what is kelp?

Summary

Kelp is a type of algae, one of the oldest organisms on Earth, thriving in watery environments and producing most of the world’s oxygen. Unlike plants, it lacks roots, stems, and leaves but has simple structures that serve similar functions. Kelp goes through two life phases—sexual and asexual—creating genetic clones half of the time. This limited genetic diversity might make it a great candidate for genetic engineering, which I’ll explore in my next post!

It might seem like a simple question: kelp is seaweed, right? And yes, that’s true…but what is seaweed, anyway? Well, it’s a type of algae.

What Even Is Seaweed?

Algae are simple, aquatic, photosynthetic organisms without the complex structures—like roots, stems, or leaves—that terrestrial plants have. They’re among the oldest eukaryotic organisms, meaning their DNA is neatly organized into little envelopes called nuclei. In contrast, prokaryotes (like bacteria) have DNA freely floating around in the cell. Fossil records show algae are over a billion years old, while land plants only date back about 450–500 million years (Delwiche et al., 2015).

Algae thrive in damp or aquatic environments, and they photosynthesize just like plants. They come in all shapes and sizes, from tiny, single-celled phytoplankton floating in oceans and lakes to larger, multicellular organisms like kelp. Algae were among the earliest organisms to perform photosynthesis and release oxygen into Earth’s atmosphere, which eventually allowed for more complex life forms, including terrestrial plants. Today, algae—both big and small—are responsible for producing around 90% of the world’s oxygen. While we often think of trees and forests as Earth’s “lungs,” it’s actually algae’s sheer abundance in our oceans that drives most of this oxygen production!

It’s worth noting that large algae, or macroalgae, come in three main “flavors”: brown algae (Phaeophyceae), green algae (Chlorophyta), and red algae (Rhodophyta). I’ll mostly focus on brown algae on this site, so when I say “kelp” or “seaweed,” I’m talking about brown algae unless stated otherwise. And I’ll keep scientific names to a minimum—only if absolutely necessary.

Seaweed Anatomy 101

We won’t dive too deeply into kelp’s evolution and phylogeny (although it’s fascinating and still evolving), but here’s the main takeaway: algae and plants are very, very distant relatives. It’s no surprise that seaweed and terrestrial plants share only a thin family connection. While both photosynthesize, they’re actually quite different. Seaweed lacks the complex structures of most land plants, like true roots, stems, and leaves. Instead, it has simple parts that serve similar functions. For example, the body of seaweed, called a thallus (like “tree” describes the whole plant), has holdfasts—basically, roots that anchor it to the ocean floor. Blades (aka kelp leaves) are attached to the thallus by stipes (branches). Kelp also has a pneumatocyst, which literally translates to “air bubble” (pneumato = air, cyst = bladder in Greek). This air-filled part keeps the body upright, bringing the blades toward sunlight.

Credit: Beaty Biodiversity Museum via Bridgette Clarkson by way of a fantastic marine ecology blog called That Marine Detective run by Jackie Hildering, a Canada-based biology teacher/diver/underwater photographer/researcher.

Love & Life Cycles

But perhaps the biggest difference from plants is that macroalgae go through two distinct life phases—a process called alternation of generations. This means kelp cycles through a sexual phase, where fertilization of a sperm and an egg occurs, and an asexual phase, where spores develop into the thallus once they attach to the seabed. In asexual reproduction, the spores carry the complete genetic material from the single parent, so there’s no genetic variation. Sexual reproduction for kelp, however, depends on an abundance of sperm and egg cells in the water, which, with shrinking kelp forests, becomes less likely. Yet, the genetic variation from sexual reproduction is essential for the evolution and adaptation of any species. Alternation of generations introduces this variation slowly, as every other lifecycle produces clones, and when kelp do finally get around to sexual reproduction, there might not be enough sperm and egg cells in the water to meet.

But, could what holds kelp back in nature (its variation-resistant lifecycle) be the very thing that makes it a prime candidate for genetic engineering?

I’ll dig into this in my next post!