Metabolic versatility of Caldarchaeales from geothermal features of Hawai'i and Chile as revealed by five metagenome-assembled genomes.

Members of the archaeal order Caldarchaeales (previously the phylum Aigarchaeota) are poorly sampled and are represented in public databases by relatively few genomes. Additional representative genomes will help resolve their placement among all known members of Archaea and provide insights into their roles in the environment. In this study, we analyzed 16S rRNA gene amplicons belonging to the Caldarchaeales that are available in public databases, which demonstrated that archaea of the order Caldarchaeales are diverse, widespread, and most abundant in geothermal habitats. We also constructed five metagenome-assembled genomes (MAGs) of Caldarchaeales from two geothermal features to investigate their metabolic potential and phylogenomic position in the domain Archaea. Two of the MAGs were assembled from microbial community DNA extracted from fumarolic lava rocks from Mauna Ulu, Hawai'i, and three were assembled from DNA obtained from hot spring sinters from the El Tatio geothermal field in Chile. MAGs from Hawai'i are high quality bins with completeness >95% and contamination <1%, and one likely belongs to a novel species in a new genus recently discovered at a submarine volcano off New Zealand. MAGs from Chile have lower completeness levels ranging from 27 to 70%. Gene content of the MAGs revealed that these members of Caldarchaeales are likely metabolically versatile and exhibit the potential for both chemoorganotrophic and chemolithotrophic lifestyles. The wide array of metabolic capabilities exhibited by these members of Caldarchaeales might help them thrive under diverse harsh environmental conditions. All the MAGs except one from Chile harbor putative prophage regions encoding several auxiliary metabolic genes (AMGs) that may confer a fitness advantage on their Caldarchaeales hosts by increasing their metabolic potential and make them better adapted to new environmental conditions. Phylogenomic analysis of the five MAGs and over 3,000 representative archaeal genomes showed the order Caldarchaeales forms a monophyletic group that is sister to the clade comprising the orders Geothermarchaeales (previously Candidatus Geothermarchaeota), Conexivisphaerales and Nitrososphaerales (formerly known as Thaumarchaeota), supporting the status of Caldarchaeales members as a clade distinct from the Thaumarchaeota.

Globally-distributed microbial eukaryotes exhibit endemism at deep-sea hydrothermal vents.

Single-celled microbial eukaryotes inhabit deep-sea hydrothermal vent environments and play critical ecological roles in the vent-associated microbial food web. 18S rRNA amplicon sequencing of diffuse venting fluids from four geographically- and geochemically-distinct hydrothermal vent fields was applied to investigate community diversity patterns among protistan assemblages. The four vent fields include Axial Seamount at the Juan de Fuca Ridge, Sea Cliff and Apollo at the Gorda Ridge, all in the NE Pacific Ocean, and Piccard and Von Damm at the Mid-Cayman Rise in the Caribbean Sea. We describe species diversity patterns with respect to hydrothermal vent field and sample type, identify putative vent endemic microbial eukaryotes, and test how vent fluid geochemistry may influence microbial community diversity. At a semi-global scale, microbial eukaryotic communities at deep-sea vents were composed of similar proportions of dinoflagellates, ciliates, Rhizaria, and stramenopiles. Individual vent fields supported distinct and highly diverse assemblages of protists that included potentially endemic or novel vent-associated strains. These findings represent a census of deep-sea hydrothermal vent protistan communities. Protistan diversity, which is shaped by the hydrothermal vent environment at a local scale, ultimately influences the vent-associated microbial food web and the broader deep-sea carbon cycle.

Mission Architecture Using the SpaceX Starship Vehicle to Enable a Sustained Human Presence on Mars.

