E-Book, Englisch, 712 Seiten
ISBN: 978-0-12-407153-7
Verlag: Elsevier Reference Monographs
Format: PDF
Kopierschutz: Adobe DRM (»Systemvoraussetzungen)
Features up-to-date knowledge of DOM, including five new chaptersThe only published work to synthesize recent research on dissolved organic carbon in the Mediterranean SeaIncludes chapters that address inputs from freshwater terrestrial DOM
Autoren/Hrsg.
Weitere Infos & Material
Foreword
As one of Earth’s largest exchangeable carbon reservoirs, similar in scale to atmospheric CO2, the biogeochemical behavior of marine dissolved organic matter (DOM) has major significance for the carbon cycle, climate, and global habitability. As Ducklow (2002) wrote in the first edition of this book: “Oceanic DOM is now recognized as an important component of the biogeochemical system and possibly a barometer of global change.” Accordingly, marine DOM research has undergone a renaissance, moving rapidly beyond issues of measurement methodology to critically important spatial-temporal mapping of DOC distribution in the global ocean (Hansell et al., 2009), and now poised to address the underlying regulatory mechanisms. Despite important strides, our understanding of the biogeochemical behavior of marine DOM is still in its exciting “early exponential growth phase.” Fundamental questions of the nature and sources of DOM and mechanisms of its production, transformation, and respiration remain unanswered. So, there is much to do, and there are fresh ideas and powerful new study tools, as reflected in this book. Below, I formulate some problems that I suggest must be solved to understand DOM behavior and to predict the future biogeochemical state of the ocean. I hope that some young—and not so young—scientists will take on the challenge to solve them. The lifetimes of DOM constituents range from minutes to millennia: some are mineralized rapidly by heterotrophic microbes (labile; LDOM); others less readily (semi-labile; SLDOM); while an incredible diversity of molecules, 1012-1015 (Hedges, 2002) have accumulated over time to comprise the huge (~ 642 PgC) but intriguing refractory DOM pool. A challenge is to understand the biological and physicochemical forces that mediate and regulate the biogeochemical behavior of various DOM components. Most oceanic DOM is ultimately derived from primary production, and owes some of its chemical complexity to it, as phytoplankton generate enormous molecular diversity at the expense of CO2 and just a few inorganic and trace nutrients in order to serve their diverse adaptive needs. Biochemical processing of primary production by genetically diverse bacteria further adds to the chemical complexity of DOM. Intriguingly, most of the biomass generated by primary production is particulate (POM) yet on average about one-half—but a variable fraction—of primary production becomes DOM within the upper mixed layer, assessed conservatively as bacterial carbon demand. The POM-DOM transition is a critical step in the flow of reduced carbon in the global ocean and the capacity of the ecosystem to retain elements in the upper ocean for air-sea exchange; yet we currently lack knowledge of the underlying mechanisms or the means of direct quantification. There is extensive literature showing that multiple physiological, biochemical, and trophic interactions cause the release of DOM from the particulate phase—including living organisms. Sloppy feeding and exudation were long believed to be the major mechanisms, but with discoveries of new links in the food web there has also been recognition of additional mechanisms of DOM production. The list now includes microbial ectohydrolase activity, viral lysis of phytoplankton and heterotrophic microbes, cellular release of transparent exopolymer particles and other gel particles, programmed cell death, and microbe- microbe antagonism. It is probable that all mechanisms—both those listed and unlisted—cause the POM-DOM transition, but their relative quantitative significance varies in time and location. In view of the critical importance of the POM-DOM transition for predictive models of carbon flow and sequestration in the ocean, I stress the need to develop quantitative methods and a better mechanistic understanding of the production of marine DOM. The utilization side of the marine DOM dynamics is deceptively simple, since essentially all DOM uptake is due to bacteria and Archaea, the dominant osmotrophic heterotrophs. The strength of this coupling is a critical variable in the regulation of DOM utilization and respiration, and subsequent air-sea exchange of CO2. Tight coupling also prevents excursions in DOM concentration, so this regulation has major ecological and biogeochemical implications. How do microbes manage to biochemically couple so tightly with primary production; and what biochemical and behavioral (e.g., chemokinesis) mechanisms regulate the nature and strength of bacteria-organic matter coupling? What