Page 109 - Algae Anatomy, Biochemistry, and Biotechnology

92 Algae: Anatomy, Biochemistry, and Biotechnology

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 CO 2 and CO. Oxygen
and CO 2 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 O 2 and CO 2 .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 struc-
tures (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 23 , about one eighth of that of water, and if sufficient gas-filled struc-
tures 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 low-
light 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 pro-
duction, 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 Ana-
baena flos-aquae, with a critical collapse pressure distributed about a mean of 6 bars. In Trichodes-
mium 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; carbo-
hydrate 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 incom-
pressible; 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
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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
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from a high concentration of NH 4 ions in its large vacuoles, exclusion of relatively heavy divalent
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ions, especially sulfate, and a high intracellular content of Na ions relative to K . As a result, the