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Diatoms can survive long periods in dark, anoxic sediments by forming resting spores or resting cells. These have been considered dormant until recently when resting cells of Skeletonema marinoi were shown to assimilate nitrate and ammonium from the ambient environment in dark, anoxic conditions. Here, we show that resting cells of S. marinoi can also perform dissimilatory nitrate reduction to ammonium (DNRA), in dark, anoxic conditions. Transmission electron microscope analyses showed that chloroplasts were compacted, and few large mitochondria had visible cristae within resting cells. Using secondary ion mass spectrometry and isotope ratio mass spectrometry combined with stable isotopic tracers, we measured assimilatory and dissimilatory processes carried out by resting cells of S. marinoi under dark, anoxic conditions. Nitrate was both respired by DNRA and assimilated into biomass by resting cells. Cells assimilated nitrogen from urea and carbon from acetate, both of which are sources of dissolved organic matter produced in sediments. Carbon and nitrogen assimilation rates corresponded to turnover rates of cellular carbon and nitrogen content ranging between 469 and 10,000 years. Hence, diatom resting cells can sustain their cells in dark, anoxic sediments by slowly assimilating and respiring substrates from the ambient environment., (© 2024 The Authors. Environmental Microbiology published by John Wiley & Sons Ltd.)

Colony formation in phytoplankton is often considered a disadvantage during nutrient limitation in aquatic systems. Using stable isotopic tracers combined with secondary ion mass spectrometry (SIMS), we unravel cell-specific activities of a chain-forming diatom and interactions with attached bacteria. The uptake of 13 C-bicarbonate and 15 N-nitrate or 15 N-ammonium was studied in Chaetoceros affinis during the stationary growth phase. Low cell-to-cell variance of 13 C-bicarbonate and 15 N-nitrate assimilation within diatom chains prevailed during the early stationary phase. Up to 5% of freshly assimilated 13 C and 15 N was detected in attached bacteria within 12 h and supported bacterial C- and N-growth rates up to 0.026 h -1 . During the mid-stationary phase, diatom chain-length decreased and 13 C and 15 N-nitrate assimilation was significantly higher in solitary cells as compared to that in chain cells. During the late stationary phase, nitrate assimilation ceased and ammonium assimilation balanced C fixation. At this stage, we observed highly active cells neighboring inactive cells within the same chain. In N-limited regimes, bacterial remineralization of N and the short diffusion distance between neighbors in chains may support surviving cells. This combination of "microbial gardening" and nutrient transfer within diatom chains represents a strategy which challenges current paradigms of nutrient fluxes in plankton communities., (© 2023. The Author(s).)

Two events share the stage as main drivers of the Cretaceous-Paleogene mass extinction-Deccan Traps volcanism, and an asteroid impact recorded by the Chicxulub crater. We contribute to refining knowledge of the volcanic stressor by providing sulfur and fluorine budgets of Deccan lavas from the Western Ghats (India), which straddle the Cretaceous-Paleogene boundary. Volcanic fluorine budgets were variable (400 to 3000 parts per million) and probably sufficient to affect the environment, albeit only regionally. The highest sulfur budgets (up to 1800 parts per million) are recorded in Deccan lavas emplaced just prior (within 0.1 million years) to the extinction interval, whereas later basalts are generally sulfur-poor (up to 750 parts per million). Independent evidence suggests the Deccan flood basalts erupted in high-flux pulses. Our data suggest that volcanic sulfur degassing from such activity could have caused repeated short-lived global drops in temperature, stressing the ecosystems long before the bolide impact delivered its final blow at the end of the Cretaceous.

