Abstract
Nostoc rivulare was grown in batch cultures under controlled CO2 and NO3- concentrations to modulate the photosynthetic source:sink relationship. Increasing CO2 supply accelerated the accumulation of chlorophyll (Chl) a, i.e., supplemental CO2 combined with double concentrations of NO3- more than doubled the amounts of Chl a relative to those of the original medium. Photosynthetic oxygen evolution and respiratory oxygen uptake were both enhanced by elevated CO2 and NO3-. Contents of soluble sugars and starch (total non-structural saccharides) as well as insoluble saccharides (structural fraction) were affected by altering CO2-NO3- combinations. Uptake as well as reduction of either NO3- or NO2- was inhibited by CO2 deprivation. Expanding the sink size via increasing NO3- supply enhanced photosynthesis and thus the sink (NO3-) acted as a feed forward stimulator of the source (photosynthesis). The regulatory role of nitrate on photosynthesis was most influential in CO2-deprived cultures since it could enhance photosynthesis to higher levels than CO2-supplemented, nitrate-free cultures.