Tidal Triggers and Hot-Spot Switches in Coastal Marsh

Coastal marshes lie at the dynamic interface between land and sea, where materials from multiple sources are mixed and transformed, and marsh productivity is extraordinarily high. Among the organisms supported by marsh plant productivity are highly diverse microbial communities, some of which support the plants themselves, and others that recycle nutrients, produce or consume greenhouse gases, or even contribute to carbon storage in marsh sediment. A powerful feature of coastal tidal marshes is the presence of a highly simplified plant community; single plant species can blanket marsh platforms, meaning that what is learned from studying that species’ interactions with microbes

microbe

Microbes are tiny living things that are found all around us and are too small to be seen by the naked eye. They live in water, soil, and in the air.

Source: NIH Source

and the local environment is more easily expanded to represent the whole marsh system. We plan to capitalize on this naturally simplified system at a brackish, coastal marsh vegetated with the cattail Typha angustifolia, where last July a remarkably rapid whole-system-scale switch in biogeochemical function was detected by micrometeorological measurements. Early in the season, emission of the greenhouse gas methane

methane

A hydrocarbon that is a primary component of natural gas. Methane is also a greenhouse gas (GHG), so its presence in the atmosphere affects the earth’s temperature and climate system

Source: EPA Source

from the brackish marsh to the atmosphere was quite high – approximately 200 nmol m-2 s-1. But because of drought, a flooding tide in mid-July was particularly saline, and within hours of marsh flooding by that saline tide, methane emissions to the atmosphere dropped to near zero. A whole marsh scale switch had been triggered. Lab, field, and modeling work are planned that target small-scale “hot spot/moment“ mechanisms we hypothesize underlie this system-scale switch that stopped methane emissions for months. 
We will test four hypotheses:
• H1. Salinity

salinity

Salinity is the dissolved salt content of a body of water. It is a strong contributor to conductivity and helps determine many aspects of the chemistry of natural waters and the biological processes within them.

Source: EPA Source

stress induces increased catalase activity in roots of Typha angustifolia, leading to root oxygen release from hydrogen peroxide produced during stress. Aerobic methanotrophy may therefore increase in rhizosphere

rhizosphere

The soil region influenced by plant roots where complex biological and ecological processes occur.

Source: EMSL Source

“hot-spots” of enhanced oxygen availability.
• H2. Anaerobic methane oxidation in the upper 10 cm (“hot-layer”) of sediment is stimulated by saline inundation.

inundation

Inundation is the flooding of land that is normally dry. It can be caused by a variety of factors, including heavy rain, snowmelt, or a storm surge. Inundations can be a major hazard, causing damage to property and infrastructure, and even loss of life.

Source: Weather.gov Source

ANME archaea in consortia with sulfate reducing bacteria are potential actors.
• H3: Methylotrophic methanogenesis

Methanogenesis

An anaerobic respiration that generates methane as the final product of metabolism.

Source: National Library of Medicine Source

produces methane despite high porewater sulfate concentration in the brackish marsh. 
• H4. Typha angustifolia roots produce glycine betaine as a compatible osmolyte. Glycine betaine, once fermented to trimethylamine, could preferentially support methylotrophic methanogens producing methane in Typha marsh.
Experiments testing these four hypotheses will inform modeling of coastal marsh systems within an E3SM-PFLOTRAN modeling framework. Understanding the hot-spot/hot-moment causes and consequences of the dramatic system tipping point that we observed is essential if coastal system functions such as greenhouse gas emissions are to be predicted, particularly as sea level rises and storm precipitation becomes more intense and sporadic.