Constraints on the utility of MnO2 cartridge method for the extraction of radionuclides: A case study using \u3csup\u3e234\u3c/sup\u3eTh

Large volume (102–103 L) seawater samples are routinely processed to investigate the partitioning of particle reactive radionuclides and Ra between solution and size-fractionated suspended particulate matter. One of the most frequently used methods to preconcentrate these nuclides from such large volumes involves extraction onto three filter cartridges (a prefilter for particulate species and two MnO2-coated filters for dissolved species) connected in series. This method assumes that the extraction efficiency is uniform for both MnO2-coated cartridges, that no dissolved species are removed by the prefilter, and that any adsorbed radionuclides are not desorbed from the MnO2-coated cartridges during filtration. In this study, we utilized 234Th-spiked coastal seawater and deionized water to address the removal of dissolved Th onto prefilters and MnO2-coated filter cartridges. Experimental results provide the first data that indicate (1) a small fraction of dissolved Th (\u3c6%) can be removed by the prefilter cartridge; (2) a small fraction of dissolved Th (\u3c5%) retained by the MnO2 surface can also be desorbed, which undermines the assumption of uniform extraction efficiency for Th; and (3) the absolute and relative extraction efficiencies can vary widely. These experiments provide insight on the variability of the extraction efficiency of MnO2-coated filter cartridges by comparing the relative and absolute efficiencies and recommend the use of a constant efficiency on the combined activity from two filter cartridges connected in series for future studies of dissolved 234Th and other radionuclides in natural waters using sequential filtration/extraction methods

Randomized lasso links microbial taxa with aquatic functional groups inferred from flow cytometry

High-nucleic-acid (HNA) and low-nucleic-acid (LNA) bacteria are two operational groups identified by flow cytometry (FCM) in aquatic systems. A number of reports have shown that HNA cell density correlates strongly with heterotrophic production, while LNA cell density does not. However, which taxa are specifically associated with these groups, and by extension, productivity has remained elusive. Here, we addressed this knowledge gap by using a machine learning-based variable selection approach that integrated FCM and 16S rRNA gene sequencing data collected from 14 freshwater lakes spanning a broad range in physicochemical conditions. There was a strong association between bacterial heterotrophic production and HNA absolute cell abundances (R-2 = 0.65), but not with the more abundant LNA cells. This solidifies findings, mainly from marine systems, that HNA and LNA bacteria could be considered separate functional groups, the former contributing a disproportionately large share of carbon cycling. Taxa selected by the models could predict HNA and LNA absolute cell abundances at all taxonomic levels. Selected operational taxonomic units (OTUs) ranged from low to high relative abundance and were mostly lake system specific (89.5% to 99.2%). A subset of selected OTUs was associated with both LNA and HNA groups (12.5% to 33.3%), suggesting either phenotypic plasticity or within-OTU genetic and physiological heterogeneity. These findings may lead to the identification of system-specific putative ecological indicators for heterotrophic productivity. Generally, our approach allows for the association of OTUs with specific functional groups in diverse ecosystems in order to improve our understanding of (microbial) biodiversity-ecosystem functioning relationships. IMPORTANCE A major goal in microbial ecology is to understand how microbial community structure influences ecosystem functioning. Various methods to directly associate bacterial taxa to functional groups in the environment are being developed. In this study, we applied machine learning methods to relate taxonomic data obtained from marker gene surveys to functional groups identified by flow cytometry. This allowed us to identify the taxa that are associated with heterotrophic productivity in freshwater lakes and indicated that the key contributors were highly system specific, regularly rare members of the community, and that some could possibly switch between being low and high contributors. Our approach provides a promising framework to identify taxa that contribute to ecosystem functioning and can be further developed to explore microbial contributions beyond heterotrophic production

Intense winter heterotrophic production stimulated by benthic resuspension

Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/109851/1/lno20004571672.pd

Possible link between Earth's rotation rate and oxygenation

The biotic and abiotic controls on major shifts in atmospheric oxygen and the persistence of low-oxygen periods over a majority of Earth’s history remain under debate. Explanations of Earth’s stepwise pattern of oxygenation have mostly neglected the effect of changing diel illumination dynamics linked to daylength, which has increased through geological time due to Earth’s rotational deceleration caused by tidal friction. Here we used microsensor measurements and dynamic modelling of interfacial solute fluxes in cyanobacterial mats to investigate the effect of changing daylength on Precambrian benthic ecosystems. Simulated increases in daylength across Earth’s historical range boosted the diel benthic oxygen export, even when the gross photosynthetic production remained constant. This fundamental relationship between net productivity and daylength emerges from the interaction of diffusive mass transfer and diel illumination dynamics, and is amplified by metabolic regulation and microbial behaviour. We found that the resultant daylength-driven surplus organic carbon burial could have shaped the increase in atmospheric oxygen that occurred during the Great and Neoproterozoic Oxidation Events. Our suggested mechanism, which links the coinciding increases in daylength and atmospheric oxygen via enhanced net productivity, reveals a possible contribution of planetary mechanics to the evolution of Earth’s biology and geochemistry

