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ARTICLE IN PRESS Deep-Sea Research II 57 (2010) 1429–1432 Contents lists available at ScienceDirect Deep-Sea Research II journal homepage: www.elsevier.com/locate/dsr2 Editorial Introduction to: ‘‘Ecological and biogeochemical interactions in the dark ocean’’ The deep sea, a vast, dark realm featuring distinctive organisms and serving as a massive reservoir of carbon, is the largest and least explored ecosystem on Earth (Fig. 1). At a time when the ocean is responding to anthropogenic forcings (Sabine et al., 2004; Richardson, 2008; Doney et al., 2009), we note that considerably less is known about ecological and biogeochemical processes in the dark ocean (the dim mesopelagic or ‘twilight zone’ plus the aphotic bathypelagic zone below) than in the euphotic zone– the focus of several prior major interdisciplinary studies. The biological pump connects surface processes to the deepest ocean layers (Volk and Hoffert, 1985; Ducklow et al., 2001), where biological processes occur at low rates, and the biota may adapt unique metabolisms. These deep layers are characterized by significant decomposition, recycling, and repackaging of particulate and dissolved organic matter (see manuscripts in this special issue). Thus, the interplay between biological and geochemical processes at depth can have significant effects on the magnitude and efficiency of the biological pump, which regulates in part atmospheric CO2 and, hence, climate (Cermeño et al., 2008; Riebesell et al., 2009; Lomas et al., 2010). Most of the biogenic material exported from the euphotic zone is remineralized within the mesopelagic zone (Martin et al., 1987; Buesseler et al., 2007; 100–1000 m), with recent interdisciplinary field studies of that system focusing on particle fluxes (Buesseler et al., 2008; Lee et al., 2009). The structure of the planktonic food web exerts control on the vertical transport, cycling, and composition of this particulate and dissolved organic matter (Legendre and Lefevre, 1995; Carlson, 2002; Boyd and Trull, 2007; Steinberg et al., 2008; Buesseler and Boyd, 2009), but there are still many questions concerning how microbial and metazoan diversity is linked to function in this zone. In addition, ecological and biogeochemical approaches to estimating metabolic rates need to be reconciled (Arı́stegui et al., 2005; Burd et al., 2010). While important processes regulating organic matter transformations and remineralization in the mesopelagic can be tightly coupled with the euphotic zone, the vast differences in time and space scales of events within these two zones make holistic study of the biological pump a challenge, particularly in terms of its ability to sequester C in the deep ocean. With residence times in the bathypelagic zone (taken here to be inclusive of all ocean depths 41000 m) ranging from centennial to millennial, and spanning the globe spatially, this zone is only slowly ventilated and circulated (Van Aken, 2007). Biogeochemical signals in the bathypelagic are thus integrative of unique ecosystem and metabolic processes occurring slowly over very long periods. Biological processes in the deepest ocean layers 0967-0645/$ - see front matter & 2010 Elsevier Ltd. All rights reserved. doi:10.1016/j.dsr2.2010.02.012 are intimately tied to particle dynamics and microbial food webs, much of which remain to be characterized (Arı́stegui et al., 2009; Nagata et al., 2010). These processes, while occurring at low rates, can play important roles in global marine elemental cycling by virtue of the incredible volume of the deep ocean, with feedbacks to the climate system. Changes in ocean stratification, for example, will elicit changes in the ecosystem functioning of the deepest layers that will be reflected in changing biogeochemical fields (e.g., dissolved oxygen concentrations). Slowly occurring processes, which may happen throughout the ocean depths, are hidden from view by the strong impacts of more dynamic processes emanating from the upper ocean. Signals for these cryptic metabolic and biophysical processes may only emerge in the deepest layers where vertical and