During my
Ph.D. studies, fellow graduate student Brian Fry (the son of isotope chemist,
Arthur Fry) worked with Pat Parker showing that “you are what you eat” in an
elegant grasshopper and plant community study (Fry et al., 1978). This study
included detailed diet measurements and mass balance equations using the
mixtures of C3 and C4 plants
found in his study area. Brian’s paper remains a classic. At the same time,
Michael DeNiro and Sam Epstein were growing mice in the laboratory and finding
similar results (DeNiro and Epstein, 1977). In 1980, I was intrigued by these
studies and set out to determine if hydrogen isotopes could also be used as a
tracer for animal diet. My study included a laboratory-based experiment with
mice, as well as with marine snails and their potential algal food sources from
a natural environment study on the intertidal coast of Maine (Estep and
Dabrowski, 1981). Essentially, I determined that the primary source of hydrogen
in the organic tissues of animals originated from their food, rather than their
drinking water. The paper, published in Science, attracted the wrath of
DeNiro and Epstein, who submitted a technical comment to Science arguing that
my findings were invalid owing to hydrogen exchange (DeNiro and Epstein, 1981).
For a quiet, shy young woman from New
Jersey and coastal Texas, being challenged by Caltech folks was unsettling. Tom
Hoering supported me in writing my rebuttal: “At present, there is some
uncertainty about whether the isotopic composition of the hydrogen in prey and
predators can be used to follow food chains, but similar criticism may also be
applied to the use of carbon or nitrogen isotopes in analogous studies” (Estep,
1981). DeNiro and Epstein’s technical comment included data on hydrogen
exchange experiments with mouse tissue that had been freeze-dried, ground,
steamed at 100°C, then analyzed. It was no surprise that their experiment showed
considerable isotopic exchange with the hydrogen in steam. I argued that
because tissues were treated carefully and at room temperature in my studies,
exchange was not the major controlling factor in the hydrogen isotopic
composition of animal tissues. With time, it has been shown that about 20-30%
of the hydrogen in animal tissues comes from drinking water---not by random
exchange, but by direct incorporation. I was correct all along.
Interestingly, although my original
paper was the first to show the utility of hydrogen for tracing diets, it has
only been cited about 129 times since it was published in 1981. This work had
little impact until almost twenty years later when the methods for measuring δ2H became easier using a thermo-chemolysis procedure that is fully
automated. Although methodology was an important factor in delaying the impact
for my work, another reason why this paper was largely ignored by the
ecological community studying animal migration is that they did not, and still
don’t, care about hydrogen in food. Those isotope biogeochemists in the know
realize that food reflects local precipitation, as water is the only source of
hydrogen available to primary producers to build organic tissues. Broadening
the ecological community’s perspective on the influence of hydrogen from all of
the major biochemical sources available to an animal has been a challenge.
During my two-year postdoctoral
fellowship funded by the Carnegie Corporation of New York, I started to apply
for permanent positions in academia. I had produced solid work on the
enzymology and isotope fractionation by Rubisco. I had transitioned to a new
isotope system at the Geophysical Lab and showed I had become independent from
my three Texas advisors and from Tom Hoering. My undergraduate degree was in
Biology and my Ph.D. was in Botany (Marine Science), and now I was completing a
postdoc at a prestigious earth science laboratory. With several papers and
presentations under my belt, I began sending out applications for faculty
positions. At that time (1978-1979), it was not an advantage to be a woman in
almost any scientific discipline. Today, women scientists with talent often
compete well for positions. Not so in the 1970s. Further, I didn’t fit easily
into either a regular biology department or an earth science department. Marine
science positions were relatively rare, and if they did exist, many did not
have the type of analytical support (i.e. mass spectrometers) that I needed to
do my work. At that time, it wasn’t clear to me that being an interdisciplinary
scientist could have its drawbacks.
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