Competition between Heterotrophic and Autotrophic Microplankton for Dissolved Nutrients Essay

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.

Introduction

Phosphorus (P) is the key biomass limiting nutrient in many lakes [13], and kinetic studies beginning with those of Rhec [8] 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 [12].

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 [2] 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.

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