horizontal inputs are greatly reduced. The collection of papers in this volume on the ‘dark ocean’ are a product of the first IMBER (Integrated Marine Biogeochemistry and Ecosystem Research) IMBIZO (a Zulu word that means ‘‘gathering’’ or ‘‘meeting’’), entitled ‘‘Integrating Biogeochemistry and Ecosystems in a Changing Ocean’’ and held in Coconut Grove, Florida, USA, during November 9–13, 2008. The week-long international conference involved participants in three concurrent and interacting workshops of 25–40 people each, who contributed to presentations and extensive discussions pertaining to each workshop topic. The contributions are a mixture of synthesis papers and new data papers from participants in two of the workshops: ‘Ecological and Biogeochemical Interactions in the Mesopelagic Zone’ and ‘Biogeochemistry and Microbial Dynamics of the Bathypelagic Zone’. The papers highlight the current state of our knowledge and our uncertainties about deep-ocean foodweb processes, particle flux and dynamics, and biogeochemical cycling, and identify opportunities to be pursued in future research of the dark ocean. The contributions cover a cross section of disciplines, including biogeochemistry, organic geochemistry, microbial and plankton ecology, genomics, technology, and modeling – all of which advance our understanding of the dark ocean. This special issue begins with a series of papers on the biogeochemistry and organic geochemistry of the deep ocean, with a focus on dissolved organic matter (DOM). Dissolved organic carbon (DOC) is the largest pool of organic matter in the ocean, is intimately tied to microbial dynamics, and contributes significantly to C export via ocean ventilation (Hansell et al., 2009). In the first paper, ‘‘Dissolved organic carbon export and subsequent remineralization in the mesopelagic and bathypelagic realms’’, Carlson et al. (2010) present DOC concentration gradients in the deep North Atlantic ARTICLE IN PRESS 1430 Editorial / Deep-Sea Research II 57 (2010) 1429–1432 Fig. 1. Graphic representation of ocean volume relative to bottom depth, illustrating that the deep ocean is the largest living space on Earth. The curve shows the global volume of water above seafloor depths ranging from 0 to 10.9 km (the Mariana Trench). The edge of the continental shelf at roughly 200 m is the traditional boundary of the deep sea. Ecological depth zones of the oceanic water column: epipelagic, upper 150–200 m; mesopelagic, down to 1000 m; bathypelagic, 1–4 km; abyssopelagic, 4–8 km; hadal, below 8 km. The displayed volumes of terrestrial and benthic environments include the surface areas of the dry land and the seafloor and the air or water above to a height of 1 km. (From Robison, 2009; Data source: UN Atlas of the Oceans - www.oceansatlas.org/). Ocean to develop a comprehensive picture of the transport and decay of DOC, the contribution of DOC to apparent oxygen utilization (AOU), and net DOC export rates from the euphotic zone into the dark ocean. In addition to spatial DOC distributions, temporal changes in DOC in the dark ocean are important for determining DOC export and controls on utilization. Santinelli et al. (2010), in ‘‘DOC dynamics in meso- and bathypelagic layers of the Mediterranean Sea’’, survey DOC distributions in this basin and from those they infer dynamics. They highlight the role of deep-water formation events in the export of DOC, finding that 490% of the AOU in recently-ventilated deep water is due to DOC remineralization. They also report very high rates of DOC consumption at depth. These very high contributions and rates differ from the results described by Carlson et al. (2010), where DOC makes a smaller contribution to AOU and it is consumed more slowly; thus these papers indicate contrasting system functionality and opportunity for further study. Meador et al. (2010) further explore DOC dynamics as they relate to prokaryotic activity in the deep Mediterranean in ‘‘Biogeochemical relationships between ultrafiltered dissolved organic matter and picoplankton activity in the Eastern Mediterranean Sea’’, revealing important relationships between the chemical composition of DOM