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Abstract BackgroundDiatoms are a class of algae that play an essential role in global ecology and produce valuable chemicals. They are known for forming intricate nanostructured silica cell walls (frustules). The introduction of non-siliceous elements like aluminum into diatoms induces properties such as a lower dissolution rate of the frustule, increasing the specific surface area of the frustule and enhancing metabolism. Previous studies have focused primarily on characterizing physiological impacts, leaving the genetic response(s) to non-siliceous elements largely unexplored. ResultsThis study investigates the transcriptional response of the pennate diatom,Entomoneis vertebralisto dissolved aluminum. Our findings reveal that in the presence of added 10 µM aluminum, biogenic silica content of the cell wall increases approximately twofold along with significant changes to core metabolism. An increase in transcription of genes encoding nitrate transporters has been observed despite the apparent downregulation of nitrogen assimilation pathways. Additionally, increased transcription of genes involved in carbon fixation were noted. Amino acid and protein motif analyses identified proteins that were differentially regulated shared amino acid compositions and motifs characteristic to silicification-associated genes. A unique structure-based analysis pipeline revealed that some of these proteins have a conserved structural core while being diverse in sequence, which could be features associated with biosilicification. ConclusionDifferential expression transcriptomics has provided insight into diatom metabolism when exposed to aluminum, highlighting potential targets for metabolic engineering. Furthermore, we identified potential silicification-associated genes using tools based on structure and amino acid composition, advancing our understanding of diatom silicification. 

Abstract The oceanic biogeochemical cycling of iron is globally important yet difficult to fully understand due to the many chemical processes involved. There is potential to use scandium, which has a similar ionic size and charge density to trivalent iron but lacks redox cycling, as a simpler analog for specific parts of the iron cycle, if we can sufficiently develop our understanding of scandium's reactivity. Here we move closer to this understanding. We look at particle reactivity and solubility through a 24‐hr incubation experiment: 5 nmol/kg of dissolved scandium and/or iron were added to filtered and unfiltered California Current System water. Particulate scandium formed only in the unfiltered treatments, at a quantity unlikely to have been taken up biologically. This is the first direct observation of scavenging of scandium, an attribute shared with iron. Our results also serve as the first test of scandium solubility in seawater: 1.9 nmol/kg of dissolved scandium was stable in the filtered treatment, 50 times more than the highest natural concentrations so far observed. This indicates that, in contrast to iron, scandium's oceanic cycling is unlikely to be influenced by solubility limits. We also compare particulate depth profiles: labile particulate iron was disproportionally higher than that of scandium in shelf‐influenced samples, likely due to iron reductively dissolving in the sediments, which scandium cannot do, and then precipitating in oxic seawater. Due to this combination of behaviors, our results suggest that paired observations of scandium and iron may help distinguish between iron sourced from sediment resuspension and reductive dissolution. 

The GEOTRACES program has greatly expanded measurements of trace elements, which serve as key nutrients, harmful contaminants, and tracers of ocean processes and past conditions. Many elements tend to associate with particulate matter, and GEOTRACES has been particularly valuable for growing our understanding of this fraction. Focusing on the micronutrient iron as an example, GEOTRACES data demonstrate that the majority of iron in the ocean is particulate. Chemically labile particulate iron, likely available for biological use, is also often more abundant than dissolved forms, particularly near continents and in the deep sea. This highlights the need to consider the particulate fraction in conceptual and numeric ocean models. Direct comparisons of particle-sampling methods highlight both the abundance of small particles (<0.45–0.8 μm), whose biogeochemical roles are still poorly known, and the difficulty in consistently capturing large, faster-sinking particles. In situ pumps with 0.8 μm filters often capture less small particulate iron than bottle-collected samples filtered onto 0.45 μm filters, but they can also capture more material near some sources. GEOTRACES datasets contain nearly sevenfold more dissolved than particulate iron measurements, and ongoing efforts to pair these measurements are needed in order to fully understand the cycles of iron and other important elements. 

