Hidden Allee effect in photosynthetic organisms

Area of Science:

• Ecology and population biology
• Mathematical modeling
• Microbiology

Background:

• The Allee effect describes reduced population growth at low densities, a phenomenon rarely observed in asexually reproducing microorganisms.
• Photosynthetic organisms, like plants, are often considered insensitive to the Allee effect, impacting ecological theories.
• Understanding low-density effects in photosynthetic microbes is crucial for food chain dynamics.

Purpose of the Study:

• To develop a novel, flexible mathematical model for density dependence, incorporating the Allee threshold.
• To investigate the presence and characteristics of the Allee effect in photo-autotrophic organisms, specifically green paramecia and cyanobacteria.
• To determine if genetic mutations can influence Allee effect sensitivity in autotrophs.

Main Methods:

• Developed a novel mathematical equation for a flexible density effect model with an Allee threshold.
• Monitored the optical growth of green paramecia under photo-autotrophic and photo-heterotrophic conditions.
• Assessed the Allee effect sensitivity of wild-type cyanobacteria (Synechocystis sp. Strain PCC6803) and a mutant lacking the photosynthesis-related pxcA gene.

Main Results:

• Green paramecia exhibited an Allee-like weak low-density effect under both photo-autotrophic and photo-heterotrophic growth.
• Wild-type cyanobacteria (PCC6803) showed no sensitivity to the low-density effect, confirming existing knowledge.
• A mutant cyanobacteria strain with a defective pxcA gene displayed sensitivity to the Allee effect, failing to propagate at low densities.

Conclusions:

• The study introduces a new mathematical model for density-dependent population dynamics with an Allee threshold.
• Green paramecia demonstrate an Allee-like effect, expanding the known occurrences of this phenomenon.
• This research provides the first evidence that a single-gene mutation can confer Allee effect sensitivity in an autotrophic organism, with implications for microbial ecology.