Prokaryotes, including bacteria and archaea, are typically considered single-celled organisms. This contrasts with eukaryotes, which include both single-celled forms like yeast and complex multicellular organisms such as plants and animals. Biology often presents prokaryotes as exclusively unicellular, a characteristic distinguishing them from multicellular life. This raises questions about whether this view fully captures prokaryotic diversity and prompts a closer examination of their biological organizations.
Understanding Multicellularity
True multicellularity requires a biological organization where cells cooperate and depend on one another. A key aspect is cell-cell adhesion, where cells remain physically connected. Multicellular organisms also exhibit a division of labor, with different cells performing specialized functions.
This specialization creates cellular interdependence, as individual cells rely on others for the organism’s survival. Multicellularity often includes irreversible differentiation, where specialized cells cannot easily revert to a less specialized state. Growth and development are coordinated, often involving programmed cell death to shape the organism.
Complex Prokaryotic Associations
Many prokaryotes exhibit complex social behaviors and structural organizations resembling multicellularity.
Biofilms are a common example, where bacterial cells attach to surfaces and each other, forming communities within a self-produced extracellular matrix. Within biofilms, cells experience nutrient and oxygen gradients, leading to localized metabolic differences and a rudimentary division of labor.
Filamentous cyanobacteria, like Anabaena and Nostoc, show defined cellular specialization. They form cell chains where some differentiate into heterocysts for nitrogen fixation, protected by thickened walls. Others become akinetes, dormant spore-like cells that survive harsh conditions and later germinate. This demonstrates a clear division of labor and interdependence.
Myxobacteria, such as Myxococcus xanthus, display collective behaviors during nutrient scarcity. Cells aggregate and swarm, forming complex fruiting bodies. Some differentiate into resistant myxospores, while others lyse to nourish developing spores. This involves cell-cell communication and programmed cell death for the collective.
Distinguishing from Eukaryotic Multicellularity
Despite complex prokaryotic associations, most scientists do not classify them as truly multicellular like eukaryotes. A key difference is the degree of cellular interdependence and differentiation irreversibility. Prokaryotic specialization is generally less complex and often reversible; specialized cells can revert to a less differentiated state more readily than in eukaryotes. This contrasts with the highly integrated, permanent specialization in multicellular eukaryotes.
Prokaryotic associations also lack hierarchical organization into true tissues, organs, and organ systems found in complex eukaryotes. Even organized prokaryotic structures, like myxobacterial fruiting bodies, do not achieve the structural and functional complexity of eukaryotic tissues.
Prokaryotic cell-cell communication and adhesion mechanisms are generally simpler than the sophisticated systems in eukaryotes. Eukaryotic cells use diverse adhesion molecules and signaling pathways for precise interactions.
Complex eukaryotic multicellular organisms undergo elaborate developmental programs involving precise cell divisions, migrations, and programmed cell death to form their characteristic shapes. While some prokaryotic collectives show coordinated development, these processes are far less intricate and lack the extensive morphogenetic events seen in eukaryotes.
Thus, while prokaryotes show collective behaviors mimicking multicellularity, they generally lack the irreversible, highly integrated specialization and tissue-level organization characteristic of complex eukaryotic multicellularity.