Slime molds are a unique group of organisms that challenge traditional classifications of life, often appearing as amorphous blobs that move and grow across surfaces. These intriguing entities are not fungi, plants, or animals, yet they exhibit characteristics that can resemble all three at different stages of their life cycles. They are found in diverse damp environments, such as rotting wood, leaf litter, and even in animal dung, playing a role in decomposition by consuming bacteria and other microorganisms. The study of slime molds offers insights into fundamental biological processes, including cell movement and communication.
The Two Main Types of Slime Molds
Slime molds are broadly categorized into two main types: plasmodial (acellular) slime molds and cellular slime molds. Plasmodial slime molds, like Physarum polycephalum, exist as a single, large cell containing millions of nuclei, forming a visible, amorphous mass called a plasmodium. This multinucleate structure lacks individual cell walls, allowing for a continuous flow of cytoplasm.
In contrast, cellular slime molds, such as Dictyostelium discoideum, spend most of their lives as individual, single-celled amoeboid organisms called myxamoebae, feeding on bacteria. When food becomes scarce, these individual cells aggregate, responding to chemical signals, to form a multicellular structure known as a pseudoplasmodium or “slug”. Unlike the plasmodium, this slug is a collection of distinct cells that temporarily move as a cohesive unit.
How Plasmodial Slime Molds Move
Plasmodial slime molds move through cytoplasmic streaming, involving the rhythmic flow of their internal contents. The plasmodium extends fan-like projections at its leading edge as it forages for food. Within this extensive network of vein-like tubes, cytoplasm streams back and forth.
This back-and-forth movement is driven by the periodic contraction and relaxation of an actomyosin network within the plasmodium’s ectoplasmic tubes. Actin and myosin, proteins similar to those found in human muscles, interact to generate force and create hydraulic pressure gradients throughout the cell. These pressure differences propel the low-viscosity endoplasm in one direction more than the other, resulting in the overall amoeboid migration. The continuous reorganization of this internal network allows the organism to navigate and explore its environment.
How Cellular Slime Molds Move
Cellular slime molds exhibit a movement strategy tied to their two distinct life stages. During periods of abundant food, individual myxamoebae move independently, much like typical amoebas, by extending pseudopods and engulfing microorganisms through phagocytosis.
When their food supply dwindles, these individual cells aggregate into a multicellular “slug” or pseudoplasmodium. This aggregation is triggered by the release of chemical signals, particularly cyclic AMP (cAMP). The resulting slug then migrates as a cohesive unit, leaving behind a trail of slime. The coordinated movement within the slug is also guided by waves of cAMP, directing cells towards the anterior tip, enabling the entire structure to search for a suitable location for spore dispersal.
What Guides Their Movement
Both plasmodial and cellular slime molds respond to environmental cues, guiding their movement toward resources and away from unfavorable conditions. Chemotaxis, the movement in response to chemical signals, is a primary driver for both types. Plasmodial slime molds are attracted to food sources like bacteria, sugars, and proteins, and are repelled by substances such as salt, caffeine, or high pH. They can sense food at a distance and adjust their movement accordingly.
Phototaxis, the response to light, also plays a role in slime mold navigation. Plasmodial slime molds avoid light. However, when preparing to form spores, they will switch their behavior and become attracted to light to facilitate spore dispersal. Cellular slime mold slugs also show phototaxis, moving towards light to find an open, lit place for fruiting body development. Thermotaxis, or movement in response to temperature, influences the direction of cellular slime mold slugs, as they seek optimal thermal environments.