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1. Calibration of temperature-dependent ocean microbial processes in the cGENIE.muffin (v0.9.13) Earth system model

   https://doi.org/10.5194/gmd-14-125-2021  

   Crichton, Katherine A. ; Wilson, Jamie D. ; Ridgwell, Andy ; Pearson, Paul N. ( January 2021 , Geoscientific Model Development)

   null (Ed.)

   Abstract. Temperature is a master parameter in the marine carbon cycle, exerting a critical control on the rate of biological transformation of a variety of solid and dissolved reactants and substrates. Although in the construction of numerical models of marine carbon cycling, temperature has been long recognised as a key parameter in the production and export of organic matter at the ocean surface, its role in the ocean interior is much less frequently accounted for. There, bacteria (primarily) transform sinking particulate organic matter (POM) into its dissolved constituents and consume dissolved oxygen (and/or other electron acceptors such as sulfate). The nutrients and carbon thereby released then become available for transport back to the surface, influencing biological productivity and atmospheric pCO2, respectively. Given the substantial changes in ocean temperature occurring in the past, as well as in light of current anthropogenic warming, appropriately accounting for the role of temperature in marine carbon cycling may be critical to correctly projecting changes in ocean deoxygenation and the strength of feedbacks on atmosphericpCO2. Here we extend and calibrate a temperature-dependent representation ofmarine carbon cycling in the cGENIE.muffin Earth system model, intended forboth past and future climate applications. In this, we combine atemperature-dependent remineralisation scheme for sinking organic matterwith a biological export production scheme that also includes a dependenceon ambient seawater temperature. Via a parameter ensemble, we jointlycalibrate the two parameterisations by statistically contrasting model-projected fields of nutrients, oxygen, and the stable carbon isotopicsignature (δ13C) of dissolved inorganic carbon in the oceanwith modern observations. We additionally explore the role of temperature inthe creation and recycling of dissolved organic matter (DOM) and hence itsimpact on global carbon cycle dynamics. We find that for the present day, the temperature-dependent version showsa fit to the data that is as good as or better than the existing tuned non-temperature-dependent version of the cGENIE.muffin. The main impact ofaccounting for temperature-dependent remineralisation of POM is in drivinghigher rates of remineralisation in warmer waters, in turn driving a morerapid return of nutrients to the surface and thereby stimulating organicmatter production. As a result, more POM is exported below 80 m but onaverage reaches shallower depths in middle- and low-latitude warmer waterscompared to the standard model. Conversely, at higher latitudes, colderwater temperature reduces the rate of nutrient resupply to the surface andPOM reaches greater depth on average as a result of slower subsurface ratesof remineralisation. Further adding temperature-dependent DOM processeschanges this overall picture only a little, with a slight weakening ofexport production at higher latitudes. As an illustrative application of the new model configuration andcalibration, we take the example of historical warming and briefly assessthe implications for global carbon cycling of accounting for a more completeset of temperature-dependent processes in the ocean. We find that betweenthe pre-industrial era (ca. 1700) and the present (year 2010), in response to asimulated air temperature increase of 0.9 ∘C and an associatedprojected mean ocean warming of 0.12 ∘C (0.6 ∘C insurface waters and 0.02 ∘C in deep waters), a reduction inparticulate organic carbon (POC) export at 80 m of just 0.3 % occurs (or 0.7 % including a temperature-dependent DOM response). However, due to this increased recycling nearer the surface, the efficiency of the transfer of carbon away from the surface (at 80 m) to the deep ocean (at 1040 m) is reduced by 5 %. In contrast, with no assumed temperature-dependent processes impacting production or remineralisation of either POM or DOM, global POC export at 80 m falls by 2.9 % between the pre-industrial era and the present day as a consequence of ocean stratification and reduced nutrient resupply to the surface. Our analysis suggests that increased temperature-dependent nutrient recycling in the upper ocean has offset much of the stratification-induced restriction in its physical transport. 

