Mold is a ubiquitous microscopic fungus that naturally breaks down organic matter. The question of whether this organism can thrive when fully submerged underwater is commonly misunderstood by the public. While mold requires moisture to grow, conditions deep below the water surface present unique obstacles. The answer depends entirely on the specific species and the exact physical conditions of the aquatic environment. Understanding the difference between common household molds and specialized aquatic organisms is necessary to explain this phenomenon.
The Biological Needs of Terrestrial Mold
The fuzzy, discoloring growth typically seen on water-damaged materials in homes belongs to true fungi, such as species from the genera Aspergillus or Penicillium. Like most organisms that utilize aerobic respiration, these molds require a constant supply of oxygen to metabolize their food sources and support growth. They feed by secreting digestive enzymes into their environment to break down complex organic materials like cellulose in wood, paper, and drywall. This process allows them to colonize and degrade structural materials. For a mold spore to germinate and for its microscopic filaments, or hyphae, to spread, it needs a food source, appropriate temperature, and moisture. A key requirement is available atmospheric oxygen, which is easily accessible when materials are damp but not saturated. The need for free atmospheric oxygen makes widespread mold growth difficult when the organism is completely enveloped by water.
The Limiting Factor of Dissolved Oxygen
Water itself is composed of oxygen, but the oxygen required for mold metabolism must be available in its dissolved, gaseous form (\(\text{O}_2\)). The concentration of dissolved oxygen (DO) in water is vastly lower than the concentration of oxygen in the air. Air is approximately 21% oxygen by volume, while fully saturated fresh water at room temperature contains less than 1% oxygen. This low concentration of DO generally limits the vigorous, widespread growth characteristic of common molds. The metabolic rate of most terrestrial molds requires a higher oxygen availability than typically found in submerged environments. While some molds are tolerant and can grow slowly at extremely low dissolved oxygen levels, strict anaerobic conditions inhibit the growth of common molds. Furthermore, the depth and movement of the water column influence DO levels; stagnant or deep water often has significantly lower oxygen, further restricting the viability of aerobic organisms.
When Mold-Like Organisms Thrive Underwater
The organisms people often observe growing underwater are frequently not true molds but specialized fungi or fungal-like organisms with adaptations for aquatic life. A prominent example is the group Oomycetes, commonly called “water molds,” which are genetically distinct from true fungi, though they share a similar filamentous growth structure. Oomycetes, such as Saprolegnia, are perfectly adapted to utilize the dissolved oxygen in aquatic habitats. True aquatic fungi, such as aquatic hyphomycetes, also possess specialized features that allow them to decompose submerged organic matter. These fungi produce uniquely shaped conidia, or spores, which are often branched or tetraradiate, enabling them to attach to submerged debris like decaying leaves. These adaptations allow them to function as primary decomposers in stream and river ecosystems, a role terrestrial molds cannot easily fill. Therefore, the cotton-like growth seen on a submerged log or a dead fish is most likely one of these specialized aquatic inhabitants, rather than common household mold.
Real-World Examples of Submerged Growth
The scientific principles governing oxygen availability are demonstrated in real-world scenarios where water saturation is managed. For example, the historic practice of storing timber in ponds, known as ponding, effectively prevents the decay caused by wood-rotting terrestrial fungi. The saturation of the wood starves the fungi of the atmospheric oxygen they need for their metabolic processes. In home environments, visible mold growth on building materials is typically observed at the waterline or in saturated, but not fully submerged, areas. This occurs after events like flooding, where materials like drywall and carpet remain damp but are exposed to the air, providing both moisture and atmospheric oxygen. Water molds, such as Saprolegnia, are frequently observed in aquaculture and aquariums, where they cause infections on fish and their eggs. Their growth is facilitated by the dissolved oxygen present in the water, which they are metabolically equipped to use.