A main goal of human space exploration is to develop humanity into a multi-planet species where civilization extends beyond planet Earth. Establishing a self-sustaining human presence on Mars is key to achieving this goal. In situ resource utilization (ISRU) on Mars is a critical component to enabling humans on Mars to both establish long-term outposts and become self-reliant. This article focuses on a mission architecture using the SpaceX Starship as cargo and crew vehicles for the journey to Mars. The first Starships flown to Mars will be uncrewed and will provide unprecedented opportunities to deliver ∼100 metric tons of cargo to the martian surface per mission and conduct robotic precursor work to enable a sustained and self-reliant human presence on Mars. We propose that the highest priority activities for early uncrewed Starships include pre-placement of supplies, developing infrastructure, testing of key technologies, and conducting resource prospecting to map and characterize water ice for future ISRU purposes.

Organic Material Distribution in Mars-Analog Volcanic Rocks, as Determined with Ultraviolet Laser-Induced Fluorescence Spectroscopy.

Understanding the distribution of trace organic material in a rocky environment is a key to constraining the material requirements for sustaining microbial life. We used an ultraviolet laser-induced fluorescence (LIF) spectroscopy instrument to characterize the distribution of organic biosignatures in basalts collected from two Mars-analog environments. We correlated the fluorescence results with alteration-related sample properties. These samples exhibit a range of alteration conditions found in the volcanic environments of Hawai'i Volcanoes National Park, Hawai'i (HI), and Craters of the Moon National Monument, Idaho (ID), including fumarolic systems. LIF mapping of the sample surfaces and interiors showed a heterogeneous distribution of areas of highly fluorescent material (point[s]-of-interest [POIs])-with fluorescence characteristics indicative of organic material. Results suggest that POIs are associated with secondary alteration mineral deposits in the rock's vesicles, including zeolites and calcite. Scanning electron microscopy with electron-dispersive X-ray spectroscopy was used to characterize the mineralogy present at POIs and support the evidence of carbon-bearing material. Overall, samples collected proximate to active or relict meteoric fumaroles from Hawai'i were shown to contain evidence for organic deposits. This suggests that these minerals are measurable spectroscopic targets that may be used to inform sample-site selection for astrobiology research.

Biosignatures Associated with Freshwater Microbialites.

Freshwater microbialites (i.e., lithifying microbial mats) are quite rare in northern latitudes of the North American continent, with two lakes (Pavilion and Kelly Lakes) of southeastern BC containing a morphological variety of such structures. We investigated Kelly Lake microbialites using carbon isotope systematics, phospholipid fatty acids (PLFAs) and quantitative PCR to obtain biosignatures associated with microbial metabolism. δ13CDIC values (mean δ13CDIC -4.9 ± 1.1‱, n = 8) were not in isotopic equilibrium with the atmosphere; however, they do indicate 13C-depleted inorganic carbon into Kelly Lake. The values of carbonates on microbialite surfaces (δ13C) fell within the range predicted for equilibrium precipitation from ambient lake water δ13CDIC (-2.2 to -5.3‱). Deep microbialites (26 m) had an enriched δ13Ccarb value of -0.3 ± 0.5‱, which is a signature of photoautotrophy. The deeper microbialites (>20 m) had higher biomass estimates (via PLFAs), and a greater relative abundance of cyanobacteria (measured by 16S copies via qPCR). The majority of PLFAs constituted monounsaturated and saturated PLFAs, which is consistent with gram-negative bacteria, including cyanobacteria. The central PLFA δ13C values were highly depleted (-9.3 to -15.7‱) relative to δ13C values of bulk organic matter, suggesting a predominance of photoautotrophy. A heterotrophic signature was also detected via the depleted iso- and anteiso-15:0 lipids (-3.2 to -5.2‱). Based on our carbonate isotopic biosignatures, PLFA, and qPCR measurements, photoautotrophy is enriched in the microbialites of Kelly Lake. This photoautotrophy enrichment is consistent with the microbialites of neighboring Pavilion Lake. This indication of photoautotrophy within Kelly Lake at its deepest depths raises new insights into the limits of measurable carbonate isotopic biosignatures under light and nutrient limitations.

