Motivation
Research Questions
Approach
Research Findings
Motivation
Organisms in nature must respond to a wide array of environmental conditions in order to survive. Changes in temperature, salinity, light, and food supply--to name just a few--can cause changes in how a plant or animal feeds, reproduces, or can even cause its death. Physiological effects on the individual can potentially translate into changes in populations and entire communities. We are investigating how marine organisms cope at a molecular, cellular, and whole-organism level to environmental stress. Information about how organisms respond physiologically to their environment will help us understand how organisms may respond to environmental change, including global climatic change.
Research Questions
- How do intertidal and subtidal organisms respond physiologically to stress?
- How do nearshore oceanographic conditions affect organismal physiology?
- How does temperature stress affect species interactions?
- Do sub-lethal physiological stresses have community-wide impacts?
Approach
PISCO scientists are using three main approaches to answer these questions.
RNA:DNA measurements
This measurement indicates whether an organism is growing at a
given time. Most animals have a fairly constant amount of DNA
(the "blueprints" for growth), and only increase the amount of
RNA (the "construction sites" for growth within the cell) when
growing. Therefore, a high ratio of RNA to DNA indicates that a
lot of "construction" is going on and the animal is growing. By
using this technique, researchers have shown that intertidal
mussels will respond very quickly at the cellular level to an
increase in their food supply caused by a phytoplankton bloom.
PISCO scientists are currently monitoring the RNA:DNA levels of
mussels all along the Pacific Coast.
Heat shock proteins
When organisms become hot or otherwise stressed, the proteins in
their cells become damaged. Damaged proteins cause harm by
sticking to healthy proteins and interfering with their function.
Heat shock proteins (HSPs) are a form of "chaperone" protein that
binds to damaged proteins and prevents them from improper
interactions with other proteins. By measuring the amount of HSP
within the cells of organisms, investigators hope to reveal
patterns of how much environmental stress organisms experience and
how they are using cellular mechanisms to cope with stress. Many
questions remain about how production of HSPs varies between
individuals, local conditions, and sites. For example, do mussels
in the high intertidal express more heat shock protein than those
in the low? Can we use HSPs to characterize how stressful a site
is in comparison to other sites? How much does the amount of heat
shock protein change from one individual to another? Currently, we
are conducting a study at two sites in Oregon--Boiler Bay and
Strawberry Hill--Oregon on California mussels
(Mytilus californianus) to help answer
these questions.
Carbon:Nitrogen
Plants require many nutrients to grow, reproduce, and survive.
Often the growth of plants is limited by the amount of certain
nutrients such as nitrogen and phosphorous. Past research indicates
that intertidal seaweeds can be nutrient limited. Scientists can
examine the nutrient status of seaweeds and other plants by
quantifying the amount of nitrogen (N) stored within their tissues.
Nitrogen is commonly measured in comparison to the amount of carbon
(C) in the tissues and expressed as a ratio, C:N. We are using the
measurement of C:N to examine how the feather boa kelp
(Egregia menziesii) responds to changes
in oceanographic conditions along
the Oregon and California coastlines. This study will give us
information on how ocean conditions affect primary productivity on
rocky shores.
Research Findings
RNA:DNA ratios
PISCO Research Fellow Elizabeth Dahlhoff has led our efforts to uncover the first evidence of a functional relationship between a physiological index of organismal condition and large-scale oceanographic patterns. Monthly samples of mussels taken in 1999 and 2000 from Oregon through southern California show that RNA:DNA ratios vary strikingly, both between the major oceanographic regions (north of Cape Blanco, Cape Blanco to Point Conception, and south of Point Conception), and within regions. Variation in RNA:DNA ratios is closely related to mussel growth, which depends on the concentration of food (phytoplankton and detritus) in the waters of the inner shelf. Some evidence suggests that south of Point Conception, mussel growth may also depend on water temperature. These findings indicated that
RNA:DNA ratios can be a powerful tool to measure the recent
nutritional state in marine organisms. To further hone this tool,
we initiated laboratory experiments to isolate the effects of
food on RNA:DNA ratios, while controlling other factors such as
temperature.
Another advance in the use of the RNA:DNA index involves the development of microplate technology, which dramatically improves the efficiency of processing samples. Microplate technology gives ecologists the flexibility to collect and analyze many more samples, which improves our understanding of fine-scale spatial and temporal variation in growth potential.
Heat shock protein (Hsp) responses
Scientists expect thermal stress to increase in marine communities as the Earth warms. The synthesis and activities of heat-shock proteins that respond to heat stress may represent a significant energy cost to an organism, directing energy away from growth and towards repair. PISCO scientists are leading the way in studying the response of ecologically important species such as mussels, whelks, limpets, grazing snails and sea stars to thermal stress, at local to geographical scales. Building on earlier studies at OSU, former Postdoctoral Researcher Patricia Halpin, Research Fellow Gretchen Hofmann and others have demonstrated that both mussel and limpet heat shock protein (Hsp 70) levels increase with height on the shore, but that responses differ between sites. While we continue to explore variation in heat shock response to stressful conditions in space and time, results thus far indicate that this response will be a valuable tool in understanding and predicting how marine communities will respond to global warming.
In a specific application of this approach, Postdoctoral Research
Lars Tomanek and other PISCO scientists compared snails of the
genus Tegula
and found that subtidal species are incapable of
synthesizing proteins at the highest temperatures encountered by
intertidal congeners. Thus, differences in the thermal tolerance
of protein synthesis may restrict the vertical distributions of
subtidal species. Moreover, unlike their intertidal relatives,
subtidal species of Tegula
exposed to a thermal stress typical of
the mid-intertidal zone are incapable of completing the heat-shock
response during the subsequent high tide period, to ready them for
the next period of heat stress during low tide. Although
intertidal species are more capable than subtidal species of
coping with heat stress-as indexed by their higher lethal
temperatures and abilities to mount a heat-shock response
rapidly-these species face higher energy costs from heat stress
due to the need to devote energy to repair and replacement of
heat-damaged proteins. These heightened costs of living in the
intertidal zone may be important in restricting the upper vertical
limits of intertidal species. Reduced times for feeding (shorter
periods of immersion) and higher costs for protein synthesis could
limit the vertical range of intertidal organisms. On-going field
studies of growth rates and heat-shock protein synthesis may
provide additional insights into this important issue.
Measuring stress using gene chips
PISCO has pioneered the use of a DNA microarray ("gene chip") to evaluate the effects of short-term changes in the environment on diverse physiological systems. Our work is the first exploitation of DNA microarray technology for the study of environmental physiology in an estuarine fish, the longjaw mudsucker (Gillichthys mirabilis). Dramatic changes in gene expression occurred in response to environmental change. Notably, stress led to reductions in expression of genes associated with protein synthesis and cell growth. The promise of this technique for evaluating the influences of environmental stresses, such as ambient oxygen availability and salinity encourages our continuing work using DNA microarrays with "non-model" species, those for which genomic information does not yet exist.
By Patricia Halpin, Former Postdoctoral Researcher, and Renee Davis-Born, and Lydia Bergen, Policy Coordinators