and heterotrophic metabolism. Oxygen consumption and prokaryotic activity were correlated with components of ultrafiltered DOM (e.g., amino acids, monosaccharides) to varying degrees, uniquely relating biochemical composition of DOM to its utilization in deep water and contributing to our understanding of DOM respiration and turnover. The composition of deep-ocean DOM is furthered explored in two subsequent papers. Yamashita et al. (2010), in ‘‘Fluorescence characteristics of dissolved organic matter in the deep waters of the Okhotsk Sea and the northwestern North Pacific Ocean’’, identify one protein-like, two humic-like, and one uncertain fluorophore in these waters. By comparing fluorescence intensities and ratios of the humic-like fluorophores with depth and with AOU in the bathypelagic, differences in the composition of humic-like fluorophores in epi- vs. meso- vs. bathypelagic waters were noted. Both humic-like components may be produced in situ as organic matter is catabolized by organisms. In ‘‘Polymer dynamics of DOC networks and gel formation in seawater’’, Verdugo and Santschi (2010) explore the paradox of how deep-ocean DOM, that due to its low concentration should be virtually unavailable to microorganisms, may become available to bacteria – namely by DOM assembly/dispersion dynamics resulting in formation of microscopic gels. These self-assembled microgels form patches of enriched nutrients that can be readily utilized by bacteria. In this paper they explain the kinetics and thermodynamics of DOC assembly/dispersion processes that result in the formation of microgels. The last paper pertaining to biogeochemistry of DOM in the dark ocean is entitled ‘‘Constraining the 2-component model of marine dissolved organic radiocarbon’’, by Beaupré and Aluwihare (2010). These authors evaluated multiple Keeling plots of DOC concentration and D14C depth profiles as tools to investigate DOC delivery throughout the water column. DOC variability in the epipelagic is dominated by biogeochemistry rather than advection, but at greater depths the redistribution of D14C-enriched DOC occurs primarily via water mass movement. These findings highlight specific processes to be evaluated in the deeper reaches of the ocean relative to those dominating the upper ocean. The next set of contributions addresses deep-sea food webs and biogeochemical cycles. In ‘‘Mesopelagic zone ecology and biogeochemistry – a synthesis’’, Robinson et al. (2010) review what is known about the diversity and distribution of microbes and metazoa (from viruses to fish) in relation to their activity and impact on global biogeochemical cycles. They show how metazoans and microbes play different roles in controlling vertical transport, cycling, and composition of POM and DOM, and emphasize the importance of linking mesopelagic microbial and metazoan diversity to function for understanding and predicting the effects of global change on deep-ocean food webs and C sequestration. Recent findings regarding deep-sea microbial diversity, distribution, and function are reviewed in Nagata et al. (2010), ‘‘Emerging concepts on microbial processes in the bathypelagic ocean – ecology, biogeochemistry, and genomics’’. This analysis highlights the remarkable diversity and adaptations of microbes ARTICLE IN PRESS Editorial / Deep-Sea Research II 57 (2010) 1429–1432 to high pressure, low temperatures, and complex organic matter substrates, and covers the current state of knowledge of the bathypelagic microbial food web from viruses, through Bacteria and Archaea, to heterotrophic nanoflagellates (HNF) and ciliates. Recently discovered communities of bathypelagic microbes and their novel metabolic capabilities will undoubtedly fill gaps in our understanding of biogeochemical cycling in the deep ocean. An important question concerning microbial dynamics in the dark ocean is how communities differ over large spatial scales. In ‘‘Fulldepth profiles of prokaryotes, heterotrophic nanoflagellates, and ciliates along a transect from the equatorial to the subarctic central Pacific Ocean’’, Sohrin et al. (2010) demonstrate that latitudinal differences in heterotrophic