Using Science-Driven Analog Research to Investigate Extravehicular Activity Science Operations Concepts and Capabilities for Human Planetary Exploration.

Biologic Analog Science Associated with Lava Terrains (BASALT) is a science-driven exploration program seeking to determine the best tools, techniques, training requirements, and execution strategies for conducting Mars-relevant field science under spaceflight mission conditions. BASALT encompasses Science, Science Operations, and Technology objectives. This article outlines the BASALT Science Operations background, strategic research questions, study design, and a portion of the results from the second field test. BASALT field tests are used to iteratively develop, integrate, test, evaluate, and refine new concepts of operations (ConOps) and capabilities that enable efficient and productive science. This article highlights the ConOps investigated during BASALT in light of future planetary extravehicular activity (EVA), which will focus on scientific exploration and discovery, and serves as an introduction to integrating exploration flexibility with operational rigor, the value of tactical and strategic science planning and execution, and capabilities that enable and enhance future science EVA operations.

Strategic Planning Insights for Future Science-Driven Extravehicular Activity on Mars.

Short-term and long-term science plans were developed as part of the strategic planning process used by the Biologic Analog Science Associated with Lava Terrains (BASALT) science team to conduct two Mars-simulation missions investigating basalt habitability at terrestrial volcanic analog sites in 2016. A multidisciplinary team of scientists generated and codified a range of scientific hypotheses distilled into a Science Traceability Matrix (STM) that defined the set of objectives pursued in a series of extravehicular activity (EVA) campaigns performed across multiple field deployments. This STM was used to guide the pre-deployment selection of sampling stations within the selected Mars analog sites on the Earth based on precursor site information such as multispectral imagery. It also informed selection of hand-held instruments and observational data to collect during EVA to aid sample selection through latency-impacted interaction with an Earth-based Science Support Team. A significant portion of the pre-deployment strategic planning activities were devoted to station selection, ultimately the locations used for sample collection and EVA planning. During development of the EVAs, the BASALT science team identified lessons learned that could be used to inform future missions and analog activities, including the critical need for high-resolution precursor imagery that would enable the selection of stations that could meet the scientific objectives outlined in the STM.

A Low-Diversity Microbiota Inhabits Extreme Terrestrial Basaltic Terrains and Their Fumaroles: Implications for the Exploration of Mars.

A major objective in the exploration of Mars is to test the hypothesis that the planet hosted life. Even in the absence of life, the mapping of habitable and uninhabitable environments is an essential task in developing a complete understanding of the geological and aqueous history of Mars and, as a consequence, understanding what factors caused Earth to take a different trajectory of biological potential. We carried out the aseptic collection of samples and comparison of the bacterial and archaeal communities associated with basaltic fumaroles and rocks of varying weathering states in Hawai'i to test four hypotheses concerning the diversity of life in these environments. Using high-throughput sequencing, we found that all these materials are inhabited by a low-diversity biota. Multivariate analyses of bacterial community data showed a clear separation between sites that have active fumaroles and other sites that comprised relict fumaroles, unaltered, and syn-emplacement basalts. Contrary to our hypothesis that high water flow environments, such as fumaroles with active mineral leaching, would be sites of high biological diversity, alpha diversity was lower in active fumaroles compared to relict or nonfumarolic sites, potentially due to high-temperature constraints on microbial diversity in fumarolic sites. A comparison of these data with communities inhabiting unaltered and weathered basaltic rocks in Idaho suggests that bacterial taxon composition of basaltic materials varies between sites, although the archaeal communities were similar in Hawai'i and Idaho. The taxa present in both sites suggest that most of them obtain organic carbon compounds from the atmosphere and from phototrophs and that some of them, including archaeal taxa, cycle fixed nitrogen. The low diversity shows that, on Earth, extreme basaltic terrains are environments on the edge of sustaining life with implications for the biological potential of similar environments on Mars and their exploration by robots and humans.

