Photo courtesy of Courtney Miller

The birds and the bees of algae

The British Broadcasting Alga
Bladderwrack is a dioecious organism, meaning that male and female gametes are produced on two different organisms. The organism requires water for reproduction, as it produces flagellated sperm that move toward the egg to fertilize it via the water, this is called broadcast spawning. You can learn more about flagella of another alga (a green algae) by visiting the site on Chlamydomonas reinhardtii.

Even algae love the summer
Fucus vesiculosus
only produces its gametes during two seasons out of the year, those seasons being spring and summer. The rest of the year the offspring grow and mature, and the mature bladderwracks just enter a vegetative state. The thing that is odd about this is that a species that is very closely related to bladderwrack, Fucus serratus, reproduces at the exact opposite time, possibly helping to prevent polyspermy. This is discussed later on in this section.

Conceptacles on a receptacle. Photo courtesy of Ester SerraoNot very good odds for the guys...
Sperm and eggs are produced in structures called conceptacles that are small structures on a receptacle. You can see both of these structures in the picture to the left. The overall structure is a receptacle, and the little bumps are the conceptacles. The receptacles are swellings at the end of the thallus when the organism is about to reproduce. The structures that reside in the conceptacles that produce the female gametes are called oogonia and contain eight eggs each. The male reproductive structures are called antheridia, which also reside in the conceptacles. The antheridia carry 64 sperm cells each before they release the cells into the water.

Female receptacle releasing an oogonia - Courtesy of Ester SerraoBladderwrack is obviously the smart cousin
In addition to only releasing its eggs and sperm during summer and spring, F. vesiculosus also has a developed a timing mechanism for release of its gametes during the day. Relative to its very close cousin F. serratus, F. vesiculosus releases a lot more eggs later in the day and at lower tide than its cousin. Researchers speculate that bladderwrack has developed this because it is preventing rapid dilution of its gametes that require sperm to fertilize them. This fertilization process would become a lot less probable if the eggs were spread out and diluted, as would happen at higher tides.

Sperm and egg pronuclei just before fusion - Courtesy of Ester SerraoFemales and their "perfume"
You may wonder how the sperm find the egg to fertilize it. The process is obviously not passive, like spores of certain trees, since the sperm are flagellated. It's actually a lot like how humans attract one another: pheromones. The female egg actually releases a species non-specific pheromone that attracts sperm in its direction. Because the pheromone is species non-specific, the egg may attract a male gamete that it is not compatible with. The way the sperm ends up knowing whether it can fertilize the egg are the oligosaccharides on the cell wall of the egg. The photo to the left is of a sperm and egg pronuclei (the dark spots) just before fusion and the creation of a zygote. The cell is stained with hemotoxylin which stains the chromatin of the nucleus.

Living in the "rough neighborhood" 
Fucus vesiculosus
seems to thrive in areas where other marine species fail to reproduce. The reason why so many marine species are unable to live in these areas that Bladderwrack can survive in is because of the low salinity. Low salinity has multiple effects on the male gametes, which bladderwrack has seem to overcome. Low salinity can decrease the motility and length of life of gametes of most organisms, and also increase the rate of polyspermy. Polyspermy is lethal to almost all species, but bladderwrack has a few mechanisms toFertilized (white) and unfertilized (dark) eggs - Courtesy of Ester Serrao overcome this. The first is a fast block. The fast block is a change in membrane potential of an egg, and happens in a few milliseconds of sperm-egg fusion. This reduces the chance of another sperm fusing with the cell membrane of the egg. Another polyspermy block is a slow-block, which happens over the course of a couple hours, and is basically the growth of a cell wall around the organism. The image to the right was taken by Ester Serrao. The blue/whitish eggs are stained with calcifuor white, which turns the cell wall white. As stated earlier, the cell wall is formed after fertilization, so the blue/whitish eggs are fertilized, and the dark eggs are unfertilized.

The Circle of Life for F. vesiculosus
The picture below shows the entire life cycle of Fucus vesiculosus. The conceptacles first release either antheridia or oogonia depending on the sex of the plant. The antheridia then release 64 sperm cells and the oogonia release eight eggs each. The sperm cell then fuses with an egg and fertilizes it to create a zygote. The zygote then goes through mitosis to eventually turn into a small bladderwrack and then the organism eventually grows into sexual maturity with the signature pairs of bladders, and the cycle then repeats itself every year.

Life cycle of Fucus vesiculosus - Created by Author


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