The question of what salt marsh grass “eats” misunderstands how plants acquire nourishment. Salt marsh grasses, primarily species like cordgrass (Spartina), are foundational organisms in their coastal environment. They do not consume food like animals but instead produce their own energy and acquire inorganic materials. This involves two distinct mechanisms: generating energy from light and collecting nutrients from the saturated mud.
The Primary Energy Source: Sunlight and Carbon Dioxide
Salt marsh grasses, like most plants, are autotrophs, meaning they create their own fuel through photosynthesis. They capture sunlight using chlorophyll to convert water and atmospheric carbon dioxide into simple sugars. These sugars serve as the plant’s fundamental energy source for growth, repair, and reproduction.
Many dominant salt marsh grasses, such as Spartina, employ the highly efficient C4 pathway of photosynthesis. This adaptation allows the grass to thrive in the high-light, high-temperature conditions common to coastal marshes. The C4 mechanism concentrates carbon dioxide, minimizing water loss while maximizing energy conversion efficiency. This high productivity allows the grasses to build the structural tissues that make up the vast marsh ecosystem.
Essential Nutrients from Saturated Soil
While sugars provide the energy, the grass requires inorganic nutrients absorbed from the soil to construct its cells and regulate vital functions. The plant’s root system, consisting of dense, spreading rhizomes, anchors it in the soft, waterlogged mud and facilitates material uptake. This soil is often anaerobic (lacking oxygen), which changes nutrient availability compared to dry land.
Nitrogen and phosphorus are two sought-after nutrients, frequently limiting factors in coastal productivity. Nitrogen is primarily absorbed as ammonium (\(\text{NH}_4^+\)), which is abundant in the oxygen-poor marsh sediment. Uptake rates for ammonium are significantly higher than for nitrate (\(\text{NO}_3^-\)), which is less available in these saturated conditions. The root systems must work in conjunction with a complex microbial community.
Bacteria associated with the grass roots play a cooperative role in nutrient exchange. These microbes help mineralize organic matter in the sediment, converting materials into bioavailable forms the plant can absorb. Furthermore, microbial factories, including sulfur-oxidizing and sulfate-reducing bacteria, help the plant manage the toxic sulfide byproduct generated in the anaerobic mud. This symbiotic relationship ensures a steady supply of inorganic building blocks essential for robust growth.
Specialized Adaptations for a Salty Environment
The defining characteristic of salt marsh grass is its ability to survive and flourish in a highly saline environment, which complicates nutrient and water uptake. These plants are classified as halophytes, meaning they are inherently salt-tolerant. High salt concentrations in the surrounding water make it difficult for the plant to absorb freshwater due to the osmotic gradient.
To counteract this, the grasses utilize specialized biological mechanisms. One primary strategy is salt exclusion, where the roots actively block the uptake of excess sodium ions from the surrounding soil water. This allows the plant to maintain a healthier internal water balance necessary for cellular function. Exclusion prevents the salt from entering the plant’s vascular system in large quantities.
For the salt that inevitably penetrates the exclusion barrier, the grass has a second line of defense: specialized salt glands located on the surfaces of its leaves. These glands actively excrete the accumulated salt, which is sometimes visible as white crystals on the leaf blades after the tide recedes. This constant removal prevents toxic levels from building up in the plant’s tissues, allowing the grass to continue generating energy and absorbing nutrients despite the surrounding brackish water.