A Flexible Telecommunication Architecture for Human Planetary Exploration Based on the BASALT Science-Driven Mars Analog.

There is a synergistic relationship between analog field testing and the deep space telecommunication capabilities necessary for future human exploration. The BASALT (Biologic Analog Science Associated with Lava Terrains) research project developed and implemented a telecommunications architecture that serves as a high-fidelity analog of future telecommunication capabilities for Mars. This paper presents the architecture and its constituent elements. The rationale for the various protocols and radio frequency (RF) link types required to enable an interdisciplinary field mission are discussed, and the performance results from the BASALT field tests are provided. Extravehicular Informatics Backpacks (EVIB) designed for BASALT and tested by human subjects are also discussed, and the proceeding sections show how these prototype extravehicular activity (EVA) information systems can augment future human exploration. The paper concludes with an aggregate analysis of the data product types and data volumes generated, transferred, and utilized by the ground team and explorers over the course of the field deployments.

Developing Future Deep-Space Telecommunication Architectures: A Historical Look at the Benefits of Analog Research on the Development of Solar System Internetworking for Future Human Spaceflight.

Exploration analog field tests, missions, and deployments enable the integration and validation of new and experimental concepts and/or technologies through strategic experimental design. The results of these operations often create new capabilities for exploration and increase confidence in, and credibility of, emerging technologies, usually at very low cost and risk to the test subjects involved. While these experiments resemble missions 10-30 years into the future, insights obtained are often of immediate value. Knowledge gained in the field translates into strategic planning data to assist long-range exploration planners, and planners influence the experimental design of field deployments, creating a synergistic relationship. The Biologic Analog Science Associated with Lava Terrains (BASALT) communication architecture is a high-fidelity analog program that emulates conditions impacting future explorers on the martian surface. This article provides (1) a brief historical review of past analog operations that deliberately used elements of a flight-like telecommunication infrastructure to add fidelity to the test, (2) samples of the accomplishments made through analog operations, and (3) potentially significant deep-space telecommunication insights gained from the BASALT program in support of future extravehicular activity exploration of Mars. This article is paired with and complements Miller et al. in this issue which focuses on the telecommunication infrastructure utilized by the BASALT team during the field deployment.

Requirements for Portable Instrument Suites during Human Scientific Exploration of Mars.

Human explorers on the surface of Mars will have access to a far wider array of scientific tools than previous crewed planetary exploration missions, but not every tool will be compatible with the restrictions of this exploration. Spectrometers on flyby, orbital, and landed missions are currently used to determine the composition and mineralogy of geological materials of various types and sizes, from small fragments to celestial bodies in the solar system. Handheld spectrometers that are capable of in situ analyses are already used for geological exploration on Earth; however, their usefulness for human exploration missions and how data from multiple handheld instruments could be combined to enhance scientific return must be further evaluated. As part of the Biologic Analog Science Associated with Lava Terrains (BASALT) research project, we incorporated two handheld instruments, a visible-near infrared spectrometer and an X-Ray Fluorescence spectrometer, into simulated Mars exploration missions conducted on basaltic terrains in Idaho and Hawai'i. To understand the data quality provided by these handheld spectrometers, we evaluated their performance under varying conditions of measurement time, distance, angle, atmosphere, and sample matrix, and we compared data quality between handheld instruments and laboratory techniques. Here, we summarize these findings, provide guidelines and requirements on how to effectively incorporate these instruments into human exploration missions to Mars, and posit that future iterations of these instruments will be beneficial for enhancing science returned from human exploration missions.

Opportunities and Challenges of Promoting Scientific Dialog throughout Execution of Future Science-Driven Extravehicular Activity.

