Jan. 17, 2021
Hello friend! Welcome to Age, a bi-weekly special edition of Scrap Facts.
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Kelp and its advanced life cycle.
In conversation with Sean Grace, a marine ecologist at Southern Connecticut State University.
We humans busy ourselves with a variety of personal and professional pursuits, but most of life just has a single goal: reproduction. The majority of creatures on Earth expire shortly after they reach sexual maturity. For some species, their final days are quite dramatic; salmon, for instance, die shortly after making a mad dash upriver to spawn. Wheat are the same way, albeit there’s no death march involved. (This is called semelparity) Most of life gets the chance to reproduce a handful of times—called iteroparity—but still, their days after reaching sexual maturity are numbered.
Kelp are a semelparous algae. On the east coast of the US in the Atlantic Ocean, they live for just one year; in the colder months in the northern hemisphere (so, now), their spores are settling and finding rocks to cling to. Over the summer, they grow at a truly alarming rate to the tune of 20 inches (50 cm) in per day as they get to their reproductive stage.
"Kelp is interesting because it has what’s known as an alteration of generation,” says Grace. Although kelp reproduce sexually—meaning, with male and female gametes, just like us—not all kelp do.
Some kelp grow up to be sporophytes. This means that they produce spores that can go on and become other other kelp. This is a version of asexual reproduction. There’s no gene combination between parents; it’s essentially cloning a parent.
Kelp spores, however, go on to produce another stage of kelp, called gametophytes. These kelp produce the sperm and egg cells that go on to mix to form a little kelp zygote somewhere in the water column—nearby, or as far as 200 miles away. This is a version of sexual reproduction, which makes sure that kelp mix up their gene pools to stay healthy as a species.
If you were to meander into a kelp forest in the shallow ocean waters, however, you wouldn’t be able to the difference between the sporophytes and gametophytes. “They look exactly the same,” says Grace.
It may seem like a ridiculous extra step to grow a whole other algae to create future generations of kelp, but it’s a survival mechanism. “It ensures reproduction.” If marine animals were to eat kelp to survive (as they tend to do), or if a kelp’s thallus (main leafy-looking body) were damaged in any way, it wouldn’t matter if it were a sporophyte or gametophyte—there would always be backup.
After they release their spores or gametes, kelp die. Their cells senesce, just like ours, which is when they essentially stop trying to multiple and repair their DNA. Instead, they realize their time has come, and they die—settling to the bottom of the sea floor to become sediment. (They bring all their carbon with them—another massive form of carbon storage we probably take for granted.)
But having this advanced life cycle—breaking up reproduction, in other words, “is a form of fitness,” Grace says. Kelp are a brown algae that have an advanced life cycle; many species of red algae do the same. Generally, the more ways a species can reproduce, the better for the survival of the species.
Other species with advanced life cycles don’t experience time the way we do. Although kelp age and die, just like we do, there are some other creatures with advanced life cycles, like some coral, that don’t. As long as they have a stable environment and food, they simply forget to age. Some of them have spent thousands of years on Earth. This feat that is tied to all the ways they can reproduce, which we’ll get into next issue.
In addition to teaching us about advanced life cycles, kelp can also teach us a thing or two about living in times of uncertainty. Read my story for Quartz here.
That’s all for now—stay curious, friend ❤️
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Top image by Rachel Couch; headshot by Matt Anzur.