Anabaena

Anabaena demonstrates that bacteria can evolve multicellular organization with division of labor. This filamentous cyanobacterium grows as chains of cells where most cells photosynthesize while specialized 'heterocysts' fix nitrogen. Photosynthesis produces oxygen, which inactivates the nitrogen-fixing enzyme; nitrogen fixation requires oxygen-free conditions. By spatially separating these incompatible processes into different cell types, Anabaena achieves what no single cell can: simultaneous photosynthesis and nitrogen fixation.

Heterocyst differentiation represents genuine cellular specialization. These cells develop thick walls blocking oxygen diffusion, lose the capacity for photosynthesis, and become dependent on neighboring cells for carbon. They're terminally differentiated—they cannot divide or return to normal function. The sacrifice is permanent. In return, heterocysts provide fixed nitrogen to the entire filament. This division of labor creates a cooperative system more capable than independent cells.

Anabaena's organization anticipates multicellular life. Cells communicate through channels connecting adjacent cells in the filament. They coordinate behavior through chemical signals. Spacing of heterocysts is regulated to optimize nitrogen distribution—too many heterocysts waste photosynthetic capacity; too few create nitrogen-starved regions. The regulatory mechanisms ensuring appropriate heterocyst frequency resemble those controlling cell differentiation in plants and animals. Anabaena reminds us that multicellularity and division of labor evolved repeatedly, always for similar reasons: specialization enables capabilities impossible for generalists.