Science-driven, human spaceflight missions of the future will rely on regular and interactive communication between Earth- and space-based teams during activity in which astronauts work directly on Mars or other planetary surfaces (extravehicular activity, EVA). The Biologic Analog Science Associated with Lava Terrains (BASALT) project conducted simulated human missions to Mars, complete with realistic one-way light time (OWLT) communication latency. We discuss the modes of communication used by the Mars- and Earth-based teams, including text, audio, video, and still imagery. Real-time communication between astronauts in the field (extravehicular, EV) and astronauts in a communication relay station (intravehicular, IV) was broadcast over OWLT, providing important contextual information to the Science Backroom Team (SBT) in Mission Control. Collaborative communication between the Earth- and Mars-based teams, however, requires active communication across latency via the Mission Log. We provide descriptive statistics of text communication between IV and SBT in a high-fidelity, scientifically driven analog for human space exploration. Over an EVA, the SBT sent an average of ∼23 text messages containing recommendations, requests, and answers to questions, while the science-focused IV crew member (IV2) sent an average of ∼38 text messages. Though patterns varied, communication between the IV and SBT teams tended to be highest during ∼50-150 min into the EVA, corresponding to the candidate sample search and presampling instrument survey phases, and then decreased dramatically after minute ∼200 during the sample collection phase. Generally, the IV2 and SBT used ∼4.6 min to craft a reply to a direct question or comment, regardless of message length or OWLT, offering a valuable glimpse into actual time-to-reply. We discuss IV2-SBT communication within the context of case examples from an EVA during which communication failures affected operations in the field. Finally, we offer recommendations for communication practices for use in future analogs and, perhaps, science-driven human spaceflight.

The BASALT Research Program: Designing and Developing Mission Elements in Support of Human Scientific Exploration of Mars.

The articles associated with this Special Collection focus on the NASA BASALT (Biologic Analog Science Associated with Lava Terrains) Research Program, which aims at answering the question, "How do we support and enable scientific exploration during human Mars missions?" To answer this the BASALT team conducted scientific field studies under simulated Mars mission conditions to both broaden our understanding of the habitability potential of basalt-rich terrains on Mars and examine the effects of science on current Mars mission concepts of operations. This article provides an overview of the BASALT research project, from the science, to the operational concepts that were tested and developed, to the technical capabilities that supported all elements of the team's research. Further, this article introduces the 12 articles that are included in this Special Collection.

Basaltic Terrains in Idaho and Hawai'i as Planetary Analogs for Mars Geology and Astrobiology.

Field research target regions within two basaltic geologic provinces are described as Earth analogs to Mars. Regions within the eastern Snake River Plain of Idaho and the Big Island of Hawai'i, the United States, provinces that represent analogs of present-day and early Mars, respectively, were evaluated on the basis of geologic settings, rock lithology and geochemistry, rock alteration, and climate. Each of these factors provides rationale for the selection of specific targets for field research in five analog target regions: (1) Big Craters and (2) Highway lava flows at Craters of the Moon National Monument and Preserve, Idaho, and (3) Mauna Ulu low shield, (4) Kilauea Iki lava lake, and (5) Kilauea caldera in the Kilauea Volcano summit region and the East Rift Zone of Hawai'i. Our evaluation of compositional and textural attributes, as well as the effects of syn- and posteruptive rock alteration, shows that basaltic terrains in Idaho and Hawai'i provide a way to characterize the geology and major geologic substrates that host biological activity of relevance to Mars exploration. This work provides the foundation to better understand the scientific questions related to the habitability of basaltic terrains, the rationale behind selecting analog field targets, and their applicability as analogs to Mars.

The Complete Genome and Physiological Analysis of the Microbialite-Dwelling Agrococcus pavilionensis sp. nov; Reveals Genetic Promiscuity and Predicted Adaptations to Environmental Stress.

