Jonathan P. Zehr - Marine Nitrogen Fixation

Cover illustration: Photograph of the Bruny Island tombolo, Tasmania, Australia. Photo by Jonathan P. Zehr, with thanks to Lev Bodrossy.

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The oceans, with a global surface area of >70% and an average depth of over 3500 m, are the largest habitat for living organisms on Earth, and microbial activity there plays a major role in the global N cycle. In the late nineteenth and early part of the twentieth centuries, N2 fixation by marine microbes was little studied and was thought to be a minor contribution to the Earths N cycle. In early attempts in the late 1800s at assessing the biomass and concentration of the cyanobacterium Trichodesmiumn, before the advent of flowmeters, and before it was known to be able to fix N2, nets were lowered vertically with the mouth closed to 200 m, then pulled up through the water column with the mouth open. Later, in the 1970s, mechanical Clark-Bumpus opening/closing nets were towed at selected depths to assess vertical population structure. More recently, the populations can be assessed underway using digital optical counting devices. In the early 1960s, Richard Dugdale and coworkers, who introduced enriched 15N2 as a tracer for marine N2 fixation, demonstrated that Trichodesmium was able to fix N2. There were doubters however because Trichodesmium didnt possess heterocysts, anoxic cells found in some cyanobacteria that were then known to be the sites of N2 fixation, and they labeled the observation as Nitrogen Fiction. With the advent of rapid sea-going gas chromatographic methods for indirectly measuring nitrogenase activity in the early 1970s, it was shown that N2 fixation in the global ocean was a significant part of the marine N cycle. This led to updated concepts of marine N cycling, diazotrophic organisms, and N2 fixation rates by several contributing authors to the book Nitrogen in the Marine Environment in 1983 (Carpenter and Capone 1983) and in a subsequent edition in 2008 (Capone et al. 2008).

This volume, written by Jon Zehr and Doug Capone, colleagues in the quest to put together new discoveries on the marine N cycle and how it functions biologically, is a beautiful and rich compendium of the state of knowledge of marine N2 fixation today. The chapters start with an introduction and review of the fundamentals of the process of N2 fixation and the history of marine N2 fixation research. Subsequent chapters describe habitats and organisms responsible for N2 fixation and the rates of N2 fixation in different ecosystems. The factors controlling N2 fixation rates, such as water temperature, nutrient availability, and other environmental controls, are presented along with a chapter on biogeography and rates of N2 fixation in different ocean basins (Chap. , entitled Marine N2 Fixation, Global Change, and the Future. The authors point out the very complex changes occurring in regard to CO2 concentrations, pH, temperature, and Aeolian dust input on a variety of diazotrophic species and call for more research on this important topic. I think this book belongs on the bookshelf of anyone interested in ocean biogeochemistry and marine ecosystem biology.

Throughout the entire book, the authors provide richly colored figures to visually convey complex information on global N processes. The marine community owes a major Thank You to Jon and Doug for this detailed, expertly written, and timely volume.