Archaeal membrane lipids known as glycerol dibiphytanyl glycerol tetraethers (GDGTs) are the basis of the TEX 86 paleotemperature proxy. Because GDGTs preserved in marine sediments are thought to originate mainly from planktonic, ammonia-oxidizing Thaumarchaeota, the basis of the correlation between TEX 86 and sea surface temperature (SST) remains unresolved: How does TEX 86 predict surface temperatures, when maximum thaumarchaeal activity occurs below the surface mixed layer and TEX 86 does not covary with in situ growth temperatures? Here we used isothermal studies of the model thaumarchaeon Nitrosopumilus maritimus SCM1 to investigate how GDGT composition changes in response to ammonia oxidation rate. We used continuous culture methods to avoid potential confounding variables that can be associated with experiments in batch cultures. The results show that the ring index scales inversely (R 2 = 0.82) with ammonia oxidation rate (ϕ), indicating that GDGT cyclization depends on available reducing power. Correspondingly, the TEX 86 ratio decreases by an equivalent of 5.4°C of calculated temperature over a 5.5 fmol·cell −1 ·d −1 increase in ϕ. This finding reconciles other recent experiments that have identified growth stage and oxygen availability as variables affecting TEX 86 . Depth profiles from the marine water column show minimum TEX 86 values at the depth of maximum nitrification rates, consistent with our chemostat results. Our findings suggest that the TEX 86 signal exported from the water column is influenced by the dynamics of ammonia oxidation. Thus, the global TEX 86 -SST calibration potentially represents a composite of regional correlations based on nutrient dynamics and global correlations based on archaeal community composition and temperature.T he glycerol dibiphytanyl glycerol tetraether (GDGT) membrane lipids of Archaea are abundant in marine water columns and sediments. The major source of GDGTs to ocean sediments is thought to be planktonic, ammonia-oxidizing Archaea (AOA) affiliated with the phylum Thaumarchaeota (formerly Marine Group I Crenarchaeota) (1, 2). Thaumarchaeota play a primary role in the nitrogen cycle, performing the first and rate-limiting step of nitrification-namely, the oxidation of ammonia to nitrite (3-5). Accordingly, Thaumarchaeota are most abundant at the base of, or below, the euphotic zone (6-9). Based on the phylogeny of their ammonia monooxygenase gene, the planktonic Thaumarchaeota are divided into two distinct clusters, the Water Column Cluster A that is most abundant in the epi-and upper mesopelagic (above ∼200 to ∼500 m, depending on location) and the Water Column Cluster B that dominates thaumarchaeal assemblages in the deeper mesopelagic and bathypelagic (7-10). These clusters putatively represent thaumarchaeal ecotypes adapted to high and low ammonium flux, respectively (11,12).Thaumarchaeota produce GDGTs containing from zero to four cyclopentane rings (GDGT-0 to GDGT-4) or four cyclopentane rings and one additional cyclohexane ring (e.g., in crenarchaeol;...

No abstract

No abstract

No abstract

No abstract

Ancestral cyanobacteria are assumed to be prominent primary producers after the Great Oxidation Event [≈2.4 to 2.0 billion years (Ga) ago], but carbon isotope fractionation by extant marine cyanobacteria (α-cyanobacteria) is inconsistent with isotopic records of carbon fixation by primary producers in the mid-Proterozoic eon (1.8 to 1.0 Ga ago). To resolve this disagreement, we quantified carbon isotope fractionation by a wild-type planktic β-cyanobacterium (Synechococcus sp. PCC 7002), an engineered Proterozoic analog lacking a CO2-concentrating mechanism, and cyanobacterial mats. At mid-Proterozoic pH and pCO2 values, carbon isotope fractionation by the wild-type β-cyanobacterium is fully consistent with the Proterozoic carbon isotope record, suggesting that cyanobacteria with CO2-concentrating mechanisms were apparently the major primary producers in the pelagic Proterozoic ocean, despite atmospheric CO2 levels up to 100 times modern. The selectively permeable microcompartments central to cyanobacterial CO2-concentrating mechanisms (“carboxysomes”) likely emerged to shield rubisco from O2 during the Great Oxidation Event.

[2] Two commonly used proxies based on the distribution of glycerol dialkyl glycerol tetraethers (GDGTs) are the TEX 86 (TetraEther indeX of 86 carbon atoms) paleothermometer for sea surface temperature reconstructions and the BIT (Branched Isoprenoid Tetraether) index for reconstructing soil organic matter input to the ocean. An initial round-robin study of two sediment extracts, in which 15 laboratories participated, showed relatively consistent TEX 86 values (reproducibility 63-4 C when translated to temperature) but a large spread in BIT measurements (reproducibility 60.41 on a scale of 0-1). Here we report results of a second round-robin study with 35 laboratories in which three sediments, one sediment extract, and two mixtures of pure, isolated GDGTs were analyzed. The results for TEX 86 and BIT index showed improvement compared to the previous round-robin study. The reproducibility, indicating interlaboratory variation, of TEX 86 values ranged from 1.3 to 3.0 C when translated to temperature. These results are similar to those of other temperature proxies used in paleoceanography. Comparison of the results obtained from one of the three sediments showed that TEX 86 and BIT indices are not significantly affected by interlaboratory differences in sediment extraction techniques. BIT values of the sediments and extracts were at the extremes of the index with values close to 0 or 1, and showed good reproducibility (ranging from 0.013 to 0.042). However, the measured BIT values for the two GDGT mixtures, with known molar ratios of crenarchaeol and branched GDGTs, had intermediate BIT values and showed poor reproducibility and a large overestimation of the ''true'' (i.e., molarbased) BIT index. The latter is likely due to, among other factors, the higher mass spectrometric response of branched GDGTs compared to crenarchaeol, which also varies among mass spectrometers. Correction for this different mass spectrometric response showed a considerable improvement in the reproducibility of BIT index measurements among laboratories, as well as a substantially improved estimation of molar-based BIT values. This suggests that standard mixtures should be used in order to obtain consistent, and molar-based, BIT values.

The scale and proportions of “streetscape skeletons,” the three-dimensional spaces of streets defined by the massing and arrangement of surrounding buildings, are theoretically relevant to the way human users perceive and behave. Nonetheless, the dominant ways of measuring and identifying streets emphasize vehicular service and functionality. Moreover, existing built environment-based classifications have focused on recommended forms rather than characterizing the full range of existing conditions that must be accounted for in policy and understanding of human–environment interactions. To work toward a better streetscape measurement and classification scheme, this study investigated how large numbers of streetscapes could be efficiently measured to evaluate design patterns across and between multiple cities. Using a novel GIS-based method, 12 streetscape skeleton variables were measured on more than 120,000 block-length streetscapes in three northeastern U.S. cities: Boston, MA, New York, NY, and Baltimore, MD. Logistic regression models based on these variables were unsuccessful at distinguishing between cities, confirming that the variables were similarly applicable to each city and that the cities had comparable streetscape skeleton identities. Cluster analyses were then used to identify four streetscape skeleton classes that were also consistent between cities: upright, compact, porous, and open. These classes were distinct from the widely used highway functional class system, reinforcing the distinction between streetscape design and roadway functionality and thus the importance of accounting for them separately. The streetscape skeleton classes provide a digestible yet objective system for identifying prevalent streetscape designs that are appropriate for urban policy design, advocacy, and urban systems research.