Buoyancy Control

The alternative to swimming is to float by means of some types of buoyancy device. In some of the attached brown algae of the seashore (Fucus vesicolosus, Ascophyllum nodosum, and Sargassum sp., Heterokontophyta) the fronds gain buoyancy from air bladders (pneumatocysts) within the thallus, which stands erect when submerged. Oxygen and nitrogen, in roughly the same proportion as in air, form the bulk of the gas, but there are also small, variable amount of CO₂ and CO. Oxygen and CO₂ derive partly from the metabolic activities of the cells in the pneumatocyst wall and diurnal changes in the composition and pressure of pneumatocyst gases have been shown. However, equilibration takes place between the gases in the pneumatocyst and in the surrounding water (or air). This is the source of nitrogen in the vesicles and the major source of O₂ and CO₂. In Enteromorpha sp. (Chlorophyta), gas bubbles are entrapped in the central area of its tubular hollow thallus, which may aid in keeping the stipe upright by flotation. In other seaweeds such as Codium fragile (Chlorophyta), gas trapped among the filaments achieves the same buoyancy effect of pneumatocysts.

Buoyancy regulation in cyanobacteria involves the production of intracellular gas-filled structures (also termed vacuoles), not delimited by membranes, and made up of assemblages of hollow cylinders, whose proteinaceous walls are permeable to gas, but not to water. The density of this structure is about 0.12 g cm^-3, about one eighth of that of water, and if sufficient gas-filled structures are present in a cell, it can become positively buoyant. In cyanobacteria buoyancy is regulated by varying gas-filled structure formation and cytoplasmatic composition through synthesis and breakdown of photosynthetic products. The production of gas-filled structures is induced by lowlight conditions (e.g., in deep layers with insufficient light). Here, photosynthesis is reduced, osmotic pressure of newly synthesized sugars is small, and ballast materials such as carbohydrates are not produced at a high rate, therefore they will not increase cell density, which in turn would increase sinking.

Under these conditions, gas-filled structures can be produced at a high rate, and cells increase their buoyancy. Conversely, if cell osmotic potential is high (high sugar production, increased amount of ballast in the form of secondary photosynthetic products), turgor pressure increases, which may collapse gas-filled structure and cells become negatively buoyant and sink in the water column. The rise in turgor pressure with light irradiance has been found in many cyanobacteria; however, for this rise to result in gas-filled structure regulation, the pressure reached must exceed the lowest pressure of gas-filled structures. This occurs, for example, in Anabaena flos-aquae, with a critical collapse pressure distributed about a mean of 6 bars. In richodesmium sp. gas-filled structures can withstand pressures of 12–37 bars depending on the species, and turgor pressure collapse is not possible as a buoyancy regulation mechanism in this genus; carbohydrate ballasting is considered the only plausible mechanism for rapid buoyancy shifts in this cyanobacterium.

Other algae obtain buoyancy from liquids of lower specific gravity than seawater or freshwater in a way similar to a bathyscaphe. Liquid-filled floats have the advantage of being virtually incompressible; but because of their higher density they must comprise a much greater proportion of the organism’s overall volume than is necessary with gas-filled floats if they are to give equivalent lifts. The large central vacuole of diatoms contains cell sap of reduced density, obtained by the selective accumulation of K⁺ and Na⁺, which replace the heavier divalent ions, conferring some buoyancy. In young, fast-growing cultures, diatom cells often remain suspended or sink only very slowly, although in older cultures they usually sink more rapidly. Studies of the distribution of diatoms in the sea suggest that some species undergo diurnal changes of depth, usually rising nearer the surface during daylight and sinking lower in darkness, possibly due to slight alterations of their overall density affected by changes in specific gravity of the cell sap, or in some cases by formation or disappearance of gas vacuoles in the cytoplasm. The dinoflagellate Noctiluca also gain buoyancy from a high concentration of NH4⁺ ions in its large vacuoles, exclusion of relatively heavy divalent ions, especially sulfate, and a high intracellular content of Na⁺ ions relative to K⁺. As a result, the density of the cell sap in the vacuoles is less than that of seawater and the cells can therefore be positively buoyant and float.

When buoyancy control is not possible by these mechanisms, algae can keep afloat and regulate their orientation and depth through adaptations reducing sinking rates. The rate at which a small object sinks in water varies with the amount by which its weight exceeds that of the water it displaces, and inversely with the viscous forces between the surface of the object and the water. The viscous forces opposing the motion are approximately proportional to the surface area, and therefore, other things being equal, the greater the surface area, the slower the sinking rate.

There are a number of structural features of planktonic organisms which increase their surface area and must certainly assist in keeping them afloat. The majority of planktonts are of small size, and therefore have a large surface to volume ratio. In many cases, modifications of the body surface increase its area with very little increase in weight. These modifications generally take two forms: a flattening of the body, or an expansion of the body surface into spines, bristles, knobs, wings, or fins.

A great range of flattened or elaborately ornamented shapes occurs in diatoms such as Chaetoceros sp. In dinoflagellates also, the cell wall in some cases is prolonged into spines (Ceratium) or wings (Dinophysis). Among the Chlorophyceae, the wall of the peripheral cells of Pediastrum colonies may bear clusters of very long and delicate chitinous bristles regarded as buoyancy devices. In Scenedesmus also the cells are clothed by a large number of bristles with a complex structure, which seem to help keep the cells in suspension.

Reduction of the sinking rate is also obtained by an increase in lipid content, which has a density of about 0.86 g cm^-3. Oil droplets are common inclusions in the cytoplasm of algae; lipids stored in this form are present in the Chrysophyceae and Phaeophyceae (Heterokontophyta), and in the Haptophyta, Cryptophyta, and Dinophyta. The thermal expansion of these compounds may be of some significance in affecting diurnal depth changes, through reduction of cell density, but without producing neutral buoyancy.