Abstract Antarctic Bottom Water has been warming in recent decades throughout most of the oceans and freshening in regions close to its Indian and Pacific sector sources. We assess warming rates on isobars in the eastern Pacific sector of the Southern Ocean using CTD data collected from shipboard surveys from the early 1990s through the late 2010s together with CTD data collected from Deep Argo floats deployed in the region in January 2023. We show cooling and freshening in the temperature‐salinity relation for water colder than ∼0.4°C. We further find a recent acceleration in the regional bottom water warming rate vertically averaged for pressures exceeding 3,700 dbar, with the 2017/18 to 2023/24 trend of 7.5 (±0.9) m°C yr⁻¹nearly triple the 1992/95 to 2023/24 trend of 2.8 (±0.2) m°C yr⁻¹. The 0.2°C isotherm descent rate for these same time periods nearly quadruples from 7.8 to 28 m yr⁻¹. 

Biogeochemical cycles constitute Earth’s life support system and distinguish our planet from others in this solar system. Microorganisms are the primary drivers of these cycles. Understanding the controls on marine microbial dynamics and how microbes will respond to environmental change is essential for building and assessing model-based forecasts and generating robust projections of climate change impacts on ocean productivity and biogeochemical cycles. An international community effort has been underway to create a global-scale marine microbial biogeochemistry research program to tackle gaps in this understanding. The BioGeoSCAPES: Ocean Metabolism and Nutrient Cycles on a Changing Planet program will identify and quantify how marine microbes adjust to a changing climate and assess the consequences for global biogeochemical cycles. This article summarizes the ongoing efforts to launch BioGeoSCAPES. 

Abstract Particulate phases transport trace metals (TM) and thereby exert a major control on TM distribution in the ocean. Particulate TMs can be classified by their origin as lithogenic (crustal material), biogenic (cellular), or authigenic (formed in situ), but distinguishing these fractions analytically in field samples is a challenge often addressed using operational definitions and assumptions. These different phases require accurate characterization because they have distinct roles in the biogeochemical iron cycle. Particles collected from the upper 2,000 m of the northwest subtropical Atlantic Ocean over four seasonal cruises throughout 2019 were digested with a chemical leach to operationally distinguish labile particulate material from refractory lithogenics. Direct measurements of cellular iron (Fe) were used to calculate the biogenic contribution to the labile Fe fraction, and any remaining labile material was defined as authigenic. Total particulate Fe (PFe) inventories varied <15% between seasons despite strong seasonality in dust inputs. Across seasons, the total PFe inventory (±1SD) was composed of 73 ± 13% lithogenic, 18 ± 7% authigenic, and 10 ± 8% biogenic Fe above the deep chlorophyll maximum (DCM), and 69 ± 8% lithogenic, 30 ± 8% authigenic, and 1.1 ± 0.5% biogenic Fe below the DCM. Data from three other ocean regions further reveal the importance of the authigenic fraction across broad productivity and Fe gradients, comprising ca. 20%–27% of total PFe. 

Abstract Iron is a key micronutrient for ocean phytoplankton, and the availability of iron controls primary production and community composition in large regions of the ocean. Pennate diatoms, a phytoplankton group that responds to iron additions in low-iron areas, can have highly variable iron contents, and some groups such as Pseudo-nitzschia, are known to use ferritin to store iron for later use. We quantified and mapped the intracellular accumulation of iron by a natural population of Pseudo-nitzschia from the Fe-limited equatorial Pacific Ocean. A total of 48 h after iron addition, nearly half of the accumulated iron was localized in storage bodies adjacent to chloroplasts believed to represent ferritin. Over the subsequent 48 h, stored iron was distributed to the rest of the cell through subsequent growth and division, partially supporting the iron contents of the daughter cells. This study provides the first quantitative view into the cellular trafficking of iron in a globally relevant phytoplankton group and demonstrates the unique capabilities of synchrotron-based element imaging approaches. 

The biology and ecology of marine microbial eukaryotes is known to be constrained by oceanic conditions. In contrast, how viruses that infect this important group of organisms respond to environmental change is less well known, despite viruses being recognized as key microbial community members.