101
Anatomy
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 also 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 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 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, and 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), hence turgor pressure increases, it may
collapse gas-filled structure, and cells become negatively buoyant 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 bar. In Trichodesmium sp., gas-filled structures can withstand pressures of 12–37 bar 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.
TABLE 2.1
Swimming and Gliding Speeds of Some Planktonic Algae
Name
Mean Speed
Gymnodium gracilentum
500 μm* s −1
Symbiodinium sp.
250 μm* s −1
Tetraflagellochloris mauritanica
200 μm* s −1
Tetraselmis suecica
180 μm* s −1
Euglena gracilis
100 μm* s −1
Chattonella sp.
120 μm* s −1
Chlorarachnion reptans
110 μm* s −1
Dunaliella salina
95 μm* s −1
Ochromonas danica
80 μm* s −1
Bigelowiella natans
70 μm* s −1
Pavlova salina
50 μm* s −1
Synechococcus
25 μm* s −1
Oscillatoria spp.
10 μm* s −1
Leptolyngbya spp.
0.004 μm* s −1
Anatomy
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 also 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 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 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, and 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), hence turgor pressure increases, it may
collapse gas-filled structure, and cells become negatively buoyant 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 bar. In Trichodesmium sp., gas-filled structures can withstand pressures of 12–37 bar 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.
TABLE 2.1
Swimming and Gliding Speeds of Some Planktonic Algae
Name
Mean Speed
Gymnodium gracilentum
500 μm* s −1
Symbiodinium sp.
250 μm* s −1
Tetraflagellochloris mauritanica
200 μm* s −1
Tetraselmis suecica
180 μm* s −1
Euglena gracilis
100 μm* s −1
Chattonella sp.
120 μm* s −1
Chlorarachnion reptans
110 μm* s −1
Dunaliella salina
95 μm* s −1
Ochromonas danica
80 μm* s −1
Bigelowiella natans
70 μm* s −1
Pavlova salina
50 μm* s −1
Synechococcus
25 μm* s −1
Oscillatoria spp.
10 μm* s −1
Leptolyngbya spp.
0.004 μm* s −1