Floats Keep Fronds Buoyant

stalks of yellow kelp fill the frame underwater, each stem is crowded with long skinny leaves and small gas-filled balls

Biological Strategy

Brown algae

Alexandra Ralevski

Image: Ed Bierman / Flickr / Some rights reserved

Move in/on Liquids

Water is not only the most abundant liquid on earth, but it’s vital to life–so it’s no surprise that the majority of life has evolved to thrive on and under its surface. Moving efficiently in and on this dense and dynamic substance presents unique challenges and opportunities for living systems. As a result, they have evolved countless solutions to optimize drag, utilize surface tension, fine tune buoyancy, and take advantage of various types of currents and fluid dynamics. For example, sharks can slide through water by reducing drag due to their streamlined shape and specially shaped features on their skin.

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Modify Buoyancy

Buoyancy is an upward force exerted by air or liquid on a solid object that works against the object’s weight. A hawk glides through air and a duck floats on water due to buoyancy. Some living systems (such as fish eggs) can remain sedentary and therefore maintain the same buoyancy at all times. But most must adjust their buoyancy because they can’t survive long unless they change position. These living systems require strategies to not only be buoyant, but also adjust buoyancy level. They often modify buoyancy by adding or decreasing lift, or by changing their weight. For example, birds like condors that soar for a long time shift the directional orientation of their wingtip feathers to manage buoyancy. Fish alter the amount of air in their swim bladders to increase or decrease their weight, thus altering their buoyancy.

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Plants

Phylum Plantae (“plants”): Angiosperms, gymnosperms, green algae, and more

Plants have evolved by using special structures within their cells to harness energy directly from sunlight. There are currently over 350,000 known species of plants which include angiosperms (flowering trees and plants), gymnosperms (conifers, Gingkos, and others), ferns, hornworts, liverworts, mosses, and green algae. While most get energy through the process of photosynthesis, some are partially carnivores, feeding on the bodies of insects, and others are plant parasites, feeding entirely off of other plants. Plants reproduce through fruits, seeds, spores, and even asexually. They evolved around 500 million years ago and can now be found on every continent worldwide.

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Gas-filled floats help keep kelp fronds near the water surface to enhance photosynthesis.

Kelp (brown algae seaweed) possess gas-filled floats known as pneumatocysts that enable fronds to float on the water surface, maximizing exposure to sunlight and enhancing . The gas content of the pneumatocysts can vary, but are usually filled with a combination of oxygen, nitrogen, and carbon dioxide. Depending on the species, the kelp may contain one large pneumatocyst or several smaller pneumatocysts distributed throughout the kelp.

Image: Jomegat / wikimedia commons /

Photo showing drift deposits of kelp (giant seaweeds) : large floats belong to the algal species Nereocystis luetkeana, tiny floats, blades and entangled cords belong to Macrocystis pyrifera. Taken in San Diego

Image: Shane Anderson / Public Domain - No restrictions

Floating kelp (Macrocystis pyrifera) with many pneumatocysts

Last Updated September 5, 2018

References

“…gas-filled floats keep [kelp] fronds at or near the sea surface…” (Koehl and Wainwright 1977:1067)

Journal article

Mechanical adaptations of a giant kelp

Limnology and Oceanography, 22(6): 1067–1071 | 01/11/1977 | Koehl MAR; Wainwright SA

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Reference

“[The floats] contain a mixture of oxygen, nitrogen, and carbon dioxide in varying relative proportions depending on the physiological status of the plant and the partial pressure of the particular gas in the surrounding medium. These gas-filled pneumatocysts provide buoyancy and let entire plants float to the sea surface” (Thiel et al. 2005:186).

Journal article

The ecology of rafting in the marine environment

Oceanography and Marine Biology. 43:181-264. | 01/01/2005 | Thiel M, Gutow L.

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