









































Impacts of Climate Change on 
Mycorrhizal Fungi in Salt Marsh 

Habitats 
Cathilyn McIntosh, Gina Wimp, Ph.D. 

Volume Two
Edition One
Fall 2021

 
GEORGETOWN SCIENTIFIC
RESEARCH JOURNAL

14



Georgetown	Scientific	Research	Journal	

Impacts of Climate Change on Mycorrhizal Fungi in Salt Marsh 
Habitats 

Cathilyn McIntosh, Gina Wimp 

Department of Biology, Georgetown University, Washington D.C. 
E-mail: clm315@georgetown.edu 
https://doi.org/10.48091/gsr.v2i1.28 

Abstract 

Salt marshes are coastal wetlands that cover 2-3% of land surface area.1 These habitats carry out several 
essential functions such as providing habitats for many species, acting as a buffer between terrestrial land and 
ocean waters, and, most importantly, acting as a major carbon (C) storage pool. Arbuscular mycorrhizal fungal 
(AMF) symbionts are key organisms in salt marsh habitats and are known to influence the following processes 
and factors: plant zonation, plant resource competition, plant productivity, plant genetic diversity, soil C 
sequestration, soil C:N:P ratios, saprotrophic bacterial population and diversity, soil stability, and litter 
decomposition. Under rapidly changing conditions caused by climate change, it is difficult to predict how 
AMF communities will respond, ultimately altering these factors. This review will explain the role of AMF 
communities in modulating carbon sequestration by increasing plant and fungal biomass and influencing soil 
organic matter decomposition. Additionally, this review presents the current knowledge regarding how sea 
level rise (SLR), elevated CO2 levels, and eutrophication are expected to decrease AMF abundance and 
diversity by increasing habitat fragmentation, decompositional rates, anoxic conditions and altering soil 
nutrient stoichiometry. Studying soil fungi is essential for understanding how mycorrhizal communities are 
predicted to react to a climate change and, consequently, alter salt marsh processes.  

Keywords: AMF, sea level rise, eutrophication, salt marsh 

1. Introduction
Salt marshes are coastal wetlands that are

consistently flooded and drained with salt water due 
to changing tides. These ecosystems are found 
worldwide and are often dominated by salt–tolerant 
smooth cordgrass which is essential for marsh 
stability. Additionally, salt marsh habitats perform a 
plethora of essential functions, such as acting as a 
buffer between terrestrial lands and ocean sea waters, 
providing habitats for coastal organisms, and most 
importantly, sequestering carbon (C) at unparalleled 
rates, which makes it an essential terrestrial C sink.2 

Salt marsh C sequestration can largely be 
attributed to its high net primary production and 
decomposition cycle. It is estimated that the global 
net primary production from salt marsh vegetation is 
0.44 x 1015 g C per year, equivalent to 440 x 106 
metric tons of C per year.2 Only about 30% of this 
vegetation is consumed by herbivores, leaving the 
remaining 70%, around 230 million metric tons of 
C, to enter the decomposition cycle.2 However, 
frequent tidal flooding creates saline and anoxic 
conditions that slow decomposition, leading to a C 
storage rate that is 10-100 times greater than 
terrestrial ecosystems.3 Regardless of the harsh saline 

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Georgetown	Scientific	Research	Journal	
 
 
and anoxic conditions, microbes, such as bacteria 
and fungi, play an essential role in the degradation 
of smooth cordgrass detritus, which significantly 
alters C sequestration rates. Therefore, 
understanding how these organisms mitigate C–
cycling and how they are affected by climatic 
variation is essential for predicting how salt marsh 
habitats will respond to climate change. 

Salt marsh soils contain all three soil fungal 
types: symbiotrophs, pathotrophs, and saprotrophs. 
Symbiotrophs, particularly arbuscular mycorrhizal 
fungi (AMF), are extremely significant in salt marsh 
habitats due to their governance of biological 
processes.4 AMF communities provide nutrients to 
their host plant in exchange for photosynthate.5,6.7 It 
has been found that AMF communities are able to 
maintain ecosystem balance and mitigate C 
sequestration by modulating soil organic matter 
(SOM) decomposition and nutrient mobilization.4 
However, researchers have now provided evidence to 
suggest climate change is heavily impacting plant–
fungal interactions as well as bacterial–fungal 
interactions, both of which have a profound 
influence on C sinks and SOM decomposition. 
Rising sea levels due to increasing global surface 
temperatures are significantly impacting salt marsh 
elevation gradients, leading to the habitat 
fragmentation of Spartina patens, a key high–marsh 
grass. As elevation gradients decline, Spartina 
alterniflora, a foundational low–marsh grass species, 
is rapidly replacing S. patens.8 These geographical 
changes, as well as plant species zonation, have 
leverage over fungal communities consequently 
affecting decomposition rates.8   