is the role of microbial genetic diversity and biogeochemical expressions in maintaining the strength of coupling? Genomic predictions have provided powerful constraints. They tell us the molecular interactions among DOM molecules and microbes that are possible. However, predicting the biogeochemical dynamics of the complex DOM pool also requires ecophysiological and biochemical studies of DOM-bacteria interactions to determine: what DOM transformations do take place, at what rates, by what biochemical mechanisms, subject to what regulatory forces and in what ecosystem context. This is indeed a tall order; but the problem is critical to solve because of the central role of DOM-bacteria interactions in predicting the carbon cycle of the future ocean. Ocean acidification and warming are likely to affect the nature and rates of microbial production and transformation of DOM with potential influence on the carbon balance between the ocean and the atmosphere. Understanding what renders some of the DOM semi-labile or refractory will also require such mechanistic studies. This important research on dissolved phase carbon cycling and sequestration requires new methods, model systems, and concepts (e.g., Microbial Carbon Pump; Jiao et al., 2010) addressing in situ dynamics and interactions among microbes and (DOM) molecules. Method refinement has been an important goal in marine DOM research. The field was transformed in the 1990s by the fundamental discovery that DOM was measurably dynamic, contrary to earlier thinking that the DOM pool was inert. This discovery “changed everything.” Interestingly, chemists and microbiologists reached this conclusion by different paths. Chemists worked diligently to refine the DOC method (in spite of or perhaps because of initial setbacks; Hedges, 2002; Sugimura and Suzuki, 1988), achieving ~ 1 µM precision, sufficient to show DOC gradients over days to seasons and across locations and depths (Hansell et al., 2009). Marine microbiologists had been finding as early as the 1960s that 14C labeled amino acids and sugars added to seawater as metabolic tracers were readily assimilated and respired with lifetimes of hours to days. Clearly, a DOM fraction represented by the radiotracers was highly dynamic. Remarkably, the seemingly opposite views of DOM lability coexisted for a decade. As it turned out, the microbiologists had been observing the behavior of a tiny but dynamic labile DOM (~ 1 µM) embedded within an ~ 40 to 70-fold larger recalcitrant DOM pool. Thus, the divergent impressions of the “shape of the elephant” were due to the enormous range of lifetimes of DOM components. This problem of the biogeochemical behaviors of labile and recalcitrant DOM, and implications for dissolved phase carbon sequestration, is an active research area, formalized as the Microbial Carbon Pump (Jiao et al., 2010). While high precision measurements of DOC concentrations (Sharp, 2002) has transformed marine DOM research, further method refinement and new method development is a high priority. First, achieving 1-2 µM precision still requires a magic touch; we need a plug and play method that even a microbial oceanographer could use! Second, experimental studies of DOM uptake, respiration, and sequestration require yet higher analytical precision. An order of magnitude improvement may even enable measurements of bacterial utilization of semi-labile DOC (lifetime 1.5 years; Hansell, 2013) albeit requiring long incubations. By analogy, consider if the scientists studying ocean acidification could only measure seawater pH with precision of 0.1 units! (While Dennis Hansell was waiting for me to finally finish this Foreword, X-Prize worth $2 M was announced for precise and user-friendly pH instrument: http://www.xprize.org/prize/wendy-schmidt-ocean-health-xprize.) Ultra-precise DOC, DON, and DOP methods will transform marine DOM biogeochemistry and climate predictions. Finally, we need a standardized method to measure microheterotrophic (bacteria + Archaea) respiration that does not significantly perturb the process being observed. These prokaryotes essentially monopolize DOM and their carbon growth efficiency is low, typically 10-30% (i.e., 70-90% of the labile DOM-C is respired by heterotrophic microbes). It is currently debated whether or not respiration and primary production are in balance or instead display spatial patterns of imbalance related to oligotrophic versus meso-/eutrophic systems (Ducklow and Doney, 2013). The “DOM problem” has been studied for the better part of a century against significant methodological odds, yet significant advances have been made in the last 10-20 years that have enriched the field. Today, there is strong interdisciplinary convergence and integration, as marine chemists, organic geochemists, analytical chemists, microbiologists and molecular biologists, and modelers join to address big questions of carbon cycling and sequestration, climate predictions, and...