microbial biomass, previously observed in the epipelagic zone, also occur in the mesoand bathypelagic zones. There are important differences in food web dynamics with latitude and depth as inferred from ratios of prokaryotes to their protist predators. Moving up a few trophic levels, Robison et al. (2010) in ‘‘The bathypelagic community of Monterey Canyon’’ report on the rich pelagic macrofauna dominated by gelatinous zooplankton in the eastern North Pacific off central California, as studied with a submersible remotely operated vehicle (ROV). There are distinct ranges within the bathypelagic zone for different taxa, some of which are known detritivores, while abundance of some taxa such as the crustacean grazers remained fairly constant with depth. This paper also highlights the value of advanced technologies in our sampling and interpretation of plankton community structure in the dark ocean. A number of global budgets and local field studies suggest that organic carbon utilization (respiration) exceeds organic C input to the mesopelagic and bathypelagic zones (Boyd et al., 1999; del Giorgio and Duarte, 2002; Reinthaler et al., 2006; Steinberg et al., 2008; Baltar et al., 2009). This conundrum is reviewed by Burd et al. (2010) in ‘‘Assessing the apparent imbalance between geochemical and biochemical indicators of meso- and bathypelagic biological activity: What the @$#! is wrong with present calculations of carbon budgets?’’. The imbalance implies either unaccounted for sources of organic carbon or over-estimated carbon sinks in the dark ocean. Burd et al. (2010) consider the problem from both sides, discussing the various uncertainties associated with environmental variability, measurement capabilities, conversion parameters, and processes that are not well sampled. One potential unaccounted source of C supply to the dark ocean is discussed in ‘‘Major contribution of autotrophy to microbial carbon cycling in the deep North Atlantic’s interior’’ (Reinthaler et al., 2010). Chemoautotrophic DIC fixation (measured by 14C-bicarbonate fixation) by Crenarchaeota in the dark ocean was substantial and within the same order of magnitude as heterotrophic microbial activity (measured by 3H-leucine incorporation). Thus, chemoautotrophy could be a substantial source of autochthonously produced organic carbon in the dark ocean, fueling heterotrophic microbial organic carbon demand. In ‘‘Carbon cycling and POC turnover in the mesopelagic zone of the ocean: Insights from a simple model’’, Anderson and Tang (2010) further explore carbon budgets of the dark ocean with a food web model that traces turnover of sinking detrital POC through to its respiration by particle-attached and free-living bacteria, and by detritivorous zooplankton. They report that bacteria account for most of the respiratory C demand and thus are the primary sink for POC, whereas zooplankton mainly transform the exported POC to suspended detritus or DOC rather than respire it to CO2. There have already been some significant advances in dark ocean research since our discussions during the IMBER IMBIZO (Aristegui et al., 2009; Hansell et al., 2009), with many more to come. This collection of papers is thus timely as we attempt to 1431 shed light on the biogeochemistry and ecology of that vast, dark Earth system. Acknowledgments First, we thank our co-chairs for the mesopelagic (Hiroaki Saito) and bathypelagic (Gerhard Herndl) workshops as well as the other members of the IMBIZO scientific organizing committee (Julie Hall, Coleen Moloney, Wajih Naqvi, Michael Roman, Sylvie Roy, Sharon Smith, Julie Hollenbeck, and Jing Zhang). We also thank the active members of the scientific steering committees for the mesopelagic workshop (Javier Arı́stegui, George Jackson, Carol Robinson, and Richard Sempéré) and the bathypelagic workshop (Doug Bartlett, Toshi Nagata, and Dan Repeta), and our invited plenary speakers (Richard Lampitt and David Karl). Our gratitude to all the other workshop participants, many of whom traveled half-way around the globe, for stimulating and enjoyable presentations and discussions, which led to this manuscript compilation. Many thanks to the