Soil–fungi interactions are also being remodeled 
by excessive mineral and nutrient fertilization, a 
process called eutrophication. This is mostly due to 
anthropogenic runoff from industrial waste and 
agricultural fertilizers entering coastal habitats. 
AMF’s ability to acquire nutrients, decompose 
SOM, and interact with rhizosphere bacteria and its 

host plant change depending on soil chemistry and 
nutrient abundance.4, 9,10 Hence, anthropogenic 
factors that have increased global surface 
temperatures, sea level rise (SLR), and 
eutrophication have had profound effects on 
microbial interactions in salt marsh ecosystems.10 In 
this review, I will outline AMF’s role in salt marsh 
habitats as well as describe their predicted reactions 
to climate change. Understanding how climate 
change impacts soil communities will allow us to 
better predict the outcomes of worsening climate 
conditions and employ more effective conservation 
plans in these areas.  

2. Mycorrhizal fungi in salt marsh ecosystems 
Arbuscular mycorrhizae (AMF) are extremely 

important mitigators of soil organic matter (SOM). 
Fungal species that are known to form AMF 
associations are in the genera Acaulospora, 
Entrophospora, Gigaspora, Glomus, Clecerocytis, and 
Scutellospora.11 These symbionts penetrate within the 
host plant’s root cortical cells that form arbuscular 
sacs, allowing for nutrient exchange. Furthermore, 
mycorrhizae extend their hyphae into the soil 
creating mycelial networks that increase root surface 
area which improves nutrient acquisition of nitrogen 
(N), phosphorus (P), improves plant stability in soil, 
as well as bolsters plant pathogen resistance.10 In 
return, host plants provide mycorrhizae with 10-
20% of their net photosynthate (fixed C 
compounds).5,6,7  

One of the main influential factors in 
determining AMF association is environmental 
conditions and micro-climate. Anoxic conditions 
increase with soil depth, thereby limiting AMF 
communities to upper soil layers which contain 
burrowing organisms and fluctuating water tables 
that provide enough aeration to support mycorrhizal 
colonization.12 Hence, elevated grass species, S. 
patens, can host AMF communities and have root 
colonization rates between 52-68%.13,14 On the other 

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Georgetown	Scientific	Research	Journal	
 
 
hand, S. alterniflora, a low, submerged grass with 
relatively anoxic soils, has root colonization rates of 
0-9%.14,15,16  

Marsh elevation also impacts nutrient 
availability which alters trade–offs in mutualistic 
investment. In elevated areas, reduced tidal 
inundation causes unpredictable nutrient import, 
resulting in S. patens to rely on its AMF association 
to acquire sufficient amounts of N and P.17 

Conversely, low marsh grasses are constantly 
submerged, allowing S. alterniflora to have reliable 
sources of N and P. If nutrients are readily available, 
the benefit received from AMF association is low, 
but the association still remains energetically costly, 
making the symbiont more parasitic than 
mutualistic.12  

Additionally, studies have found that plant–
fungal associations and community composition 
differ based on the genetic specificity of both AMF 
and host plants. Van Der Heijden et al. (1998)18 was 
able to show that plants not only respond to the 
presence of AMF, but response varies amongst 
fungal taxa. This would suggest that AMF diversity 
and taxonomic specificity for certain plants have the 
ability to alter host plant responses.18 Other studies, 
like Koch et al (2006)19, further examined this 
phenomenon by observing plant responses in 
relation to intraspecific genetic diversity in isolates 
from one Glomus intraradices population. They 
found that AMF genetic diversity was able to affect 
plant species richness, growth, and productivity.19 

Intraspecific variation within a single plant species 
has also been shown to alter AMF community 
composition. Eppley et al. (2009)20 found that 
different sexes of salt grass Distichlis spicata (with 
both sexes having the same growth rate) had 
dissimilar AMF colonization rates, suggesting host 
plant intraspecific genetic variation can alter AMF 
diversity and abundance. With climate change 
shifting plant community composition in salt 
marshes (i.e., replacement of S. patens patches by S. 

alterniflora) then we should expect there to be drastic 
changes to AMF species richness as well as genetic 
diversity. This will likely have significant impacts on 
SOM decomposition leading to altered C 
sequestration rates. Additionally, below ground 
alterations to fungal communities due to climate 
change will likely alter above ground responses in 
host plants.   