Members of the bacterial genus Agrococcus are globally distributed and found across environments so highly diverse that they include forests, deserts, and coal mines, as well as in potatoes and cheese. Despite how widely Agrococcus occurs, the extent of its physiology, genomes, and potential roles in the environment are poorly understood. Here we use whole-genome analysis, chemotaxonomic markers, morphology, and 16S rRNA gene phylogeny to describe a new isolate of the genus Agrococcus from freshwater microbialites in Pavilion Lake, British Columbia, Canada. We characterize this isolate as a new species Agrococcus pavilionensis strain RW1 and provide the first complete genome from a member of the genus Agrococcus. The A. pavilionensis genome consists of one chromosome (2,627,177 bp) as well as two plasmids (HC-CG1 1,427 bp, and LC-RRW783 31,795 bp). The genome reveals considerable genetic promiscuity via mobile elements, including a prophage and plasmids involved in integration, transposition, and heavy-metal stress. A. pavilionensis strain RW1 differs from other members of the Agrococcus genus by having a novel phospholipid fatty acid iso-C15:1Δ4, ß-galactosidase activity and amygdalin utilization. Carotenoid biosynthesis is predicted by genomic metabolic reconstruction, which explains the characteristic yellow pigmentation of A. pavilionensis. Metabolic reconstructions of strain RW1 genome predicts a pathway for releasing ammonia via ammonification amino acids, which could increase the saturation index leading to carbonate precipitation. Our genomic analyses suggest signatures of environmental adaption to the relatively cold and oligotrophic conditions of Pavilion Lake microbialites. A. pavilionensis strain RW1 in modern microbialites has an ecological significance in Pavilion Lake microbialites, which include potential roles in heavy-metal cycling and carbonate precipitation (e.g., ammonification of amino acids and filamentation which many trap carbonate minerals).

The Complete Genome and Physiological Analysis of the Eurythermal Firmicute Exiguobacterium chiriqhucha Strain RW2 Isolated From a Freshwater Microbialite, Widely Adaptable to Broad Thermal, pH, and Salinity Ranges.

Members of the genus Exiguobacterium are found in diverse environments from marine, freshwaters, permafrost to hot springs. Exiguobacterium can grow in a wide range of temperature, pH, salinity, and heavy-metal concentrations. We characterized Exiguobacterium chiriqhucha strain RW2 isolated from a permanently cold freshwater microbialite in Pavilion Lake, British Columbia using metabolic assays, genomics, comparative genomics, phylogenetics, and fatty acid composition. Strain RW2 has the most extensive growth range for temperature (4-50°C) and pH (5-11) of known Exiguobacterium isolates. Strain RW2 genome predicts pathways for wide differential thermal, cold and osmotic stress using cold and heat shock cascades (e.g., csp and dnaK), choline and betaine uptake/biosynthesis (e.g., opu and proU), antiporters (e.g., arcD and nhaC Na+/K+), membrane fatty acid unsaturation and saturation. Here, we provide the first complete genome from Exiguobacterium chiriqhucha strain RW2, which was isolated from a freshwater microbialite. Its genome consists of a single 3,019,018 bp circular chromosome encoding over 3,000 predicted proteins, with a GC% content of 52.1%, and no plasmids. In addition to growing at a wide range of temperatures and salinities, our findings indicate that RW2 is resistant to sulfisoxazole and has the genomic potential for detoxification of heavy metals (via mercuric reductases, arsenic resistance pumps, chromate transporters, and cadmium-cobalt-zinc resistance genes), which may contribute to the metabolic potential of Pavilion Lake microbialites. Strain RW2 could also contribute to microbialite formation, as it is a robust biofilm former and encodes genes involved in the deamination of amino acids to ammonia (i.e., L-asparaginase/urease), which could potentially boost carbonate precipitation by lowering the local pH and increasing alkalinity. We also used comparative genomic analysis to predict the pathway for orange pigmentation that is conserved across the entire Exiguobacterium genus, specifically, a C30 carotenoid biosynthesis pathway is predicted to yield diaponeurosporene-4-oic acid as its final product. Carotenoids have been found to protect against ultraviolet radiation by quenching reactive oxygen, releasing excessive light energy, radical scavenging, and sunscreening. Together these results provide further insight into the potential of Exiguobacterium to exploit a wide range of environmental conditions, its potential roles in ecosystems (e.g., microbialites/microbial mats), and a blueprint model for diverse metabolic processes.