reviewers we solicited whose comments greatly improved the quality of the manuscripts. Financial support for the IMBER IMBIZO was provided by The Scientific Committee for Ocean Research (SCOR), IMBER, the U.S. Ocean Carbon and Biogeochemistry (OCB) program, EurOceans, Universite de Bretagne Occidentale, Global Ocean Ecosystem Dynamics (GLOBEC), Centre National de la Recherche Scientifique, and the University of Miami’s Rosenstiel School of Marine and Atmospheric Science and Center for Oceans and Human Health. D.K.S. was partially supported by NSF OCE-0628444 and D.A.H. by NSF OCE-0752972 during the compilation, review, and editing of this volume. References Anderson, T.R., Tang, K.W., 2010. Carbon cycling and POC turnover in the mesopelagic zone of the ocean: Insights from a simple model. Deep-Sea Research II 57 (16), 1581–1592. Arı́stegui, J., Agustı́, S., Middelburg, J., Duarte, C.M., 2005. Respiration in the mesopelagic and bathypelagic zones of the oceans. In: Del Giorgio, P., LeB Williams, P.J. (Eds.), Respiration in Aquatic Ecosystems. Oxford Univ. Press, pp. 182–206. Arı́stegui, J., Gasol, J.M., Duarte, C.M., Herndl, G.J., 2009. Microbial oceanography of the dark ocean’s pelagic realm. Limnology and Oceanography 54, 1501–1529. Baltar, F., Aristegui, J., Gasol, J.M., Sintes, E., Herndl, G.J., 2009. Evidence of prokaryotic metabolism on suspended particulate organic matter in the dark waters of the subtropical North Atlantic. Limnology and Oceanography 54, 182–193. Beaupré, S.R., Aluwihare, L., 2010. Constraining the 2-component model of marine dissolved organic radiocarbon. Deep-Sea Research II 57 (16), 1494–1503. Boyd, P.W., Sherry, N.D., Berges, J.A., Bishop, J.K.B., Calvert, S.E., Charette, M.A., Giovannoni, S.J., Goldblatt, R., Harrison, P.J., Moran, S.B., 1999. Transformations of biogenic particulates from the pelagic to the deep ocean realm. Deep-Sea Research II 46, 2761–2792. Boyd, P.W., Trull, T.W., 2007. Understanding the export of biogenic particles in oceanic waters: Is there consensus? 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Robison, B.H., Sherlock, R.E., Reisenbichler, K.R., 2010. The bathypelagic community of the Monterey Canyon. Deep-Sea Research II 57 (16), 1551–1556. Sabine, C.L., Feely, R.A., Gruber, N., Key, R.M., Lee, K., Bullister, J.L., Wanninkhof, R., Wong, C.S., Wallace, D.W.R., Tilbrook, B., Millero, F.J., Peng, T.H., Kozyr, A., Ono, T., Rios, A.F., 2004. The oceanic sink for anthropogenic CO2. Science 305, 367–371. Santinelli, C., Nannicini, L., Seritti, A., 2010. DOC dynamics in meso- and bathypelagic layers of the Mediterranean Sea. Deep-Sea Research II 57 (16), 1446–1459. Sohrin, R., Imazawa, M., Fukuda, H., Suzuki, Y., 2010. Full-depth profiles of prokaryotes, heterotrophic nanoflagellates, and ciliates along a transect from the equatorial to the subarctic central Pacific Ocean. Deep-Sea Research II 57 (16), 1537–1550. Steinberg, D.K., Van Mooy, B.A.S., Buesseler, K.O., Boyd, P.W., Kobari, T., Karl, D.M., 2008. Bacterial vs. zooplankton control of sinking particle flux in the ocean’s twilight zone. Limnology and Oceanography 53, 1327–1338. Verdugo, P., Santschi, P.H., 2010. Polymer dynamics of DOC networks and gel formation in seawater. Deep-Sea Research II 57 (16), 1486–1493. Van Aken, H., 2007. Oceanic Thermohaline Circulation. Springer Verlag, New York, NY 326 pp. Volk, T., Hoffert, M.I., 1985. Ocean carbon pumps: Analysis of relative strengths and efficiencies in ocean-driven atmospheric CO2 changes. Geophysical Monographs 32, 99–110. Yamashita, Y., Cory, R.M., Nishioka, J., Kuma, K., Tanoue, E., Jaffé, R., 2010. Fluorescence characteristics of dissolved organic matter in the deep waters of the Okhotsk Sea and the northwestern North Pacific Ocean. Deep-Sea Research II 57 (16), 1478–1485. Deborah K. Steinberg n Virginia Institute of Marine Science, The College of William and Mary, Gloucester Point, VA 23062 E-mail address: [email protected] Dennis A. Hansell Rosenstiel School of Marine and Atmospheric Science, University of Miami, Miami, FL, 33149 Received 3 March 2010; accepted 3 March 2010 Corresponding author. Tel.: + 804 684 7838; fax: 804 684 7293.