The Secret Life of Kelp

Learning to surf in northern California, we learned to love kelp. Not so much its tendency to snag our boards while sliding down a wave, but for other reasons. Anchored to the rocky bottom with strong holdfasts, kelp provided a convenient anchor to hold onto and stay put in strong currents, and tended to prevent local winds from making the surface of the water choppy and chaotic. We also used it to condition our hair, popping open the gas-filled floats that keep kelp plants standing upright in the water, and rubbing the insides of the floats, which are coated with a slimy substance, into our locks. There are lots of hair products available that contain algae, so we figured going straight to the source would be great.

One day, my friends and I were enthusiastically smoothing algae slime onto our heads while we bobbed on our surfboards, when a fellow surfer paddled over. “You know that’s where the kelp stores its sperm and eggs, right?” he gestured to the gnocchi-sized gas bladders we were in the midst of popping. Suddenly, I second-guessed our beauty routine. Was he right? Was I preventing the adult kelp from making baby kelp? If that were true, wouldn’t kelp harvesting damage kelp reproduction?

Kelp grows in temperate water, often in thick beds called forests. It’s harvested in many areas, like off the coast of California, using lawn-mower-type boats. Kelp and other types of algae are used for a range of products, including cosmetics like shampoo and toothpaste, and food like salad dressing and ice cream. Agar, carrageenan, and alginate are some of the compounds extracted from different types of algae, and mostly used as thickeners.

Kelp is like a plant – it is photosynthetic and has structures that look like roots (the kelp holdfast), stems (the stipe) and leaves (blades)– but kelp and other algae belong to a separate kingdom of life from plants, called protists.

So: do kelp gas bladders double as reproductive organs? Well, no. The way kelp actually reproduces is much more interesting.

The mature stands of kelp that make up kelp beds don’t produce eggs and sperm at all. Instead, they produce an intermediate stage, microscopic spores produced by special blades without floats near the holdfast at the base of the adult kelp. Spores are different from seeds because they are single cells and only contain half of the chromosomes of the adult; they will mature and produce either sperm or eggs. Seeds are multi-cellular and are already fertilized, containing a full set of chromosomes; they are ready to produce an adult plant as they mature.

Once fertilized, the egg develops into a sporophyte that takes over the female gametophyte. The sporophyte eventually grows to form the large fronds many meters tall that we recognize as kelp.

Can you think of any species that live on land and reproduce similarly, with a microscopic spore stage instead of via sexual reproduction? In what kinds of environments do they tend to live?