To describe algal reproduction cycles, one must first understand that cells contain chromosomes. Some cells have duplicate sets of chromosomes, making them diploid; some have only one set, making them haploid. One cycle involves a diploid alga (with each of its cells having two sets of chromosomes). One of its cells splits by a process called meiosis, creating two haploid spores which each wind up with one set of chromosomes. Each of these spores develops into a haploid alga that forms gametes. These gametes then join together to form a diploid zygote cell; this cell grows and matures into a diploid alga, starting the cycle all over again [see illustration]. The first generation alga (the diploid alga) is called the sporophyte generation because it makes spores, and the second generation alga (the haploid alga) - which may look very different from the first generation - is called the gametophyte generation because it makes gametes. Changing reproductive strategies from one generation to the next is called alternation of generations. This kind of cycle occurs in all three seaweed groups - red, green, and brown.
A second type of reproduction cycle starts with a haploid alga that produces haploid gametes; these then join to form a diploid zygote. This zygote doesn’t become an alga but instead undergoes meiosis, creating a haploid spore. This spore then develops into a haploid alga, which starts the cycle over again. This cycle is found mainly in the green algae group.
A third kind of reproduction cycle starts with a diploid alga which undergoes meiosis, resulting in two haploid gametes. These gametes then fuse and form a diploid zygote. A diploid alga grows from this zygote, and the cycle starts again. This cycle occurs in some brown seaweeds.
A fourth cycle starts with two haploid algae that produce gametes. These two gametes then fuse and form a zygote. The zygote develops into a diploid carpospore, which then undergoes meiosis, creating two haploid spores that form into two new haploid algae. The cycle then repeats itself. This reproductive strategy is common in red algae.
Though algae aren’t very complex in their makeup, they rival land plants in the complexity of their reproduction cycles. These four cycles are but a few examples in a sea of different reproductions cycles. And those algae that utilize more than one cycle are afforded an advantage in their ever-changing ocean environment.
Simeon Smith is a high school intern at the Kachemak Bay Research Reserve.
If you have questions about Kachemak Bay, contact reserve staff at 235-4799 or visit their website at www.kbayrr.org.
Sponsored by the Kachemak Bay Research Reserve
Illustration by Catie Bursch
Complex reproduction cycles allow seaweeds to propagate via special cells called spores or gametes. While only one spore is needed to produce a new alga, it takes two gametes joining together to do the same. Each alga species has at least one way to reproduce, and some have several reproduction “tricks up their stipe.”
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