Salt tolerance

Effects of salinity on morpho-physiological features and
the rhizosphere microbiome of three marsh species

Salt marshes

Salt marshes are important coastal ecosystems because they are highly productive. They also provide valuable ecosystem services, such as the provision of food and shelter for many organisms, carbon sequestration, shoreline stabilization and storm protection, filtration of excess nutrients, and valued recreational and aesthetic opportunities. Coastal marshes are increasingly recognized to be threatened by anthropogenic induced climate change and sea level rise (SLR). In addition to direct inundation, SLR will play a potentially important role in increasing saltwater influx into coastal marsh habitats, resulting in gradually accelerating salinification. The deleterious effects of saltwater intrusion into coastal marsh habitats is still poorly understood, potentially affecting productivity, litter decomposition, and other ecosystem functions. Improved understanding of vegetation tolerances and responses to coastal salinification is, therefore, increasingly critical because of the disproportionately large range of the ecosystem services that coastal marshes offer to human societies.

Salt marsh species

Coastal salt marsh dominated coasts along the northern Gulf of Mexico (GoM) are dominated by three plant species; the main low marsh fringing species in saline waters is the grass Spartina alterniflora Loisel, in brackish areas or in mid-marsh elevations the dominant species is black needle rush, Juncus roemerianus Scheele, and in fresh water or high marsh transitions the sedge, Schoenoplectus americanus (Pers.) Volkart ex Schinz & R. Keller can be very abundant. Found in marine salinity marsh habitats, the grass S. alterniflora is a long-lived, warm season perennial that grows to 2m tall, and spreads extensively by long hollow rhizomes. Soft, spongy stems up to 1.7cm in diameter emerge from the rhizomes. The flat leaf blades are typically 30 to 50cm long, tapering to a long inward-rolled tip. The halophytic rush J. roemerianus is found growing in brackish marshes in dense zonal stands. It is a moderate growing, bunch forming, grass-like perennial rush. The plant is coarse and rigid, 0.5-1.5 m tall. The leaves are terete and stiff, with 2-4 leaves per shoot. S. americanus is found in fresh and brackish marshes. It is a perennial native herbaceous sedge growing to 2m tall. Single stems that are in small groups, sharply triangular grow up from long stout rhizomes. The leaves are firm, long, and strongly folded, sometimes flat and 2-4 millimeters wide.

Rhizosphere microbiomes

Natural tidal marshes are highly productive coastal ecosystems characterized by sediments rich in organic matter resulting from the decay of plant material and root exudation. Because of the frequent waterlogging, the abundant residual organic matter is actively decomposed via anaerobic microbial respiration through sulfate reduction. The sulfate reducers produce hydrogen sulfide and other reduced sulfur compounds that serve as electron donors for sulfur-oxidizing microorganisms, which complete the sulfur cycle. Sulfide is toxic to plant metabolism, and it is thought that marsh grasses benefit from the interaction with sulfur-oxidizing microorganisms that help to lower hydrogen sulfide levels in the rhizosphere. Plants have also developed various biochemical and physiological mechanisms to respond and adapt to stress conditions. One of them is the fostering of root-associated microbial communities that positively influence plant fitness in response to abiotic stressors. The capacity of rhizobacteria to alleviate the detrimental effect of abiotic stress is a multifactorial phenotype. Beneficial rhizobacteria improve the nutritional status of the host plant by fixing atmospheric nitrogen, solubilizing inorganic phosphate, and secreting iron-chelating siderophores. Many root-colonizing microorganisms stimulate plant organ development through the production of phytohormones, which stimulate the formation of root hairs, root growth, and branching, and can improve mineral and nutrient uptake and enhance growth following environmental stress, like elevated rhizosphere porewater salinity.

A research project of the Center for Plant Restoration and Coastal Plant Research - https://sites.google.com/site/coastalplantrestoration/