Production and Fate of Transparent Exopolymer Particles in the Ocean

The production and fate of transparent exopolymer particles (TEP) have been investigated in various oceanic regions (tropical, temperate, and polar), from the sea surface microlayer (SML) to the deep ocean. Accumulation of TEP within the mixed layer was observed even in the absence of phytoplankton blooms, indicating abiotic processes are important in TEP production. The abiotic TEP aggregation rates measured in the tropical and temperate North Pacific and the Arctic Ocean averaged between 8 and 12 μmol C L-1 d-1. Depth profiles from under sea ice in the Arctic revealed the highest TEP concentrations, potentially released by sympagic algal activity at the bottom of the sea ice. The aggregation rates in the SML, the interfacial layer between the ocean and atmosphere, were generally enhanced over those in the bulk surface waters by factors of 2 to 30. This finding further strengthens a developing consensus on the gelatinous nature of the SML, which will also affect microbial life, light penetration, and surface wave properties. We present a conceptual model implying that abiotic aggregation is an important factor for TEP production in the ocean, in particular in sea surface microlayers, while consumption by zooplankton and protists recycle TEP, providing a new pool of dissolved precursor material. Overall, TEP is recycled within the water column through heterotrophic grazing and degradation, providing a new pool of TEP precursor materials, while enhanced aggregation rates of TEP in the SML indicates the importance of this thin surface film in the marine carbon cycle

Seeking Sunlight: Rapid Phototactic Motility of Filamentous Mat-Forming Cyanobacteria Optimize Photosynthesis and Enhance Carbon Burial in Lake Huron’s Submerged Sinkholes

We studied the motility of filamentous mat-forming cyanobacteria consisting primarily ofOscillatoria-like cells growing under low-light, low-oxygen, and high-sulfur conditions in Lake Huron’s submerged sinkholes using in situ observations, in vitro measurements and time-lapse microscopy. Gliding movement of the cyanobacterial trichomes (100–10,000 μm long filaments, composed of cells ~10 μm wide and ~3 μm tall) revealed individual as well as group-coordinated motility. When placed in a petri dish and dispersed in ground water from the sinkhole, filaments re-aggregated into defined colonies within minutes, then dispersed again. Speed of individual filaments increased with temperature from ~50 μm min-1 or ~15 body lengths min-1 at 10∘C to ~215 μm min-1 or ~70 body lengths min-1 at 35∘C – rates that are rapid relative to non-flagellated/ciliated microbes. Filaments exhibited precise and coordinated positive phototaxis toward pinpoints of light and congregated under the light of foil cutouts. Such light-responsive clusters showed an increase in photosynthetic yield – suggesting phototactic motility aids in light acquisition as well as photosynthesis. Once light source was removed, filaments slowly spread out evenly and re-aggregated, demonstrating coordinated movement through inter-filament communication regardless of light. Pebbles and pieces of broken shells placed upon intact mat were quickly covered by vertically motile filaments within hours and became fully buried in the anoxic sediments over 3–4 diurnal cycles – likely facilitating the preservation of falling debris. Coordinated horizontal and vertical filament motility optimize mat cohesion and dynamics, photosynthetic efficiency and sedimentary carbon burial in modern-day sinkhole habitats that resemble the shallow seas in Earth’s early history. Analogous cyanobacterial motility may have played a key role in the oxygenation of the planet by optimizing photosynthesis while favoring carbon burial

Year-Round Measures of Planktonic Metabolism Reveal Net Autotrophy in Surface Waters of a Great Lakes Estuary

During 2009 and 2010, we quantified monthly changes in plankton metabolism and environmental variables in the surface waters of Muskegon Lake, a Great Lakes estuary connected to Lake Michigan. Muskegon Lake’s mean (±SE) annual gross plankton primary production (GPP) and respiration (R) rates were 46 ± 9 and 23 ± 4 mg C l−1 yr−1, respectively. GPP:R ratios of 0.6 to +4.8 with a yearly mean of 2.0 ± 0.3 indicated that the surface water of Muskegon Lake was net autotrophic during all but the winter months under ice cover, when it was in a near carbon balance to slightly heterotrophic state. Approximately 5% of GPP and 12% of R occurred during the winter months, highlighting winter’s potential role in nutrient regeneration. An overall positive annual net community production (NCP) rate of 28 ± 6 mg C l−1 yr−1 makes Muskegon Lake’s surface waters a net sink for carbon on an annual basis. Annual heterotrophic bacterial production (BP) rates were 5 ± 3 mg C l−1 yr−1, suggesting a substantial fraction of GPP was likely processed through the microbial food web (2 to 76%). A stepwise multiple linear regression model revealed the plausible drivers of GPP (temperature [T], photosynthetically active radiation [PAR], total phosphorus [TP], dissolved oxygen [DO], chlorophyll a [chl a]), NCP (T, PAR, TP), R (T, DO, ammonium [NH3], soluble reactive phosphorous [SRP], dissolved organic carbon [DOC]) and GPP:R (T, PAR, SRP, DOC). Year-round measurements inform us of the strong seasonality in the carbon cycle of temperate lakes

