Seagrass is a true flowering plant that forms vast underwater meadows and lives completely submerged in saltwater environments. Seagrass is primarily green, much like the terrestrial grasses it resembles. This coloration results directly from the biological machinery the plant uses to capture sunlight for energy production. The green color is a visual indicator that the plant is healthy and actively performing photosynthesis in its aquatic habitat.
The Green Hue: Chlorophyll and Light Capture
The green color of seagrass is due to the high concentration of chlorophyll in its cells. Seagrass contains both chlorophyll a and chlorophyll b, the same primary pigments found in land plants. These chlorophyll molecules are housed within chloroplasts and initiate the process of photosynthesis.
Chlorophyll functions by selectively absorbing specific wavelengths of light from the visible spectrum. Chlorophyll a and b efficiently absorb light in the blue-violet (400–500 nm) and red-orange (600–700 nm) spectra. This dual absorption leaves the middle part of the spectrum, the green light, largely unused.
The green light, which is not absorbed, is reflected back, giving the seagrass its characteristic green appearance beneath the water’s surface. Seagrasses are generally found in shallow waters where light penetration is still high. They rely heavily on this green pigment to capture the necessary solar energy for growth and survival.
Factors Causing Color Variation
While seagrass is fundamentally green, its color can shift to shades of yellow, brown, or reddish-purple under environmental stress. This color change is a direct visual cue of a physiological response to unfavorable conditions. When sunlight intensity is too high, such as in very shallow water or during low tide exposure, the plant may synthesize non-green accessory pigments for protection.
One protective mechanism involves the accumulation of anthocyanins, which are reddish or purple pigments that act as sunscreens to shield the photosynthetic machinery from damaging UV radiation. Nutrient deficiency or disease can lead to chlorosis, where chlorophyll breaks down faster than it is replaced. The resulting lack of green pigment causes the seagrass to appear pale green or yellow, often associated with plant die-off.
Color variation is also caused by carotenoids, which are yellow-orange accessory pigments. These pigments are always present but are usually masked by the abundant green chlorophyll. Under stressful conditions, such as warming water temperatures, the ratio of carotenoids to chlorophyll may increase, causing the leaves to take on a brownish or yellowish hue as part of a photoacclimation process.
Seagrass Versus Seaweed: A Key Difference
Distinguishing seagrass from seaweed is often confusing, but the difference is rooted in their biology and coloration. Seagrass is a vascular plant with true roots, stems, and veins to transport water and nutrients, much like land-based flowering plants. Seaweed is non-vascular algae, lacking complex internal transport structures, and absorbs nutrients directly from the surrounding water.
This structural difference explains the wider color palette seen in seaweeds, which are categorized as green, brown, or red algae. Brown and red seaweeds possess accessory pigments, like fucoxanthin, allowing them to absorb light wavelengths that penetrate deeper into the water. Seagrass, restricted to green chlorophyll due to its terrestrial ancestry, thrives primarily in shallower, brighter waters where red and blue light are available.
The difference in color reflects distinct evolutionary strategies for capturing light in the marine environment. Seagrass relies on its green hue to maximize light absorption in the upper water column. Seaweeds, conversely, use a range of pigments to exploit the limited light that filters down to greater depths.