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2. Characterization of the Fe metalloproteome of a ubiquitous marine heterotroph, Pseudoalteromonas (BB2-AT2): multiple bacterioferritin copies enable significant Fe storage

   https://doi.org/10.1039/d0mt00034e  

   Mazzotta, Michael G. ; McIlvin, Matthew R. ; Saito, Mak A. ( May 2020 , Metallomics)

   Fe is a critical nutrient to the marine biological pump, which is the process that exports photosynthetically fixed carbon in the upper ocean to the deep ocean. Fe limitation controls photosynthetic activity in major regions of the oceans, and the subsequent degradation of exported photosynthetic material is facilitated particularly by marine heterotrophic bacteria. Despite their importance in the carbon cycle and the scarcity of Fe in seawater, the Fe requirements, storage and cytosolic utilization of these marine heterotrophs has been less studied. Here, we characterized the Fe metallome of Pseudoalteromonas (BB2-AT2). We found that with two copies of bacterioferritin (Bfr), Pseudoalteromonas possesses substantial capacity for luxury uptake of Fe. Fe : C in the whole cell metallome was estimated (assuming C : P stoichiometry ∼51 : 1) to be between ∼83 μmol : mol Fe : C, ∼11 fold higher than prior marine bacteria surveys. Under these replete conditions, other major cytosolic Fe-associated proteins were observed including superoxide dismutase (SodA; with other metal SOD isoforms absent under Fe replete conditions) and catalase (KatG) involved in reactive oxygen stress mitigation and aconitase (AcnB), succinate dehydrogenase (FrdB) and cytochromes (QcrA and Cyt1) involved in respiration. With the aid of singular value decomposition (SVD), we were able to computationally attribute peaks within the metallome to specific metalloprotein contributors. A putative Fe complex TonB transporter associated with the closely related Alteromonas bacterium was found to be abundant within the Pacific Ocean mesopelagic environment. Despite the extreme scarcity of Fe in seawater, the marine heterotroph Pseudoalteromonas has expansive Fe storage capacity and utilization strategies, implying that within detritus and sinking particles environments, there is significant opportunity for Fe acquisition. Together these results imply an evolved dedication of marine Pseudoalteromonas to maintaining an Fe metalloproteome, likely due to its dependence on Fe-based respiratory metabolism. 

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3. The impact of marine nutrient abundance on early eukaryotic ecosystems

   https://doi.org/10.1111/gbi.12384  

   Reinhard, Christopher T. ; Planavsky, Noah J. ; Ward, Ben A. ; Love, Gordon D. ; Le Hir, Guillaume ; Ridgwell, Andy ( February 2020 , Geobiology)

   The rise of eukaryotes to ecological prominence represents one of the most dramatic shifts in the history of Earth's biosphere. However, there is an enigmatic temporal lag between the emergence of eukaryotic organisms in the fossil record and their much later ecological expansion. In parallel, there is evidence for a secular increase in the availability of the key macronutrient phosphorus (P) in Earth's oceans. Here, we use an Earth system model equipped with a size‐structured marine ecosystem to explore relationships between plankton size, trophic complexity, and the availability of marine nutrients. We find a strong dependence of planktonic ecosystem structure on ocean nutrient abundance, with a larger ocean nutrient inventory leading to greater overall biomass, broader size spectra, and increasing abundance of large Zooplankton. If existing estimates of Proterozoic marine nutrient levels are correct, our results suggest that increases in the ecological impact of eukaryotic algae and trophic complexity in eukaryotic ecosystems were directly linked to restructuring of the global P cycle associated with the protracted rise of surface oxygen levels. Our results thus suggest an indirect but potentially important mechanism by which ocean oxygenation may have acted to shape marine ecological function during late Proterozoic time.

    

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4. Reviews and syntheses: Present, past, and future of the oxygen minimum zone in the northern Indian Ocean

   https://doi.org/10.5194/bg-17-6051-2020  

   Rixen, Tim ; Cowie, Greg ; Gaye, Birgit ; Goes, Joaquim ; do Rosário Gomes, Helga ; Hood, Raleigh R. ; Lachkar, Zouhair ; Schmidt, Henrike ; Segschneider, Joachim ; Singh, Arvind ( January 2020 , Biogeosciences)

   null (Ed.)

