Adaptations for buoyancy

A. Adaptations for maintaining buoyancy: marine plants have achieved the maintenance of

buoyancy through the evolutionary process, which results in optimizing time spent in the euphotic zone, by 3 mechanisms: the reduction in size; reduction in weight; and the reduction in streamlining.
1. Reduction in size:
a. Simplest way to obtain relatively large surface area to volume ratio, thereby increasing resistance, and thus slowing the rate of sinking.

b. Stokes’ Law:
1). For a sphere, the sinking velocity is equal to a constant times the radius squared.
2). Means that the smaller the object, given that the two objects have the same specific gravity (made out of the same material, for example), the larger its surface area to volume ratio.
3). The larger the surface area to volume ratio, the greater the frictional resistance to movement through the water; hence, the slower the rate of sinking; results in spending more time in the euphotic zone.

c. The larger the surface area to volume ratio has the added advantage in that the plant can be more efficient in the absorption of nutrients, since absorption takes place through the cell membrane (the outside surface area of the plant). This is crucial, since many of the mineral salts are only available in trace amounts to plants.
1). If this were not crucial, why is there not a proliferation of multi-cellular,larger plants with bladder-like floats (like Sargassum) as many of the animal groups have evolved.
2). Sunlight in the euphotic zone is highly scattered, so that an unicellular plant is advantaged in utilizing this scattered light better than a larger multicellular plant.

2. Reduction in weight: the use of water, gas (oxygen, carbon dioxide, carbon monoxide), oils, fats, and mucous all help reduce specific gravity in plants so that the plants won't sink, or at least won't sink as fast as otherwise.

a. Bladder form, as in Coscinodiscus, a diatom, or Noctiluca, a dinoflagellate.

b. Ribbon form, as in Eucampia, a diatom, or the several species of dinoflagellates that link together into chains.

3. Reduction in streamlining: exposing more surface area to the axis of sinking, or travel

along a longer route of sinking due to the body shapes.

a. Filamentous, as in the diatom, Rhizosolenia.

b. Branching, as in the diatom, Chaetoceros, or the dinoflagellate, Ceratium. If one studies the cold water versus warm water forms of Ceratium, those living in warmer waters (these waters are less dense, hence do not offer as much support, as per Archimedes’ Principle. Have elaborate, branching horns, whereas those living in colder waters are unbranched and simpler in structure. Hence Ceratium would sink faster in the tropics than in the denser, colder waters).