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.