3. Mycorrhizal fungi mitigating C–cycling and 
decomposition 

Because salt marshes have incredibly high net 
primary productivity and litter accumulation rates, 
AMF mitigation of saprotrophic bacteria and SOM 
decomposition is essential for C–cycling.2, 21 
Research has shown that AMF communities can 
either stimulate or stunt C sequestration, depending 
on environmental conditions. However, there is 
debate as to how new climate conditions will alter 
these processes and whether they will lead to soil C 
storage or C release. 

AMF stimulation of C sequestration can occur 
through soil aggregation, which is the binding 
together of micro–and macro–soil particles bound by 
cohesive forces or organic matter. Mycorrhizae 
promote soil aggregation by releasing binding 
agents, such as glomalin–related soil proteins that 
entangle soil macro–aggregates in dense hyphal 
networks.22 Once macro–aggregates are stabilized, 
micro–aggregates can form, allowing mycelial 
networks to physically protect C pools and increase 
C retention.22 This enmeshment also harbors a large 
portion of soil microbial biomass and accounts for a 
large portion of the SOM C pool.23 Improved 
nutrient acquisition due to AMF association can 
support higher amounts of above– and below–
ground plant biomass. Consequently, this increase in 
living and dead organic matter adds to the overall C 
sink. 24, 25 

The exact mechanism of AMF decomposition is 
not completely understood, but there are a few 

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Georgetown	Scientific	Research	Journal	
 
 
proposed mechanisms. It is thought that 
mycorrhizal communities excrete hydrolytic 
enzymes, such as cellulase, pectinase, and 
xyloglucanase.22 However, there is no direct evidence 
to support that these extracellular enzymes have 
saprotrophic abilities. Another more supported 
hypothesis is priming, which is AMF’s ability to 
manipulate rhizosphere microbial communities 
indirectly thereby affecting decompositional rates. 
Mycorrhizae emit labile C exudates into the soil 
which stimulate saprotrophic bacterial 
accumulation, SOM decomposition, and CO₂ 
release.24 This causes a release of ammonium (NH4

+) 
that AMF rapidly absorbs and transfers to its host 
plant.10, 21, 26 Mycorrhizal access to both host plant C 
and soil C may make AMF communities better 
rhizosphere competitors, allowing them to alter soil 
microbe composition.12 Changing microbial 
communities would likely lead to shifts in important 
functional groups such as nitrogen fixation. As a 
result, altering the diversity and abundance of 
organisms responsible for important functional 
processes would have ramifications on soil nutrient 
levels, plant productivity rates, and plant community 
structures.12   

4. Effects of ambient CO₂ levels and sea level rise                         
Recent anthropogenic activities have caused 

ambient CO₂ levels to rise significantly, which has 
coincided with higher-than-average global surface 
temperatures.27 This has led to an net decrease in 
total sea ice, which has ultimately caused global sea 
levels to rise 0.19 m from 1901 to 2010.28  

SLR will have profound consequences for salt 
marsh ecosystems. As described before, S. patens, a 
high elevation grass, and S. alterniflora, a low 
elevation grass, exist at limited elevation ranges 
relative to mean sea levels.29 Our current rate of SLR 
is estimated to be approximately 2.5mm per year. 
However, through peat accumulation and sediment 
aggregation, salt marshes gain elevation and can 

vertically increase 2.08 mm to 4.20 mm each year, 
allowing them to remain above rising sea levels.29 
Furthermore, elevated ambient CO₂ levels are 
shown to bolster soil accretion and elevation gain.30 
Elevated CO₂ levels amplify plant growth and 
photosynthate production, resulting in an increase in 
fixed C allocations to mycorrhizae, increasing root 
and fungal biomass.10 Under higher CO₂ levels, 
heightened plant growth demands stimulate 
additional C allocation to AMF communities in 
order to support nutrient mining. In terrestrial 
systems, elevated CO₂ levels (550-700 p.p.m.) 
caused AMF density to increase by 84%.31 These 
results support Langley et al. (2009)30 which found 
that root thickness increased to 4.9 mm per year 
under supplemental CO₂ levels compared to 0.7 mm 
per year under normal ambient CO₂ conditions. If 
SLR rates and marsh elevation gain remain 
unchanged, it is predicted that salt marsh ecosystems 
will be able to persist into the next century.29 