   Abstract. Decreasing concentrations of dissolved oxygen in the ocean are considered one of the main threats to marine ecosystems as they jeopardize the growthof higher organisms. They also alter the marine nitrogen cycle, which isstrongly bound to the carbon cycle and climate. While higher organisms ingeneral start to suffer from oxygen concentrations < ∼ 63 µM (hypoxia), the marine nitrogen cycle responds to oxygenconcentration below a threshold of about 20 µM (microbial hypoxia),whereas anoxic processes dominate the nitrogen cycle at oxygenconcentrations of < ∼ 0.05 µM (functionalanoxia). The Arabian Sea and the Bay of Bengal are home to approximately21 % of the total volume of ocean waters revealing microbial hypoxia.While in the Arabian Sea this oxygen minimum zone (OMZ) is also functionallyanoxic, the Bay of Bengal OMZ seems to be on the verge of becoming so. Eventhough there are a few isolated reports on the occurrence of anoxia prior to1960, anoxic events have so far not been reported from the open northernIndian Ocean (i.e., other than on shelves) during the last 60 years.Maintenance of functional anoxia in the Arabian Sea OMZ with oxygenconcentrations ranging between > 0 and ∼ 0.05 µM is highly extraordinary considering that the monsoon reverses thesurface ocean circulation twice a year and turns vast areas of the ArabianSea from an oligotrophic oceanic desert into one of the most productiveregions of the oceans within a few weeks. Thus, the comparably lowvariability of oxygen concentration in the OMZ implies stable balancesbetween the physical oxygen supply and the biological oxygen consumption,which includes negative feedback mechanisms such as reducing oxygenconsumption at decreasing oxygen concentrations (e.g., reduced respiration).Lower biological oxygen consumption is also assumed to be responsible for aless intense OMZ in the Bay of Bengal. According to numerical model results,a decreasing physical oxygen supply via the inflow of water masses from thesouth intensified the Arabian Sea OMZ during the last 6000 years, whereas areduced oxygen supply via the inflow of Persian Gulf Water from the northintensifies the OMZ today in response to global warming. The first issupported by data derived from the sedimentary records, and the latterconcurs with observations of decreasing oxygen concentrations and aspreading of functional anoxia during the last decades in the Arabian Sea.In the Arabian Sea decreasing oxygen concentrations seem to have initiated aregime shift within the pelagic ecosystem structure, and this trend is alsoseen in benthic ecosystems. Consequences for biogeochemical cycles are asyet unknown, which, in addition to the poor representation of mesoscalefeatures in global Earth system models, reduces the reliability of estimatesof the future OMZ development in the northern Indian Ocean. 

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5. Decreasing Phanerozoic extinction intensity as a consequence of Earth surface oxygenation and metazoan ecophysiology

   https://doi.org/10.1073/pnas.2101900118  

   Stockey, Richard G. ; Pohl, Alexandre ; Ridgwell, Andy ; Finnegan, Seth ; Sperling, Erik A. ( October 2021 , Proceedings of the National Academy of Sciences)

   The decline in background extinction rates of marine animals through geologic time is an established but unexplained feature of the Phanerozoic fossil record. There is also growing consensus that the ocean and atmosphere did not become oxygenated to near-modern levels until the mid-Paleozoic, coinciding with the onset of generally lower extinction rates. Physiological theory provides us with a possible causal link between these two observations—predicting that the synergistic impacts of oxygen and temperature on aerobic respiration would have made marine animals more vulnerable to ocean warming events during periods of limited surface oxygenation. Here, we evaluate the hypothesis that changes in surface oxygenation exerted a first-order control on extinction rates through the Phanerozoic using a combined Earth system and ecophysiological modeling approach. We find that although continental configuration, the efficiency of the biological carbon pump in the ocean, and initial climate state all impact the magnitude of modeled biodiversity loss across simulated warming events, atmospheric oxygen is the dominant predictor of extinction vulnerability, with metabolic habitat viability and global ecophysiotype extinction exhibiting inflection points around 40% of present atmospheric oxygen. Given this is the broad upper limit for estimates of early Paleozoic oxygen levels, our results are consistent with the relative frequency of high-magnitude extinction events (particularly those not included in the canonical big five mass extinctions) early in the Phanerozoic being a direct consequence of limited early Paleozoic oxygenation and temperature-dependent hypoxia responses.

    

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