When a hetcrotrophic (Rhodotorula rubra) and a phototrophic (Selenastrum capricornutum) plankton were grown together in dilute phosphate (Pi) continuous cultures, coexistence occurred only when the heterotroph was growth- rate limited by organic carbon (C). Because of its higher affinity for Pi, and because C starvation does not affect the heterotrophic yeast’s ability to transport Pi, the concentration of organic carbon indirectly controlled the biomass of the phototroph. The results support a threshold model of microbial growth.
Phosphorus (P) is the key biomass limiting nutrient in many lakes , and kinetic studies beginning with those of Rhec  have shown that heterotrophic bacteria are better competitors than are phytoplankton for dissolved inorganic orthophosphate (Pi). If such kinetic studies accurately reflect multi-species competition for Pi, some resource (or inhibitor) other than Pi must restrict heterotrophic production in “phosphate- limited” systems where phytoplankton predominate. Results from at least one whole lake experiment showed that fertilization with organic carbon (sucrose) and nitrogen caused a decline in phytoplankton biomass from the higher levels observed when P was added to the lake. In addition, the carbon/phosphorus ratio of the lake water actually declined after organic carbon was added .
In this study we have investigated with laboratory experiments how a carbon-limited heterotroph may affect the growth and biomass of a P-limited autotroph. An alga known to have a moderate affinity for Pi  was forced to compete for this resource against a heterotrophic yeast known to have a higher affinity for Pi [ 10]. To allow for some growth of the autotroph, the organic carbon (C) concentration was varied from levels that were growth-rate limiting for the heterotroph to levels such that available P limited the biomass of both the yeast and alga. Our experimental design (dual species continuous culture) allowed us to investigate whether or not C starvation affects the Pi affinity of the heterotroph (i.e., does a multiplicative or threshold kinetic model best describe competition for phosphorus and carbon.