Although greater biomass and soil accretion 
rates may seem to negate the issue of SLR, these 
modified conditions may change the biological 
mechanisms that govern C sequestration and 
decomposition. For example, photosynthesis rates 
diminish with each marginal increase in CO₂ 
concentration, leading to changes in carbohydrate 
allocations to AMF communities.30 Research has 
shown that under elevated CO₂ levels, mycorrhizal 
communities exhibit amplified rates of SOM 
decomposition.10 Studies have also observed 
escalating N mobilization rates, suggesting that 
priming and decomposition rates also increased, 
leading to soil C losses.26, 10 Moreover, the 
assumption that marsh soil accretion rates neutralize 
flooding from SLR rests on the assumption that 
SLR rates will remain stagnant. However, with 
increasing ambient CO₂ concentrations, SLR rates 
are expected to increase, thereby surpassing the 
speed of soil accretion.28 Increasing litter 
accumulation will also alter soil chemistry which is 

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an important predictor of AMF’s relationships with 
plants and saprotrophic bacterial communities as 
well AMF’s regulation of decomposition rates. 

Habitat loss due to human development of 
shorelines also has added additional pressures on 
marsh habitats. In the past, salt marsh grasses have 
been able to escape the threats of SLR by retreating 
to and colonizing higher elevation areas; but the 
development of shorelines has obstructed salt marsh 

grass migration.29 Figure 1 shows plant composition 
in the Cedar and Nezera Islands, near the Long 
Island tidal wetlands, in 1974 (top) and 2008 
(bottom).32 The figure reveals that in 2008, there 
were more intense regions of fresh marsh 
development indicated by the darker pink. 
Moreover, Figure 1 displays the extreme 
fragmentation of S. patens (yellow) and the 
domination of S. alterniflora (green). This 
phenomenon has been experienced across many 
North American salt marshes and is not specific to 
this location. Since 1974, the presence of S. patens 
has decreased by 68%; in 1974 the Cedar and Nezera 
Islands consisted primarily of high marsh habitat 
(412.7 acres) but in 2008, only 11% of high marsh 
elevation remained (45.4 acres). It is important to 
restate that AMF associations are host–specific and 
non–resistant to the anoxic conditions of submerged 
marsh soils. Therefore, these changes would lead to 
reduced species richness and shifts in community 
composition of AMF,33 perhaps causing the 
extirpation of a particular taxon or a bottleneck effect 
on AMF genetic diversity.  

5. Effects of Eutrophication 
Eutrophication is characterized by excessive 

plant and algal growth due to fertilization of limiting 
growth factors such as N and P.34 Eutrophication 
occurs naturally, but anthropogenic activities, such 
as sewage runoff and agricultural activities, have 
amplified this effect.34 Fertilization of salt marshes 
greatly transfigures soil C:N:P stoichiometry 
affecting AMF–plant relationships. Numerous 
studies have found opposing results on how 
eutrophication affects AMF’s responses to grass 
zonation, microbial populations, decomposition, 
and C storage, making it difficult to predict how 
nutrient adjustments will alter AMF governed 
factors. 

Studies have predicted that anthropogenic 
addition of N and P will shift plant competitive 

Figure 1. Differences in marsh elevation in the Cedar & 
Nezera Islands near Long Island NY from 1974 (top) to 2008 
(bottom). 1974 displays a high percentage of high marsh 
habitat (412.7 ac) compared to 2008 (45.4 ac). Adapted from 
Cameron Engineering & Associates, 2015.32 

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dominance as nutrient addition alters nutrient 
foraging rates, fungal–plant relationships, and 
fungal–bacterial interactions.35 Experimental 
additions of N to S. patens and S. alterniflora patches 
displayed a reduction in below–ground root biomass, 
but an increase in above–ground biomass, suggesting 
that nutrient foraging was eased. 9, 36, 37 In addition to 
shifts in biomass production, other studies found 
that N addition caused spore biovolume and density 
of extraradical hyphae to decrease.10, 33, 38 These 
observations are consistent with the resource–ratio 
hypothesis, which states that more competitive and 
successful species will grow in habitats with lower 
resource levels. It also suggests that when limitations 
are eased, competition for resources will shift from 
below–ground to above–ground.39 For example, 
when N limitations below–ground are reduced, 
plants are likely to increase above–ground 
production to compete for light.35 Under typical 
conditions, AMF allows S. patens to be 
competitively dominant in a resource limited 
environment. However, as N limitations are eased, 
and above–ground growth is increased, S. alterniflora 
is more successful and energy–efficient.35 For S. 
patens, as soil nutrient levels increase, nutrient 
foraging services greatly diminish causing it to be 
more parasitic than mutualistic.35, 40 However, more 
recent studies found conflicting results in that N 
addition to salt marsh plots did not display signs of 
increased above–ground biomass.41 More research 
regarding eutrophication’s influence on above– and 
below–ground biomass allocation should be 
investigated. 

