Working with Ron Benner and solving salt marsh mysteries

Ron Benner (l), Marilyn, Matt McCarthy(r), Sea Ranch, California 2018

          

My involvement in the marine science community in the 1980s encouraged me to jump into studying the fate of organic matter in the natural world —the slow breakdown of living biomass as it transforms into “goo” that ultimately may form petroleum. John Hedges of the University of Washington and Ron Benner, then a postdoc at the University of Georgia, led this research area. John Hedges, a former Geophysical Lab postdoc and Univ. of Texas Marine Science graduate, was a formidable geochemist known the world over for his careful work. At the Geophysical Lab, his reputation was colored by the stories Tom Hoering told about him. John didn’t learn much chemistry in the Port Aransas Marine Lab apparently, before coming to the Carnegie after earning his Ph.D. In the 1970s, gas chromatograph instruments cooled their furnace boxes by automatically lifting up the top of the instrument and allowing hot air to escape. Hedges rested a full cup of coffee on the top of the instrument one morning, then left the lab to talk to a colleague in his office. As luck would have it, his analytical run ended; the top of the gas chromatograph rose up; and there went his entire cup of coffee drenching the inner workings of the instrument. A rookie mistake, and one that he never outlived.

Ron Benner, 1988

         I connected with Ron Benner first by telephone regarding a potential collaboration with a graduate student from Univ. of Georgia. At the end of one conversation, he remarked, “I expect to publish the data from this work,” in a tone that implied that I might be a dilettante and not be serious! I glared at the phone and icily replied that of course, I expected to publish. When we finally met in person in 1986, Ron turned out to be a friendly, generous fellow. A tall, imposing, and ruggedly handsome man (looking remotely like the actor Omar Sharif), Ron has a way of pausing for a few seconds before answering a question. His answers come in full documents with paragraph structure and complete sentences. He’s known widely for being a tough scientist as well as a fun-loving person you’d like to have on an oceanographic expedition. We remain good friends and colleagues to this day.

         At that time (1985), one of the most contentious theories involved the role of Spartina alterniflora, a C₄ grass that dominates salt marshes in North America, for sustaining the growth of important animal species living in highly productive marsh ecosystems. Salt marshes ring the edges of bays and estuaries from Massachusetts to northern Florida. Spartina is the dominant plant that you see when you drive from the mainland in places like New Jersey or North Carolina to the outer beach islands where most tourists are headed. To me, salt marshes are some of the most beautiful places in North America for their brilliant greenness next to sparkling salty waters. Mussels, clams, crabs, and fish abound there. It would make sense to anyone looking at the ecosystem that the Spartina plants should be important contributors to their well-being and be a part of their diet.

         Carbon isotopes of organic matter in the water, Spartina, sediments, phytoplankton, and invertebrates (e.g. mussels and crabs) were measured from Woods Hole, Massachusetts, all the way down to Georgia. Every study of the isotope patterns found that the carbon isotopes in the sediments and animals in the bays had carbon isotope values closer those of phytoplankton rather than those patterns measured in Spartina.  Many concluded that Spartina was just a pretty plant and not important. The interpretation of the carbon isotope data had important implications because salt marshes were threatened habitats. The idea that salt marshes were not important contributors to food web dynamics did not make sense to me. If estuarine scientists could prove that these wetlands were nurseries and energy sources for commercial seafood, they were more likely to be protected from development.

Stable isotopes solve mystery of salt marshes, 1987

         My involvement with Spartina ecosystems began in 1985. I was contacted by Kent Sprague, a student from the University of Georgia who was studying estuarine sediment deposits from thousand year old salt marsh sediments. He came to the Geophysical Laboratory with sediment cores composed of peat—a sediment type built by the remains of plants. From these cores, he picked out fragments of nearly intact Spartina that were hundreds to thousands of years old. We measured the carbon isotope patterns of these plant fragments and found values very similar to modern bulk sediment. We used pyrolysis-gas chromatograph-mass spectrometry coupled to stable isotope analysis to provide further information on the structural and chemical composition of organic matter sources from these marshes (Fogel et al., 1989). Then, I figured out a way to estimate the sources of organic matter—either from Spartina or phytoplankton—to ancient peat sediments. Sedimentary material and plant fragments plotted firmly in the space showing that Spartina contributed roughly 50% of the organic matter in sediments. Discovering why the sediments and plant fragments from Spartina had more very different values from modern Spartina involved more work.

         Ron Benner was conducting litterbag experiments with Spartina. Dried plants are weighed, sewn into nylon or polypropylene bags with defined mesh sizes, and then incubated in the environment either under aerobic (with oxygen) or anaerobic (without oxygen) conditions.  Bags are periodically (e.g. weekly or monthly) removed from the environment, and the remaining plants in the litterbag are dried and subjected to various types of analyses. Kent encouraged Ron to write to me about analyzing the Spartina in the litterbags for carbon and nitrogen isotopic compositions. Using the bulk Spartina material, Benner used chemical methods to separate the major plant structural biochemicals: lignin, cellulose, and hemicellulose.  In 1985, Ron wrote to me:

“I welcome your suggestions and I believe that the carbon isotope measurements on the chemically fractionated material is a good one. Am I correct to assume that the carbon isotope ratios among these particular fractions (cellulose, hemicellulose, lignin) will probably be indistinguishable?”

         Samples from an 18-month experiment were brought up to the lab by Kent Sprague and analyzed using laborious, sealed-tube combustion methods. The results were striking (Benner et al., 1987). Although bulk carbon isotope pattern in Spartina had one value, the biochemical fractions were very different—not at all what Benner had predicted. Cellulose and hemicellulose had carbon isotope patterns with slightly more ¹³C in them, while the lignin (the material that makes wood hard) isotope composition had considerably less ¹³C. Uncharacterized material, suberins and other insoluble material, had carbon isotope compositions close to those in lignin. As the relative proportion of lignin relative to other compounds increased in the litterbags from 10 to 15%, the carbon isotopes of the remaining Spartina showed small, but significant changes in the direction of what we had measured in sediments and old plant fragments. The change in carbon isotope signature demonstrated that as the plant decayed, labile celluloses were preferentially decomposed leaving more isotopically different lignin.

         Ron, Kent, and I widened the study to measure biochemical fractions from eight other species of plants (Benner et al., 1987). In all cases, the carbon isotope signatures of lignin showed that same depletion in ¹³C relative to bulk plant material. Carbon isotopes of the cellulose and hemicellulose fractions from the eight plants had similar patterns to what we measured in Spartina. This paper is my most highly cited publication. It was rejected first from the journal Science because a reviewer was not certain our results could be extrapolated to other plants and ecosystems. The work was then published in Nature and has survived the test of time. It has become a classic.

         Subsequent studies with compound specific isotope analysis of individual carbohydrates (Teece and Fogel, 2006) showed that the major 6-carbon sugars in celluloses, glucose, galactose, and mannose, have quite variable carbon isotope values. In higher plants, the synthesis of these sugars and their translocation to wood, stems or rhizome is probably associated with additional isotopic fractionation. Small carbohydrates (i.e. sugars) are the most biosynthetically-active and labile molecules in an organism and in the environment. Variation in their carbon isotope signatures compound specific level reflects this dynamic nature.

         Ron Benner and I after more than 30 years of working together on various projects are both entering the realm of planned retirement, trying to figure out how to slowly unwind and unravel ourselves from a life long career that we’ve loved. His advice to me has been to think about the new life ahead, the new positive challenges, and to embrace the good times we had during our scientific journeys.