Competition between heterotrophic and autotrophic microplankton for dissolved nutrients.

Area of Science:

• Microbial Ecology
• Limnology
• Biogeochemistry

Background:

• Microbial communities often involve interactions between heterotrophic and phototrophic organisms.
• Nutrient availability, particularly phosphate (Pi), is a critical factor regulating aquatic microbial growth.
• Understanding coexistence mechanisms is key to predicting microbial community dynamics.

Purpose of the Study:

• To investigate the conditions for coexistence between a heterotrophic yeast (Rhodotorula rubra) and a phototrophic alga (Selenastrum capricornutum).
• To determine the role of organic carbon (C) and phosphate (Pi) limitation in regulating the growth and interaction of these plankton species.
• To test the applicability of a threshold model for microbial growth in a mixed-culture system.

Main Methods:

• Continuous culture experiments were performed using dilute phosphate (Pi) conditions.
• The growth rates of Rhodotorula rubra (heterotroph) and Selenastrum capricornutum (phototroph) were monitored.
• The concentration of organic carbon (C) was manipulated to assess its effect on coexistence and biomass.

Main Results:

• Coexistence was observed only when the heterotrophic yeast was limited by organic carbon (C).
• The yeast exhibited a higher affinity for phosphate (Pi) compared to the alga.
• Organic carbon (C) concentration indirectly controlled the biomass of the phototroph (alga) due to its influence on yeast Pi uptake.

Conclusions:

• Organic carbon availability is a critical indirect regulator of phototrophic biomass in phosphate-limited aquatic systems.
• The heterotrophic yeast's ability to transport Pi, unaffected by C starvation, plays a key role in competitive exclusion.
• The findings support a threshold model of microbial growth, highlighting the importance of nutrient-specific limitation and inter-species interactions.