a special combination of substances, which are very different from the ones typically found in terrestrial plants and which allow them to play a distinctive role in human nutrition. Most notably, the mineral content of seaweeds is 10 times as great as that found in plants grown in soil; as a consequence, people who regularly eat seaweeds seldom suffer from mineral deficiencies. In addition, marine algae are endowed with a wide range of trace elements and vitamins. Because they contain a large volume of soluble and insoluble dietary fiber, which are either slightly, or else completely, indigestible, seaweeds also have a low calorie count.

From Seaweeds: Edible, Available, and Sustainable by Ole G. Mouritsen.

Marine algae possess a fantastic ability to take up and concentrate certain substances from seawater. For example, the iodine concentration in konbu and other types of kelp is up to 100,000 times as great in the cells of the seaweeds as in the surrounding water, and the potassium concentration is 20–30 times greater. On the other hand, the sodium content is appreciably lower than that of salt water. Depending on the species, fresh seaweeds are 70–90 percent water by weight. The composition of the dry ingredients in the different types of seaweeds can vary a great deal, but the approximate proportions are about 45–75 percent carbohydrates and fiber, 7–35 percent proteins, less than 5 percent fats, and a large number of different minerals and vitamins.

Broadly speaking, the proteins in seaweeds contain all the important amino acids, especially the essential ones that cannot be synthesized by our bodies and that we therefore have to ingest in our food. Porphyra has the greatest protein content (35 percent) and members of the order Laminariales the lowest (7 percent).

Three groups of carbohydrates are found in seaweeds: sugars, soluble dietary fiber, and insoluble dietary fiber. Many of these carbohydrates are different from those that make up terrestrial plants and, furthermore, they vary among the red, the green, and the brown species of algae. The sugars, in which we include sugar alcohols such as mannitol in brown algae and sorbitol in red algae, can constitute up to 20 percent of the seaweeds. The seaweed cells make use of several types of starch-like carbohydrates for internal energy storage; again, these vary according to species. For example, the brown algae contain laminarin, which is of industrial importance as it can be fermented to make alcohol.

From Seaweeds: Edible, Available, and Sustainable by Ole G. Mouritsen.

Soluble dietary fiber, which is situated in between the seaweed cells and binds them together, constitutes up to 50 percent of the organism. Composed of three distinct groups of carbohydrates, namely, agar, carrageenan, and alginate, fiber can absorb water in the human stomach and intestines and form gelatinous substances that aid in the digestive process. Insoluble dietary fiber derived from the stiff cell walls of the seaweeds is present in lesser quantities, typically amounting to between 2 percent and 8 percent of the dry weight. Cellulose is found in all three types of algae and xylan (another type of complex carbohydrate) in the red and green ones.

The primary mineral components in seaweeds are iodine, calcium, phosphorous, magnesium, iron, sodium, potassium, and chlorine. Added to these are many important trace elements such as zinc, copper, manganese, selenium, molybdenum, and chromium. The mineral composition, especially, varies significantly from one seaweed species to another. Konbu contains more than 100–1,000 times as much iodine as nori. On average, dulse—a widely eaten red seaweed—is the poorest choice in terms of mineral and vitamin content but, on the other hand, it is far richer in potassium salts than in sodium salts. In general, marine algae are a much better source of iron than foods such as spinach and egg yolks. An abundance of vitamins is present in seaweeds, namely, vitamins A, B (B1, B2, B3, B6, B12, and folate), C, and E, but no vitamin D. (For a nutritious crispbread recipe, click on the "+" sign in the image below.)

Seaweeds contain iodine, although the exact quantities again vary greatly by species. The iodine content is dependent on where the seaweed grew and how it has been handled after harvest. Furthermore, the iodine is not evenly distributed, being most abundant in the growing parts and least plentiful in the blades. In particular, the brown seaweeds contain large amounts of iodine. It is not known for certain why brown seaweeds contain so much iodine, but this is probably linked to their capacity for rapid growth. Recent studies of the brown seaweed species oarweed (Laminaria digitata) discovered high concentrations of inorganic iodine in the form of iodide (I¯) in the cell walls. Iodide was found to act as the main antioxidant for this tissue. In addition, the study showed that the action of iodide was not accompanied by an accumulation of organically bound iodine.

