Kelp is a kind of seaweed found only in saltwater. If you have ever seen kelp bundled up along a seashore, or drifting in large marine aquariums, you might think you are looking at a plant.
But kelp possesses interesting properties that set it apart from plants.
TL;DR (Too Long; Didn't Read)
Kelp belongs to the kingdom of organisms called protists and possesses different kinds of cells. It is therefore considered multicellular. However, it is quite different from plants despite its plantlike appearance.
Is Kelp a Plant?
Until relatively recently, kelp was classified as a kind of plant. It turns out that kelp actually is not a plant, although it is still common to see “kelp plants” used as terminology in some sources. Kelp is a kind of algae, specifically brown algae, the largest algae on Earth. Kelp is therefore considered one of the macroalgae.
Algae belong to Kingdom Protista (protists), separating its organisms from kingdoms Plantae, Animalia, Fungi and Monera. Brown algae or kelp is vastly different from red algae.
Kelp does have transport of nutrients that makes it similar to plants and uses sieve elements to help facilitate this transport. Kelp differs from plants in their means of reproduction, their physical properties and their cellular makeup. Kelp also absorbs nutrients differently, obtaining it simply from the movement of seawater.
Long-distance transport of nutrients might seem very similar between kelp and plants, but in fact they simply evolved separately. Kelp as a whole belongs to the class Phaeophyceae.
Is Kelp Multicellular?
Since kelp is a form of protist, and many protists are single-celled, it might seem natural to wonder if kelp is single-celled. Not all protists are single-celled, though. In fact, brown algae or kelp is multicellular.
Kelp possesses different cells with different functions, particularly for nutrition. Kelp stores food in laminarin. In some types of kelp such as bull kelp, three different kinds of tissues reside in the leaf-like blades. These include the central medulla, the cortex with its large cells, and the meristoderm with small cells.
As for reproductive cells, kelp possesses sperm and spores. Kelp can undergo both sexual and asexual reproduction, alternating between generations in some species. Asexual reproduction entails fragments of kelp blades growing into new organisms entirely.
Some cells of kelp can grow quite large, even as much as 1 centimeter across. The large cells aid in protein production and cell function. This is important, since kelp tends to grow rapidly.
Does Kelp Have Cell Walls?
Kelp does possess cell walls. The cell walls are made primarily of cellulose, just as in plants. Each cell wall is made of a cellulose inner layer as well as an outer layer made of a substance called algin.
Algin is an extracellular matrix, made of various polymers and monomers called alginates. Algin is gelatinous, meaning it can swell and is elastic, and so it is often used in the food industry as a thickener.
Physical Properties of Kelp
Kelp “plants” are called brown algae due to their brownish-yellow color. Despite this color, kelp does contain chlorophyll; it is simply overlaid by a pigment called fucoxanthin.
Kelp does not possess roots in the manner of plants. It uses what is called a holdfast, which is similar to a root, and attaches to whatever substrate it needs.
Extending from the holdfast, its stem or stipe stretches out and possesses blades at the end. There can be more than one blade on kelp. Some of the blades have a midrib. These blades absorb sunlight for photosynthesis. Because of this function, the blades need to be closer to the surface than other parts of the kelp.
Fortunately, kelp has the perfect feature to aid in getting blades higher in the water column. A bulbous feature resides underneath the blades, filled with gas, and that creates a flotation device for the kelp called a pneumatocyst. With this flotation the kelp blades can be held erect and therefore get greater access to sunlight. This bulb or bladder gives the kelp species "bladder wrack" its common name. Some of these bladders can extend up to 15 centimeters around.
The blades of kelp age, wear and fall off over time. They are therefore annual portions of kelp, whereas the stipe or stem is perennial. This keeps opportunistic organisms that grow on kelp from overcoming it.
Kelp unfortunately exudes a nasty stench when it rots. This happens due to the generation of a sulfurous gas called methyl mercaptan. However, fresh kelp does not have that unappealing odor. It does smell like the sea due to its production of chemicals called bromophenols, which it releases in the air.
Different Kinds of Kelp
There are many different kinds of kelp. So far, as many as 2,000 species of brown algae have been discovered.
Kelp species span from the very tiny to immense, such as giant Pacific kelp (Macrocystis pyrifera). Giant Pacific kelp stands quite literally above other types of kelp and exhibits rapid growth. It has even been recorded to grow as much as 50 centimeters a day. In contrast, Ectocarpus is a type of kelp that grows in small filaments of cells.
Some kelp plants grow in “forests” because they extend up from their substrate. Other kelp species grow in mats or cushions. An example of a mat-like kelp is Sargassum, of the famed Sargasso Sea. Sargassum also possesses the interesting feature of massive slime production. This is used for sexual reproduction to keep sperm and egg cells in close proximity.
Another prominent type of kelp is bull kelp or Nereocystis luetkeana, which is crucial to the ecology of the North Pacific Ocean.
Among the different types of kelp, species can be fanlike such as Padina, or reside along rocky beaches like Fucales. Other kinds of kelp include babberlocks (Alaria esculenta), sea whistle (Ascophyllum nodosum), sugar kelp (Laminaria saccharina) and May weed (Laminaria hyperborean), among many others.
The Importance of Kelp
Other minerals abundant in kelp include calcium, phosphorus, iron, potassium and several vitamins. In fact, the amount of iron in kelp is greater than that found in spinach. Trace elements round out the nutritional bonanza in kelp. These include:
- among others
The laminarin food storage component of kelp can also be fermented into alcohol.
- American Scientist: The Science of Seaweeds
- UCSB ScienceLine: Why Are Kelp Considered Protists?
- Annals of Botany: The Gelatinous Extracellular Matrix Facilitates Transport Studies in Kelp: Visualization of Pressure-induced Flow Reversal Across Sieve Plates
- The MarineBio Conservation Society: Forests of the Sea: Phytoplankton and Marine Plants
- Marine Education Society of Australasia: Brown Algae (Phaeophyta)
About the Author
J. Dianne Dotson is a science writer with a degree in zoology/ecology and evolutionary biology. She spent nine years working in laboratory and clinical research. A lifelong writer, Dianne is also a content manager and science fiction and fantasy novelist. Dianne features science as well as writing topics on her website, jdiannedotson.com.