Constraints imposed by the spherical form and gelatinous matrix of centimeter-thick colonies of the

cyanobacterium Nostoc zetterstedtii on its functional properties were tested by examining the scaling of its

composition, light absorption, photosynthesis, and respiration to individual size. In three summer experiments

with colonies collected from the bottom of oligotrophic lakes of low inorganic carbon concentrations (dissolved

inorganic C, DIC), metabolism and pigment density of colonies were scaled to their surface area as most algal

filaments were confined to a 2-mm-thick outer shell. Nostoc absorbed 96% of incident light from the surface to the

center because of high areal pigment density, but absorbed photons were used with low quantum efficiency (11-

38 mmol O2 mol21 photon) and photosynthesis was low relative to dark respiration (2.0-5.4). Therefore, N.

zetterstedtii is threatened by reduced light availability and only extended to lake depths receiving about 12% of

surface irradiance, whereas mosses, characeans, and angiosperms with thin photosynthetic tissues grew deeper

(3.1-7.5% of surface irradiance). Nostoc ameliorated the restrictions of low lake DIC and long diffusion paths by

active transport that could extract most external DIC, accumulate DIC in the colony 150-fold above external

concentrations, and retain respiratory CO2. The energy cost of solute transport and gel formation in Nostoc

colonies and extensive self shading restrict their potential growth, whereas colony formation should prevent

grazing and increase longevity and nutrient recirculation. Nostoc zetterstedtii has become one of rarest freshwater

macroalgae because of widespread lake eutrophication reducing water transparency and increasing competition

from taller and faster-growing stands of filamentous algae and higher plants.