The history of the discovery of iodine as an element actually begins with seaweeds. Bernard Courtois (1777–1838), a French chemist, was working in his laboratory in 1811, extracting saltpeter from seaweeds for the production of gunpowder for Napoleon’s army. He noticed that his chemical experiments with the seaweed ash gave rise to a violet-colored vapor that condensed as crystals on his copper vessels and, unfortunately, caused them to corrode. Courtois convinced first his French, and later his English, fellow chemists that his discovery had important dimensions. Their work then rapidly led to the identification of the substance that was the source of the vapors. It turned out to be a previously unknown element and, as the color violet is called iodes in Greek, the new element was given the name iodine.

Terrestrial plants are a poor source of iodine, which can result in iodine deficiency in vegetarians and vegans. The accidental discovery of iodine in seaweeds is a wonderful example of how research and an open mind on the part of the researcher can lead to results that have a major significance for the economy and for human health.

Feast or Funk for the Senses

Despite their importance to human diet, seaweeds have often been regarded with disdain. The Roman poet Virgil is credited with the saying that there is nothing more worthless than washed-up seaweed: “nihil vilior alga.” He was absolutely right insofar as dead, rotting seaweeds give off a horrible stench. That unpleasant smell is due to a number of gases that are not dangerous, but are the source of odors that we consider offensive.

From Seaweeds: Edible, Available, and Sustainable by Ole G. Mouritsen.

One of the chief culprits is a chemical substance, dimethylsulfoniopropionate (DMSP), which is found in red and green algae, where it helps regulate the osmotic balance of the cell in relation to the surrounding salt water. Some researchers think that DMSP is an important antioxidant, which provides support for the physiological functions of the algae. DMSP accumulates in those animals in the food chain that feed on seaweeds.

DMSP has no taste and no smell, but dimethyl sulfide (DMS), a volatile gas that is a by-product of DMSP breakdown, has a characteristic disagreeable odor. It is formed when DMSP is oxidized in the atmosphere or when it is degraded by bacterial action. It can also be released in the course of food preparation when fresh fish and shellfish are heated. When present in small quantities, DMS is the cause of what we often call “the smell of the sea,” but in large quantities it results in the disagreeable aroma that is associated with rotten seaweeds and with fish that is no longer fresh.

DMS is the most abundant gaseous sulfur compound emitted into the Earth’s atmosphere as a result of biological processes. When DMS is released into the atmosphere, it, in turn, is oxidized to form particulate aerosol substances. These can cause condensation of water vapor, which brings about cloud formation and thereby affects the weather. So although we may find its odor offensive, many scientists think that the DMS generated from the decomposition of marine algae, especially phytoplankton, plays a vital role in the regulation of the Earth’s climate.

When brown algae and some types of red algae decay, they can cause the formation of another sulfurous gas, methyl mercaptan. This is the gas that smells like rotten cabbage and is often added to natural gas in order to alert us to its presence. Conversely, fresh seaweeds, much like a delightfully aromatic ocean breeze, have a characteristic, agreeable smell. In both cases this is due to substances called bromophenols, which the seaweeds synthesize. They are released into the air and accumulate in ocean-dwelling fish and shellfish through their food intake.

Because there are no bromophenols in fresh water, fish that live in lakes and streams lack the same pleasant odor and taste as their saltwater cousins. That is yet one more way that seaweeds contribute agreeably and meaningfully to the human diet.


Adapted with permission from Seaweeds: Edible, Available, and Sustainable by Ole G. Mouritsen, published by the University of Chicago Press. ©2013 Ole G. Mouritsen. All rights reserved.