The Red Sea, a narrow body of water nestled between Africa and Asia, is typically a vibrant blue-green, like many clear tropical waters. However, at certain times, it takes on a reddish or reddish-brown tint. This occasional transformation, which is the scientific basis for its distinctive name, is a dynamic event linked to microscopic life within its waters.
The Microscopic Culprit
The Red Sea’s occasional reddish hue comes from a specific marine cyanobacterium, Trichodesmium erythraeum. Often called “sea sawdust” due to its filamentous structure and tendency to form visible colonies, this photosynthetic prokaryote plays a significant role in marine ecosystems. It converts atmospheric nitrogen into a usable form for other marine life. While widespread in tropical waters, it thrives abundantly in the Red Sea’s unique conditions.
Trichodesmium erythraeum can exist as single filaments or aggregate into larger colonies, sometimes forming extensive mats visible on the water’s surface. Its defining characteristic is its pigment composition, which includes chlorophyll A and a reddish photosynthetic pigment called phycoerythrin. This phycoerythrin gives the cyanobacterium its rusty red color, directly contributing to the sea’s coloration when present in high concentrations.
How the Color Appears
The Red Sea’s reddish coloration appears during “blooms” of Trichodesmium erythraeum. A bloom is a rapid increase in the cyanobacteria population, leading to their dense accumulation near the water’s surface. As these organisms congregate, the red pigment phycoerythrin within their cells becomes concentrated enough to visibly alter the water’s appearance. This pigment absorbs blue and green light while reflecting red light, creating the reddish or reddish-brown tint.
The effect is particularly noticeable when calm, elevated water temperatures allow the cyanobacteria to form thick surface scums. When these blooms die off, the decaying organisms further contribute to the reddish-brown discoloration. Satellite images can capture these vast, pigmented marine regions. The intensity and duration of these color changes vary, transforming sections of the sea from their typical blue-green to a distinct reddish hue.
Beyond the Bloom
The Red Sea’s reddish color is a transient phenomenon resulting from periodic Trichodesmium erythraeum blooms. The occurrence and intensity of these blooms are influenced by specific environmental conditions, including water temperature, salinity, and nutrient levels. High temperatures, often exceeding 27°C, and sufficient nutrient supply, particularly iron and phosphorus, favor their growth.
While cyanobacteria are the primary cause, other theories about the Red Sea’s name have circulated. Some suggest the name comes from reflections of surrounding red mountains or red-colored minerals leaching into the water. However, these factors are not considered the dominant scientific explanation for the sea’s overall “red” appearance. The Red Sea’s typical state is blue-green, and the reddish tint is a temporary biological event.