Algar
Christopher K.
Algar
Christopher K.
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ArticleRelease of multiple bubbles from cohesive sediments(American Geophysical Union, 2011-04-20) Algar, Christopher K. ; Boudreau, Bernard P. ; Barry, Mark A.Methane is a strong greenhouse gas, and marine and wetland sediments constitute significant sources to the atmosphere. This flux is dominated by the release of bubbles, and quantitative prediction of this bubble flux has been elusive because of the lack of a mechanistic model. Our previous work has shown that sediments behave as elastic fracturing solids during bubble growth and rise. We now further argue that bubbles can open previously formed, partially annealed, rise tracts (fractures) and that this mechanism can account for the observed preferential release at low tides in marine settings. When this mechanical model is applied to data from Cape Lookout Bight, NC (USA), the results indicate that methanogenic bubbles released at this site do indeed follow previously formed rise tracts and that the calculated release rates are entirely consistent with the rise of multiple bubbles on tidal time scales. Our model forms a basis for making predictions of future bubble fluxes from warming sediments under the influence of climate change.
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ArticlePredicting microbial nitrate reduction pathways in coastal sediments(Inter-Research, 2014-01-23) Algar, Christopher K. ; Vallino, Joseph J.We present an ecosystem model that describes the biogeochemistry of a sediment nitrate reducing microbial community. In the model, the microbial community is represented as a distributed metabolic network. Biogeochemical pathways are controlled through the synthesis and allocation of biological structure that serves to catalyze each process. Allocation is determined by way of a thermodynamically constrained optimization according to the principle of maximum entropy production (MEP). According to the MEP principle, ecosystems will organize so as to maximize the dissipation of free energy based upon the available resources (nutrients and electron donors and acceptors). In the model, 3 nitrate reduction pathways, viz. heterotrophic denitrification, anammox, and dissimilatory nitrate reduction to ammonium, compete for electron acceptors (nitrate and nitrite). The model predicts switches in the dominant nitrate cycling pathways based upon the ratio of carbon to nitrate supply. An advantage of this approach over a traditional organism-centric kinetic approach is that the Monod growth parameters, maximum uptake rate (νmax), and half saturation (kM) constants are determined during the optimization procedure as opposed to parameters specified a priori. Such a model is therefore useful for applications where these kinetic parameters are unknown and difficult to measure, such as the marine subsurface, or when ecosystems undergo large-scale environmental perturbations resulting in shifts in the dominant organisms.
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ArticleInitial rise of bubbles in cohesive sediments by a process of viscoelastic fracture(American Geophysical Union, 2011-04-14) Algar, Christopher K. ; Boudreau, Bernard P. ; Barry, Mark A.An understanding of the mechanics of bubble rise in sediments is essential because of the role of bubbles in releasing methane to the atmosphere and the formation and melting of gas hydrates. Past models to describe and predict the rise of other buoyant geological bodies through a surrounding solid (e.g., magmas and hydrofractures) appear not to be applicable to bubbles in soft sediments, and this paper presents a new model for gas bubble rise in soft, fine-grained, cohesive sediments. Bubbles in such sediments are essentially “dry” (little if any free water) and grow through a process of elastic expansion and fracture that can be described using the principles of linear elastic fracture mechanics, which assume the existence of a spectrum of flaws within the sediment fabric. By extending this theory, we predict that bubbles initially rise by preferential propagation of a fracture in a (sub) vertical direction. We present a criterion for initial bubble rise. Once rise is initiated, the speed of rise is controlled by the viscoelastic response of the sediments to stress. Using this new bubble rise model, we estimate rise velocities to be of the order of centimeters per second. We again show that capillary pressure plays no substantive role in controlling bubble growth or rise.
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ArticleSubseafloor microbial communities in hydrogen-rich vent fluids from hydrothermal systems along the Mid-Cayman Rise(John Wiley & Sons, 2016-01-21) Reveillaud, Julie ; Reddington, Emily ; McDermott, Jill M. ; Algar, Christopher K. ; Meyer, Julie L. ; Sylva, Sean P. ; Seewald, Jeffrey S. ; German, Christopher R. ; Huber, Julie A.Warm fluids emanating from hydrothermal vents can be used as windows into the rocky subseafloor habitat and its resident microbial community. Two new vent systems on the Mid-Cayman Rise each exhibits novel geologic settings and distinctively hydrogen-rich vent fluid compositions. We have determined and compared the chemistry, potential energy yielding reactions, abundance, community composition, diversity, and function of microbes in venting fluids from both sites: Piccard, the world's deepest vent site, hosted in mafic rocks; and Von Damm, an adjacent, ultramafic-influenced system. Von Damm hosted a wider diversity of lineages and metabolisms in comparison to Piccard, consistent with thermodynamic models that predict more numerous energy sources at ultramafic systems. There was little overlap in the phylotypes found at each site, although similar and dominant hydrogen-utilizing genera were present at both. Despite the differences in community structure, depth, geology, and fluid chemistry, energetic modelling and metagenomic analysis indicate near functional equivalence between Von Damm and Piccard, likely driven by the high hydrogen concentrations and elevated temperatures at both sites. Results are compared with hydrothermal sites worldwide to provide a global perspective on the distinctiveness of these newly discovered sites and the interplay among rocks, fluid composition and life in the subseafloor.
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ArticleSeafloor incubation experiment with deep-sea hydrothermal vent fluid reveals effect of pressure and lag time on autotrophic microbial communities(American Society for Microbiology, 2021-04-13) Fortunato, Caroline S. ; Butterfield, David A. ; Larson, Benjamin I. ; Lawrence-Slavas, Noah ; Algar, Christopher K. ; Zeigler Allen, Lisa ; Holden, James F. ; Proskurowski, Giora ; Reddington, Emily ; Stewart, Lucy C. ; Topçuoğlu, Begüm D ; Vallino, Joseph J. ; Huber, Julie A.Depressurization and sample processing delays may impact the outcome of shipboard microbial incubations of samples collected from the deep sea. To address this knowledge gap, we developed a remotely operated vehicle (ROV)-powered incubator instrument to carry out and compare results from in situ and shipboard RNA stable isotope probing (RNA-SIP) experiments to identify the key chemolithoautotrophic microbes and metabolisms in diffuse, low-temperature venting fluids from Axial Seamount. All the incubations showed microbial uptake of labeled bicarbonate primarily by thermophilic autotrophic Epsilonbacteraeota that oxidized hydrogen coupled with nitrate reduction. However, the in situ seafloor incubations showed higher abundances of transcripts annotated for aerobic processes, suggesting that oxygen was lost from the hydrothermal fluid samples prior to shipboard analysis. Furthermore, transcripts for thermal stress proteins such as heat shock chaperones and proteases were significantly more abundant in the shipboard incubations, suggesting that depressurization induced thermal stress in the metabolically active microbes in these incubations. Together, the results indicate that while the autotrophic microbial communities in the shipboard and seafloor experiments behaved similarly, there were distinct differences that provide new insight into the activities of natural microbial assemblages under nearly native conditions in the ocean.