There is also debate about the effects of N 
addition on elevation gradients and decomposition 
rates in marsh habitats. Past studies have found that 
N addition promotes above–ground biomass and 
decomposition rates.9, 36 As N is added to soils, there 
is a subsequent release of CO₂, indicating there is an 
increase in microbial respiration.37 This is likely due 
to a reduction in bacterial–fungal competition. As 

AMF biomass diminishes with increasing N 
addition, competition for N between fungi and 
saprotrophic bacteria is eased, leading to an increase 
in bacterial populations and decompositional 
activities.42 Studies have found that, under these 
conditions, plants produced lower amounts of 
polyphenols, which regulate soil microbes that were 
likely linked to their overall observation of increased 
CO₂ soil emissions.1 Furthermore, it was observed 
that for every atom of nitrogen added to salt marsh 
soils, 6.1 moles of CO₂ were released.42 These 
changes in decomposition rates may negatively affect 
the soil accretion rates that protect salt marsh 
habitats from SLR. Yet, more recent studies report 
that N addition to salt marsh plots did not display 
signs of increased above–ground biomass.41 While 
this study did find that fertilization increased 
respiration rates, decomposition rates remained 
stagnant.41 

N enrichment alone is not the sole determinant 
in salt marsh grass root biomass.9 In P–limited soils, 
N enrichment increases AMF root colonization, 
spore biovolume, and density of extraradical 
hyphae.10 N enrichment to P–limited soils increases 
plant biomass as it shifts soil N:P ratios leading to P 
limitations.10 But, most saline marine systems are N–
limited and P–rich meaning N addition would cause 
AMF associations to be unbeneficial because of a 
decrease in competition with soil mircobes.36 Future 
research is needed to accurately predict how 
eutrophication will affect these ecosystems long term 
and elucidate conflicting results.  

6. Conclusion 

AMF fungal communities are foundational 
species in salt marsh habitats, moderating C 
sequestration, plant zonation, nutrient acquisition, 
soil stability, microbial populations, and 
decomposition. Recent anthropogenic activities 
have elevated ambient CO₂ levels, leading to SLR 
which poses risks to plant community structure as 

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Georgetown	Scientific	Research	Journal	
 
 
well as AMF processes.10, 26, 27 SLR is predicted to 
diminish AMF abundance, increase decomposition 
and diminish C pools.10 Furthermore, 
anthropogenic N fertilization of marsh soils has led 
to decreased root and fungal biomass causing 
competition to shift from below–ground to above–
ground.39 Additionally, it is believed that reductions 
in AMF biomass and increased N availability will 
increase microbial respiration and decomposition 
rates. However, there is still debate about how 
decomposition rates are impacted by N 
fertilization.41 Additional research should be 
conducted to elucidate the exact mechanisms of 
AMF driven decomposition, which would allow us 
to create a clearer picture of how decompositional 
processes function in salt marsh habitats. Moreover, 
future studies should observe how climate change 
conditions such as elevated CO2 levels, SLR, and 
eutrophication will impact AMF and plant 
community structures, decompositional rates, and C 
storage rates in order to create more consistent 
results within the current literature as well as provide 
more concrete directions for conservation efforts.  

Acknowledgments 
This work was supported by Georgetown’s 

Center for Research and Fellowships – Kalorama 
Fellowship.  

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23




	Table of Contents
	Letter From the Editors
	Letter From the Editor-in-Chief Emerita
	Energy Security in Poland: Where the Energy Sector Falls Short and Where It Can Go
	Impacts of Climate Change on Mycorrhizal Fungi in Salt Marsh Habitats
	CRISPR and COVID-19: Lessons Learned to Prepare for the Next Pathogen
	The Impacts on Well-being of Undergraduate College Students Serving in a Resident Assistant Role
	About the Authors
	Acknowledgements