Metagenomic Analysis Suggests Modern Freshwater Microbialites Harbor a Distinct Core Microbial Community.

Modern microbialites are complex microbial communities that interface with abiotic factors to form carbonate-rich organosedimentary structures whose ancestors provide the earliest evidence of life. Past studies primarily on marine microbialites have inventoried diverse taxa and metabolic pathways, but it is unclear which of these are members of the microbialite community and which are introduced from adjacent environments. Here we control for these factors by sampling the surrounding water and nearby sediment, in addition to the microbialites and use a metagenomics approach to interrogate the microbial community. Our findings suggest that the Pavilion Lake microbialite community profile, metabolic potential and pathway distributions are distinct from those in the neighboring sediments and water. Based on RefSeq classification, members of the Proteobacteria (e.g., alpha and delta classes) were the dominant taxa in the microbialites, and possessed novel functional guilds associated with the metabolism of heavy metals, antibiotic resistance, primary alcohol biosynthesis and urea metabolism; the latter may help drive biomineralization. Urea metabolism within Pavilion Lake microbialites is a feature not previously associated in other microbialites. The microbialite communities were also significantly enriched for cyanobacteria and acidobacteria, which likely play an important role in biomineralization. Additional findings suggest that Pavilion Lake microbialites are under viral selection as genes associated with viral infection (e.g CRISPR-Cas, phage shock and phage excision) are abundant within the microbialite metagenomes. The morphology of Pavilion Lake microbialites changes dramatically with depth; yet, metagenomic data did not vary significantly by morphology or depth, indicating that microbialite morphology is altered by other factors, perhaps transcriptional differences or abiotic conditions. This work provides a comprehensive metagenomic perspective of the interactions and differences between microbialites and their surrounding environment, and reveals the distinct nature of these complex communities.

Climate change and mercury accumulation in Canadian high and subarctic lakes.

Mercury (Hg) profiles were compared to profiles of climate indicators including microfossil remains and algal-derived or S2 carbon (C) in dated sediment cores from 14 lakes spanning latitudinal and longitudinal gradients across the Canadian high and subarctic. Hg fluxes increased postindustrialization (post-∼1850) in 11 of these lakes (postindustrialization Hg fluxes (ΔHgF(F)) = 2-24 µg m(-2) y(-1)). Correction of HgF(F) for catchment contributions demonstrated that Hg deposition originating from catchment-independent factors, such as atmospheric deposition, increased since industrialization in all 14 lakes. Several of these lakes also showed postindustrial shifts in algal assemblages consistent with climate-induced changes. Eleven lakes showed post-1850s increases in S2F(F), suggesting that lake primary productivity has recently increased in the majority of our sites (ΔS2F(F) = 0.1-4 g m(-2) y(-1)). Other studies have interpreted significant relationships between Hg:S2 concentrations in Arctic sediment as support for the algal scavenging hypothesis, which postulates that Hg fluxes to Arctic sediments are largely driven by S2. However, in six of our lakes we observed no Hg:S2 relationship, and in one lake a significant negative Hg:S2 relationship was observed due to increased Hg and decreased S2 C deposition during the postindustrialization period. In six of the seven lakes where a significant positive Hg:S2 relationship was observed, algal assemblages either did not change through time or the timing of the shifts did not correspond to changes in Hg deposition. Our results demonstrate that, although Arctic lakes are experiencing a myriad of changes, including increased Hg and S2 deposition or changing algal assemblages, increased lake primary productivity does not appear to be driving changes in Hg fluxes to sediments.