Recent Icelandic rifting events have illuminated the roles of centralized crustal magma reservoirs and lateral magma transport 1-4 , important characteristics of mid-ocean ridge magmatism 1,5 . A consequence of such shallow crustal processing of magmas 4,5 is the overprinting of signatures that trace the origin, evolution and transport of melts in the uppermost mantle and lowermost crust 6,7 . Here we present unique insights into processes occurring in this zone from integrated petrologic and geochemical studies of the 2021 Fagradalsfjall eruption on the Reykjanes Peninsula in Iceland. Geochemical analyses of basalts erupted during the first 50 days of the eruption, combined with associated gas emissions, reveal direct sourcing from a near-Moho magma storage zone. Geochemical proxies, which signify different mantle compositions and melting conditions, changed at a rate unparalleled for individual basaltic eruptions globally. Initially, the erupted lava was dominated by melts sourced from the shallowest mantle but over the following three weeks became increasingly dominated by magmas generated at a greater depth. This exceptionally rapid trend in erupted compositions provides an unprecedented temporal record of magma mixing that filters the mantle signal, consistent with processing in near-Moho melt lenses containing 10 7 -10 8  m 3 of basaltic magma. Exposing previously inaccessible parts of this key magma processing zone to near-real-time investigations provides new insights into the timescales and operational mode of basaltic magma systems., (© 2022. The Author(s).)

The ~31-km-wide Hiawatha structure, located beneath Hiawatha Glacier in northwestern Greenland, has been proposed as an impact structure that may have formed after the Pleistocene inception of the Greenland Ice Sheet. To date the structure, we conducted 40 Ar/ 39 Ar analyses on glaciofluvial sand and U-Pb analyses on zircon separated from glaciofluvial pebbles of impact melt rock, all sampled immediately downstream of Hiawatha Glacier. Unshocked zircon in the impact melt rocks dates to ~1915 million years (Ma), consistent with felsic intrusions found in local bedrock. The 40 Ar/ 39 Ar data indicate Late Paleocene resetting and shocked zircon dates to 57.99 ± 0.54 Ma, which we interpret as the impact age. Consequently, the Hiawatha impact structure far predates Pleistocene glaciation and is unrelated to either the Paleocene-Eocene Thermal Maximum or flood basalt volcanism in east Greenland. However, it was contemporaneous with the Paleocene Carbon Isotope Maximum, although the impact's exact paleoenvironmental and climatic significance awaits further investigation.

N 2 fixation constitutes an important new nitrogen source in the open sea. One group of filamentous N 2 fixing cyanobacteria (Richelia intracellularis, hereafter Richelia) form symbiosis with a few genera of diatoms. High rates of N 2 fixation and carbon (C) fixation have been measured in the presence of diatom-Richelia symbioses. However, it is unknown how partners coordinate C fixation and how the symbiont sustains high rates of N 2 fixation. Here, both the N 2 and C fixation in wild diatom-Richelia populations are reported. Inhibitor experiments designed to inhibit host photosynthesis, resulted in lower estimated growth and depressed C and N 2 fixation, suggesting that despite the symbionts ability to fix their own C, they must still rely on their respective hosts for C. Single cell analysis indicated that up to 22% of assimilated C in the symbiont is derived from the host, whereas 78-91% of the host N is supplied from their symbionts. A size-dependent relationship is identified where larger cells have higher N 2 and C fixation, and only N 2 fixation was light dependent. Using the single cell measures, the N-rich phycosphere surrounding these symbioses was estimated and contributes directly and rapidly to the surface ocean rather than the mesopelagic, even at high estimated sinking velocities (<10 m d -1 ). Several eco-physiological parameters necessary for incorporating symbiotic N 2 fixing populations into larger basin scale biogeochemical models (i.e., N and C cycles) are provided., (© 2021. The Author(s).)

Orbital data indicate that the youngest volcanic units on the Moon are basalt lavas in Oceanus Procellarum, a region with high levels of the heat-producing elements potassium, thorium, and uranium. The Chang’e-5 mission collected samples of these young lunar basalts and returned them to Earth for laboratory analysis. We measure an age of 1963 ± 57 million years for these lavas and determine their chemical and mineralogical compositions. This age constrains the lunar impact chronology of the inner Solar System and the thermal evolution of the Moon. There is no evidence for high concentrations of heat-producing elements in the deep mantle of the Moon that generated these lavas, so alternate explanations are required for the longevity of lunar magmatism.