Microbes, marshes, and mangroves: Implications for coastal carbon storage

Article Information: Barreto CR, Morrissey EM, Wykoff DD, Chapman SK. 2018. Co-occurring Mangroves and Salt Marshes Differ in Microbial Community Composition. Wetlands. DOI Link: https://doi.org/10.1007/s13157-018-0994-9

Salt marsh sediment
Fig. 1. Muddy salt marsh sediment. Photo Credit: Ashley Bulseco-McKim

Coastal blue carbon systems store LOTS of carbon

Mangroves and salt marshes (along with seagrasses) are often referred to as “blue carbon” systems. These are coastal wetland systems that capture and store carbon in their sediments (Fig. 1) due to lots of vegetation growth and slow decomposition, or break down, of organic material (less decomposition means more storage over time). This service is important because it helps to reduce negative consequences of climate change by removing carbon dioxide that would otherwise exist in the atmosphere and storing it in their sediments instead. Mangroves (Fig. 2) are shrubs or trees that grow upwards from their strong root systems, while salt marshes consist of flat expanses of grass and muddy sediments (Fig. 3). While both systems store carbon efficiently, the means and extent by which they achieve this service, differ.

The tides and temperatures are changing

In tropical and subtropical latitudes, salt marsh and mangrove vegetation types tend to co-exist, with relative distribution depending largely on temperature and tidal influence. However, increasing temperatures and sea level rise, both of which can be consequences of climate change, have led to the expansion of mangroves forests into salt marshes. In one site in Florida, mangrove abundance has increased by 69% over the past 7 years, drastically changing the soil carbon cycle in that area. What does this mean for carbon storage in these blue carbon systems?

Microbes show the way!

Mangrove Forest
Fig. 2. Mangrove forest in Loxahatchee, Florida. Photo credit: National Oceanic and Atmospheric Administration

A team in Florida led by Chelsea Barreto and others at Villanova set out to answer this question, but did so by examining the smallest of organisms: microbes. They wanted to know if microbes in salt marshes and mangrove roots differed, and what this meant for the ability for each vegetation type to store carbon.

Why does understanding the microbial community matter? Microbes play a fundamental role in soil carbon cycling.

Much like you and I need to consume food to survive, many microbes use carbon-rich organic matter as a source of energy. Similarly, much like us, some of these microbes use oxygen to “breathe” (aerobic respiration). However, many microbes can use other chemical elements too, such as iron, manganese, nitrate, sulfate, or carbon dioxide (anaerobic respiration), but these processes do not provide as much energy, and thus do not use as much organic matter.

Depending on the amount of soil organic matter these microbes eat, what remains behind and unprocessed is stored in the sediments as blue carbon. Hence, the more microbial activity occurs, the lower the potential for carbon storage.

salt marsh
Fig. 3. Example of a New England salt marsh. Photo credit: Ashley Bulseco-McKim

Getting to the “root” of the matter

As you can see, mangroves (Fig. 2) have much larger roots than marshes (Fig. 3), and therefore shuttle more oxygen to the soil below. The researchers hypothesized that, if mangrove area increases, it may shift the microbial community towards microbe types that can use oxygen, and that this would result in less carbon storage (because using oxygen consumes more organic matter).

They setup plots along a salt marsh-mangrove transition zone and collected soil samples to investigate the microbial community and experimentally test for carbon substrate usage between vegetation types (yellow and green dots; Fig. 4).

Fig. 4. Experimental plots setup across a salt marsh-mangrove transition zone, with green dots representing marsh-dominated vegetation (>90% cover) and yellow dots representing mangrove-dominated vegetation (>70% cover). Three blue dots represent “pure mangrove” samples and orange dots represent “pure marsh” samples for incubations. (Barreto et al. 2018)

Results supported their hypothesis that microbial communities differ between salt marshes and mangroves. While microbes in the salt marsh plots came from groups known to use less energetically favorable, anaerobic (oxygen-free) pathways, microbes in mangrove soil primarily represented aerobic (oxygen-using) microbes.

This suggested that there would be less carbon storage capacity in mangrove soils, because the microbes there had the ability to use it for energy rather than leaving it behind to be stored belowground.

How can we know if these changes in the microbial community matter? They tested for it. The research team also collected soil cores from each vegetation type (blue and orange dots; Fig. 4) and let them incubate over time to see how much carbon each would use up. Although not significant, results suggested that mangrove-soil used more organic matter than marsh soil, meaning that less would be left behind to be stored into sediment as blue carbon.

The take home message: Don’t forget the microbes

The results of this study show two things: 1) that microbes differ between marshes and mangroves and that 2) this could result in changes to soil carbon cycling and carbon storage. This is a first step in better understanding the consequences of vegetation changes from marshes to mangroves in response to climate change (and what it could mean for climate), but there are obviously many more factors to consider over the long term.

The most important message here, perhaps, is how important microbes are in understanding the function of coastal systems. Many of the changes these researchers observed would have remained insignificant and undetectable had they not examined the microbial community. So, don’t forget the microbes.

Microbial communities mediate many of the biogeochemical processes we are most interested in studying, so ignoring their influence may leave us blind to the environmental changes that are occurring.

 

 

References:

Barreto CR, Morrissey EM, Wykoff DD, Chapman SK. 2018. Co-occurring Mangroves and Salt Marshes Differ in Microbial Community Composition. Wetlands. DOI Link: https://doi.org/10.1007/s13157-018-0994-9

National Oceanic and Atmospheric Administration. 2018. https://oceanservice.noaa.gov/facts/mangroves.html. Accessed 2/1/2018.

National Oceanic and Atmospheric Administration. 2018. https://oceanservice.noaa.gov/facts/saltmarsh.html. Accessed 2/1/2018.

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Ashley Bulseco-McKim

Ashley Bulseco-McKim

Ashley is currently a postdoctoral scientist at the Marine Biological Laboratory in Woods Hole, MA studying microbial food web connectivity. She earned her Ph.D. in Ecology, Evolution, and Marine Biology at Northeastern University, and is particularly interested in better characterizing the microbial response to human-driven disturbances and its link to ecosystem function. Ashley is passionate about finding unique ways to communicate science. Outside of her research, Ashley enjoys spending time outdoors with her dogs and coaching youth softball. You can find her on Twitter @MarshMicrobe.

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