2. Cyanobacteria are photosynthetic. They use light energy 
and chlorophyll-a to convert carbon dioxide into cellular bio¬ 
mass and in the process produce oxygen. This process is es¬ 
sentially identical to photosynthesis in terrestrial plants and 
all other marine and freshwater algae. It is this characteris¬ 
tic which accounts for the other commonly used name for this 
group: blue-green algae. 
3. Although lacking external means of motility such as 
flagella or cilia, some cyanobacteria contain intracellular gas 
vacuoles. By regulating the number of these internal, gas-fill¬ 
ed vesicles, cyanobacteria are capable of vertical rates of mi¬ 
gration that rank among the highest observed for algae, up to 
2-3 m hr 1 . 
4. Unlike other algae, certain species of cyanobacteria are 
capable of utilizing nitrogen gas from the atmosphere for bio¬ 
mass and metabolic processes. Species which are capable of nit¬ 
rogen fixation are not dependent on inorganic forms of nitrogen 
found in their environment such as nitrate, nitrite, or ammonium, 
which are required by other algae. 
Although an abundant supply of nitrogen and phosphorus is 
necessary to support the high algal biomass which occurs in sum¬ 
mer, cyanobacteria blooms, it appears that certain adaptive re¬ 
sponses of the principal bloom-forming cyanobacteria, Micro ¬ 
cystis aeruginosa . are a more immediate cause of surface scum 
formation. As temperature and light availability increase in 
the spring and early summer, algal growth rates increase and 
algal biomass accumulates in the water column. Fueled by an 
abundance of nutrients, algae may become so dense that light in 
the water column is decreased by self-shading, and/or carbon 
dioxide availability is decreased by an excess of demand over 
supply. Microcystis aeruginosa has been shown to respond to 
both of these conditions by increasing its bouyancy and floating 
to the surface where the availability of both light and carbon 
dioxide are maximal (Paerl and Ustach, 1982). Unlike other al¬ 
gae which are actually inhibited by summer surface light inten¬ 
sities, M. aeruginosa responds to high light intensities by pro¬ 
ducing a pigment which protects its photosynthetic apparatus 
from the deleterious effects of too much light (Paerl et al., 
1983). It is this unusual capability of M. aeruginosa to resist 
and thereby exploit high surface light intensities which is re¬ 
sponsible for their dense accumulation at the surface. Coinci¬ 
dent with this migration to the air-water interface, M. aeru ¬ 
ginosa changes its morphology from individual small cells to a 
colonial form comprised of thousands of cells in a mucous en¬ 
velope. A secondary but significant effect of surface scum 
formation is that light availability to more desirable algae 
species distributed throughout the water column is decreased 
with a consequent reduction in their growth and abundance. 
101 