Microhabitats are associated with diversity-productivity relationships in freshwater bacterial communities

Eukaryotic communities commonly display a positive relationship between biodiversity and ecosystem function (BEF) but the results have been mixed when assessed in bacterial communities. Habitat heterogeneity, a factor in eukaryotic BEFs, may explain these variable observations but it has not been thoroughly evaluated in bacterial communities. Here, we examined the impact of habitat on the relationship between diversity assessed based on the (phylogenetic) Hill diversity metrics and heterotrophic productivity. We sampled co-occurring free-living (more homogenous) and particle-associated (more heterogeneous) bacterial habitats in a freshwater, estuarine lake over three seasons: spring, summer and fall. There was a strong, positive, linear relationship between particle-associated bacterial richness and heterotrophic productivity that strengthened when considering dominant taxa. There were no observable BEF trends in free-living bacterial communities for any diversity metric. Biodiversity, richness and Inverse Simpson's index, were the best predictors of particle-associated production whereas pH was the best predictor of free-living production. Our findings show that heterotrophic productivity is positively correlated with the effective number of taxa and that BEF relationships are associated with microhabitats. These results add to the understanding of the highly distinct contributions to diversity and functioning contributed by bacteria in free-living and particle-associated habitats

Cyanobacterial life at low O 2 : community genomics and function reveal metabolic versatility and extremely low diversity in a Great Lakes sinkhole mat

Cyanobacteria are renowned as the mediators of Earth’s oxygenation. However, little is known about the cyanobacterial communities that flourished under the low‐O 2 conditions that characterized most of their evolutionary history. Microbial mats in the submerged Middle Island Sinkhole of Lake Huron provide opportunities to investigate cyanobacteria under such persistent low‐O 2 conditions. Here, venting groundwater rich in sulfate and low in O 2 supports a unique benthic ecosystem of purple‐colored cyanobacterial mats. Beneath the mat is a layer of carbonate that is enriched in calcite and to a lesser extent dolomite. In situ benthic metabolism chambers revealed that the mats are net sinks for O 2 , suggesting primary production mechanisms other than oxygenic photosynthesis. Indeed, 14 C‐bicarbonate uptake studies of autotrophic production show variable contributions from oxygenic and anoxygenic photosynthesis and chemosynthesis, presumably because of supply of sulfide. These results suggest the presence of either facultatively anoxygenic cyanobacteria or a mix of oxygenic/anoxygenic types of cyanobacteria. Shotgun metagenomic sequencing revealed a remarkably low‐diversity mat community dominated by just one genotype most closely related to the cyanobacterium Phormidium autumnale , for which an essentially complete genome was reconstructed. Also recovered were partial genomes from a second genotype of Phormidium and several Oscillatoria . Despite the taxonomic simplicity, diverse cyanobacterial genes putatively involved in sulfur oxidation were identified, suggesting a diversity of sulfide physiologies. The dominant Phormidium genome reflects versatile metabolism and physiology that is specialized for a communal lifestyle under fluctuating redox conditions and light availability. Overall, this study provides genomic and physiologic insights into low‐O 2 cyanobacterial mat ecosystems that played crucial geobiological roles over long stretches of Earth history.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/90535/1/j.1472-4669.2012.00322.x.pd

Systematically Variable Planktonic Carbon Metabolism Along a Land-To-Lake Gradient in a Great Lakes Coastal Zone

During the summers of 2002–2013, we measured rates of carbon metabolism in surface waters of six sites across a land-to-lake gradient from the upstream end of drowned river-mouth Muskegon Lake (ML) (freshwater estuary) to 19 km offshore in Lake Michigan (LM) (a Great Lake). Despite considerable inter-year variability, the average rates of gross production (GP), respiration (R) and net production (NP) across ML (604 ± 58, 222 ± 22 and 381 ± 52 µg C L−1 day−1, respectively) decreased steeply in the furthest offshore LM site (22 ± 3, 55 ± 17 and −33 ± 15 µg C L−1day−1, respectively). Along this land-to-lake gradient, GP decreased by 96 ± 1%, whereas R only decreased by 75 ± 9%, variably influencing the carbon balance along this coastal zone. All ML sites were consistently net autotrophic (mean GP:R = 2.7), while the furthest offshore LM site was net heterotrophic (mean GP:R = 0.4). Our study suggests that pelagic waters of this Great Lakes coastal estuary are net carbon sinks that transition into net carbon sources offshore. Reactive and dynamic estuarine coastal zones everywhere may contribute similarly to regional and global carbon cycles