Otter Point Creek
Summer Research Proposal
Name: EXAMPLE
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Title:
The effect of high temperature on eelgrass (Zostera marina) and on the wasting disease
pathogen (Labyrinthula zosterae)
Background research:
Submerged aquatic vegetation (SAV), such as Zostera marina (eelgrass), is an essential
part of such aquatic environments. Eelgrass is the only true seagrass present in the Chesapeake
Bay and dominates the total submerged aquatic vegetation (SAV) biomass in winter, spring, and
summer (Moore et al., 2000).
In the 1930s, eelgrass populations along the Atlantic coasts in North America and
Europe, including those in the Chesapeake Bay (Moore, 2009), were simultaneously diminished
due to an outbreak of wasting disease resulting in widespread decline and in some areas
extinction of eelgrass (Den Hartog, 1996). Labyrinthula zosterae (the wasting disease
pathogen) has been identified as the pathogenic microorganism that likely caused the wasting
disease epidemic along the Atlantic coasts during this time (Muehlstein et al., 1988, 1991;
Raghukumar, 2002).
The genus, Labyrinthula, consists of two strains, pathogenic and non-pathogenic, which
are distributed in marine and estuarine environments worldwide (Porter, 1990; Short et al., 1987;
Muehlstein et al., 1988; Vergeer and den Hartog, 1993; Blakesey et al., 2002; Raghukumar,
2002). Labyrinthula is associated with organic detritus assisting in decomposition and with
marine algae and vascular plants having parasitic roles (Porter, 1990; Raghukumar, 2002).
Pathogenic Labyrinthula specifically grows as either an irregular, clumped formation of spindle
cells or as a closely packed colony of cells (Young, 1943; Muehlstein et al., 1991). Labyrinthula
has very different growth patterns in liquid culture as compared to agar (Muehlstein et al., 1991).
Muehlstein et al. (1991) found that in liquid cultures an intricate branching pattern composed of
narrower spindle-shaped cells forms fewer a sheet-like patterns, whereas on agar cells form
dense colonies.
There is little research on the impact of abiotic factors on Labyrinthula growth due to the
trouble associated with accurately measuring growth (Martin et al., 2009). The optimal
temperature range for growth of Labyrinthula is between 15C and 30C (Young, 1943; Sykes
and Porter, 1973). However, there is little growth above 40C or below 5C (Sykes and Porter,
1973). Yet, the extreme temperature ranges in which Labyrinthula can grow and survive in
remain unclear.
Temperature is an environmental stressor that has contributed to the decline in eelgrass
biomass without exposure to the wasting disease pathogen. Eelgrass is tolerant of temperatures
ranging from 0C to 35C (Biebel et al., 1971); however, Rasmussen (1977) suggests that at
25°C to 30°C, plants become damaged or can die. Increased frequency and duration of high
temperatures (exceeding 30C) have been shown to contribute to the significant decrease in
shoot density of eelgrass (Biebel et al., 1971), specifically in the Chesapeake Bay region (Moore
and Jarvis, 2008). Marsh et al. (1986) found that even short-term exposure to high temperatures
(above 30°C) can cause eelgrass to have limited photosynthesis and biomass. In the Chesapeake
Bay, eelgrass grows well during cooler months and dies back during warmer summer months
(Moore et al., 2006). During summer months, the rate of leaf production decreases while the rate
of senescence increases, factors which both contribute to the overall decline in the number of
leaves per shoot (Moore et al., 1996) and total SAV biomass of eelgrass (Moore et al., 2000).
Overall higher temperatures decrease eelgrass density, root production, and initiation of new
leaves (Bintz et al., 2003; Touchette et al., 2003), and eelgrass leaf lifespan (Hosokawa et al,
2009).
Long-term declines and extinction in eelgrass can also be contributed to L. zosterae,
which likely caused the wasting disease epidemic along the Atlantic coasts during the 1930s
(Muehlstein et al., 1988, 1991; Den Hartog, 1996; Raghukumar, 2002). Labyrinthula zosterae
typically infects older, outer eelgrass leaves initially and then moves to the younger, inner leaves
(Burdick et al., 1993; Hily et al., 2002). Eelgrass shows symptoms of wasting disease by the
presence of dark brown or black patches on its leaves (Muehlstein et al., 1991; Raghukumar,
2002). Environmental stressors that prompted the wasting disease outbreak of the 1930s are still
not fully understood (Moore, 2009). Stevens (1936) hypothesized that the combination of
Labyrinthula and increased coastal water temperatures worked together to damage the eelgrass
population in the 1930s. There were twice as many days with water temperatures above 20°C
from 1932 to 1951 than the previous fifteen years (Rasmussen, 1977). The increase in summer
temperatures, which began in the 1930s, probably weakened eelgrass causing the plants to be
less resistant to ever present pathogens (Rasmussen, 1977).
Hypotheses:
Biebel et al. (1971) found that eelgrass is tolerant of temperatures ranging from 0C to
35C. Moreover, Moore and Jarvis (2008) found that increased frequency and duration in high
temperatures (greater than 30°C) contributed to significant shoot dieback. I hypothesize that
eelgrass growth rate will have an inverse relationship with increasing temperature and that there
is a physiological tolerance level in which eelgrass can grow and after that threshold the growth
rate will rapidly decline. However, other environmental factors have been found to be associated
with increased temperatures, such as nutrient enrichment (Burkholder et al., 1992; Bintz et al.,
2003; Touchette et al., 2003), low levels of dissolved oxygen (DO), and low pH (Touchette et al.,
2003). I hypothesize that an increase in temperature alone will have detrimental effects on
eelgrass growth rates.
For Labyrinthula, Sykes and Porter (1973) found that the optimal temperature range for
growth is between 15C and 30C with little growth above 40C or below 5C. I hypothesize
that L. zosterae will be able to withstand higher temperatures than eelgrass. During the 1930s
epidemic, a combination of environmental stressors, including high temperatures, were probably
beneficial to the growth of the wasting disease pathogen and perhaps contributed to the decline
in eelgrass populations
Methods/procedures:
I will conduct two experiments to study the influence of high temperatures on eelgrass
and Labyrinthula zosterae. In the first, I will grow eelgrass alone in mesocosm tanks at varying
temperatures typical of those during warmer months of the year. In the second, I will grow L.
zosterae in liquid culture at varying temperatures to determine the impact of increased
temperatures on the growth of L. zosterae alone.
Objectives/goals of study:
I hope to provide evidence for the importance of increasing overall conservation efforts
of marine ecosystems and coastal shallow water habitats in order to prevent a marine disease
outbreak, such as the wasting disease epidemic of the 1930s.
References:
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Biology 8: 48-56.
Bintz J.C., S.W. Nixon, B.A. Buckley, and S.L. Granger. 2003. Impacts of temperature and
nutrients on coastal lagoon plant communities. Estuaries 26: 765-776.
Blakesley B.A., D.M. Berns, M.F. Mercello, M.O. Hall, J. Hyniova. 2002. The dynamics and
distribution of the slime mold Labyrinthula sp. and its potential impacts on
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