Zehr, J. P., and Capone, D. G. 2021. Marine Nitrogen Fixation. Springer Nature Switzerland: Cham, ISBN 978-3-030-67745-9. 10.1007/978-3-030-67746-6. Suggested retail price $119.99 hardcover, $89.00 ebook

Abstract

Biological nitrogen (N₂) fixation is critically important in the global nitrogen cycle because it is largely responsible for balancing the losses of fixed (bio-available) N by such processes as microbial denitrification. Although atmospheric lightning is a well-known natural mechanism for converting N₂ to N oxides that eventually become nitrate ions, this is a small global source (~ 5 Tg yr⁻¹) compared to other sources of fixation. For example, anthropogenic combustion of fossil fuels produces roughly six times as much fixed N as lightning (~ 30 Tg yr⁻¹). In contrast, biological fixation from all sources, including natural terrestrial and aquatic systems and legumes used in agriculture, amounts to about 260 Tg yr⁻¹ globally (Fowler et al. 2013). Appreciation for the general role of legumes in fertilizing soil has been understood at least since Roman times, and the mechanism for this, N₂ fixation, has been recognized since the mid-1800s.

Given that the oceans cover 70% of the Earth's surface, their role in fixing N₂ has interested marine scientists and biogeochemists for many decades. Although microbiologists documented the presence of N₂-fixing microbes in the oceans in the early 1900s, actual measurement of marine N₂ fixation using ¹⁵N tracer technology was not achieved until the early 1960s. Since then, a substantial quantity of research on marine N₂ fixation has occurred. The book that is the subject of this review provides a thorough and first-rate review of our state of knowledge on this topic.

The book's eight main chapters cover much more than marine N₂ fixation, including much on its biochemistry and on methods to study it in various environments. Chapter 1 is a nicely written general introduction to the nitrogen cycle, and Chapter 10 is a summary of the main points made in preceding chapters. The book concludes with 17 still-unanswered questions that the authors provide as a guide for continuing research on this topic.

The real heart of the book begins in Chapter 2, which is an extensive treatment of the fundamentals of N₂ fixation. Topics include: its microbiology—who does it, which is shown to be an eclectic range of prokaryotic organisms known as diazotrophs; the complicated nature of nitrogenase enzymes and nonstructural genes required to make the process work; the actual reduction reaction of N₂ to 2NH₃; and cellular mechanisms used to regulate formation of nitrogenase and related proteins. The majority of the 84 references in this well-documented chapter are recent (past 20 years), and the treatment is up-to-date. I found the section on regulation of nitrogenase formation revelatory and particularly interesting. A topic not covered in detail, however, is the actual structure of the active site for N₂ reduction, which is now understood at the atomic structure level. Because the nature of the intermediates between N₂ and NH₃ remains controversial, however, ab initio molecular dynamics modeling of transition state conditions is still only a future prospect (Einsle and Rees 2020).

Experimentalists will find Chapter 5, which describes methods used to study marine N₂ fixation, particularly useful. Topics range from laboratory techniques, such as gene analysis and ways to cultivate diazotrophs, to environmental observations, such as remote sensing, use of biogeochemical proxies, and mathematical modeling. Useful practical information is included on methods to measure rates of fixation, including ¹⁵N tracers, acetylene reduction, and the more recent technique of measuring H₂ production (H₂ is also a product of N₂ reduction by nitrogenase). Although I was familiar with the ¹⁵N and acetylene reduction methods, most of the content of this chapter was eye-opening to me, as I think it will be to readers who are not already experts on N₂ fixation.

As a process-oriented biogeochemist, I was especially interested in learning more about the rates of N₂ fixation in the oceans. Chapters 3, 4, and 6–8 cover various aspects on the occurrence of N₂ fixers and their activity in marine environments, including estuaries, open ocean surface waters, deep hypoxic zones, sediments, and coral reefs. Although there is much qualitative information on factors that affect N₂-fixing activity (Chapter 6), I was not able to find information expressed in conventional rate units (e.g., mass fixed per volume or per biomass per unit time). Such information would be useful in understanding how significant fixation is in supplying the nitrogen needs for primary production.

Instead, Chapter 8 focuses on global contributions of marine N₂ fixation (in Tg yr⁻¹), as synthesized from measured data. Of course, that is useful for global budgeting purposes, but it is not directly helpful in answering shorter-term ecological questions. As the authors rightly observe, scaling up individual measurements to global oceanic contributions of N₂ fixation is exceedingly difficult because of the huge temporal and spatial variability of fixation in the oceans relative to the availability of measured rates. Early estimates of global contributions, based on observed fixation rates by Trichodesmium and estimates of their mass abundance across the oceans, were far smaller than estimates of oceanic N losses by denitrification and ammonium oxidation (annamox), leading to an apparently imbalanced budget. According to the authors, such an imbalance is improbable over long time-scales. More recent estimates of oceanic N₂ fixation described in Chapter 8 rely on indirect basin-scale biogeochemical and modeling approaches. One approach is based on variations in ¹⁵N isotope natural abundance caused by the slight discrimination against using the heavier isotope (¹⁵N) that occurs in most N-cycle processes except for N₂ fixation. Another approach analyzes basin-scale deviations from the Redfield N : P ratio; net denitrification leads to lower N : P ratios, and net fixation does the opposite. Such indirect estimates yield much larger contributions (~ 200 Tg yr⁻¹, according to the authors) than upscaling measured rates, apparently balancing the budget. As the authors point out, however, such estimates are based on numerous assumptions. Consequently, much uncertainty remains in the contributions of marine N₂ fixation to the global N budget.

Given that all chapters dealing with environmental aspects of N₂ fixation focus on marine systems (open oceans, coastal regions, and sediments), the book is aptly named. As one who spent some efforts early in my career measuring N₂ fixation rates in lakes and wetlands (e.g., Brezonik and Harper 1969), however, I wish the authors had included a chapter on freshwaters. Although that literature is not as abundant as that for marine environments, substantial research on N₂ fixation in lakes, rivers, and wetlands has been accomplished and apparently has not been the topic of a general review. Out of curiosity, I did a Web of Science search and found 2577 articles associated with the term “marine nitrogen fixation,” and 1251 articles associated with N₂ fixation in lakes, wetlands, or freshwaters; a search that included the word “review” yielded no general reviews on N₂ fixation in freshwaters. Of course, not all articles that mention these search terms actually report results related to the terms; nonetheless, the above figures suggest that a review of N₂ fixation in freshwaters would be useful. Meanwhile, aquatic ecologists will find Marine Nitrogen Fixation worth reading and a useful addition to their libraries.