Lever Mark A.

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Lever
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Mark A.
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  • Article
    Response of microphytobenthic biomass to experimental nutrient enrichment and grazer exclusion at different land-derived nitrogen loads
    (Inter-Research, 2005-06-09) Lever, Mark A. ; Valiela, Ivan
    Effects of eutrophication on the relative importance of nutrients and macroherbivores as controls of microphytobenthic standing crop were examined in estuaries with different nitrogen loading rates: Sage Lot Pond (14 kg ha–1 yr–1), Green Pond (178 kg ha–1 yr–1), and Childs River (601 kg ha–1 yr–1). We selected 5 sites with similar salinity ranges on shallow-water, sandy substrates per estuary. In year-round experiments, we fertilized sediments with nitrogen + phosphorus to examine nutrient limitation. We conducted exclusion experiments to determine the significance of macroherbivores as controls of microphytobenthic biomass and examined possible interactions between nutrients and grazing in cages fertilized with nitrogen + phosphorus. Cages fertilized with nitrogen only were also included to determine if nitrogen availability was limiting. Nitrogen + phosphorus addition increased sediment chlorophyll a (chl a) content (herein used as a proxy for biomass) by a similar magnitude across estuaries. Grazer exclusion also increased chl a, but to a different extent across estuaries: the magnitude of the response increased with increasing nitrogen loading rates. We found no interactions between nutrients and grazing. Strong chl a increases in response to nitrogen only addition indicated N limitation in Sage Lot Pond and Green Pond. In the highly eutrophic Childs River estuary we found virtually no response to nitrogen-only additions, suggesting the possibility of phosphorus limitation in this estuary.
  • Article
    Heterotrophic Archaea dominate sedimentary subsurface ecosystems off Peru
    (National Academy of Sciences, 2006-02-27) Biddle, Jennifer F. ; Lipp, Julius S. ; Lever, Mark A. ; Lloyd, Karen G. ; Sorensen, Ketil B. ; Anderson, Rika E. ; Fredricks, Helen F. ; Elvert, Marcus ; Kelly, Timothy J. ; Schrag, Daniel P. ; Sogin, Mitchell L. ; Brenchley, Jean E. ; Teske, Andreas ; House, Christopher H. ; Hinrichs, Kai-Uwe
    Studies of deeply buried, sedimentary microbial communities and associated biogeochemical processes during Ocean Drilling Program Leg 201 showed elevated prokaryotic cell numbers in sediment layers where methane is consumed anaerobically at the expense of sulfate. Here, we show that extractable archaeal rRNA, selecting only for active community members in these ecosystems, is dominated by sequences of uncultivated Archaea affiliated with the Marine Benthic Group B and the Miscellaneous Crenarchaeotal Group, whereas known methanotrophic Archaea are not detectable. Carbon flow reconstructions based on stable isotopic compositions of whole archaeal cells, intact archaeal membrane lipids, and other sedimentary carbon pools indicate that these Archaea assimilate sedimentary organic compounds other than methane even though methanotrophy accounts for a major fraction of carbon cycled in these ecosystems. Oxidation of methane by members of Marine Benthic Group B and the Miscellaneous Crenarchaeotal Group without assimilation of methane–carbon provides a plausible explanation. Maintenance energies of these subsurface communities appear to be orders of magnitude lower than minimum values known from laboratory observations, and ecosystem-level carbon budgets suggest that community turnover times are on the order of 100–2,000 years. Our study provides clues about the metabolic functionality of two cosmopolitan groups of uncultured Archaea.
  • Article
    The Guaymas Basin hiking guide to hydrothermal mounds, chimneys, and microbial mats : complex seafloor expressions of subsurface hydrothermal circulation
    (Frontiers Media, 2016-02-18) Teske, Andreas ; de Beer, Dirk ; McKay, Luke J. ; Tivey, Margaret K. ; Biddle, Jennifer F. ; Hoer, Daniel ; Lloyd, Karen G. ; Lever, Mark A. ; Røy, Hans ; Albert, Daniel B. ; Mendlovitz, Howard P. ; MacGregor, Barbara J.
    The hydrothermal mats, mounds, and chimneys of the southern Guaymas Basin are the surface expression of complex subsurface hydrothermal circulation patterns. In this overview, we document the most frequently visited features of this hydrothermal area with photographs, temperature measurements, and selected geochemical data; many of these distinct habitats await characterization of their microbial communities and activities. Microprofiler deployments on microbial mats and hydrothermal sediments show their steep geochemical and thermal gradients at millimeter-scale vertical resolution. Mapping these hydrothermal features and sampling locations within the southern Guaymas Basin suggest linkages to underlying shallow sills and heat flow gradients. Recognizing the inherent spatial limitations of much current Guaymas Basin sampling calls for comprehensive surveys of the wider spreading region.