Biological activity and the Earth's surface evolution: Insights from carbon,sulfur, nitrogen and iron stable isotopes in the rock record

The search for early Earth biological activity is hindered by the scarcity of the rock record. The very few exposed sedimentary rocks have all been affected by secondary processes such as metamorphism and weathering, which might have distorted morphological microfossils and biogenic minerals beyond recognition and have altered organic matter to kerogen. The search for biological activity in such rocks therefore relies entirely on chemical, molecular or isotopic indicators. A powerful tool used for this purpose is the stable isotope signature of elements related to life (C, N, S, Fe). It provides key informations not only on the metabolic pathways operating at the time of the sediment deposition, but more globally on the biogeochemical cycling of these elements and thus on the Earth's surface evolution. Here, we review the basis of stable isotope biogeochemistry for these isotopic systems. Rather than an exhaustive approach, we address some examples to illustrate how they can be used as biosignatures of early life and as proxies for its environment, while keeping in mind what their limitations are. We then focus on the covariations among these isotopic systems during the Archean time period to show that they convey important information both on the evolution of the redox state of the terrestrial surface reservoirs and on co-occurring ecosystems in the Archean.     

Trophic structure of common marine species in the Bohai Strait,North China Sea,based on carbon and nitrogen stable isotope ratios

Trophic relationships among common coastal species in the Bohai Strait, North China Sea, were investigated in this study using stable isotope ratios of carbon (¹³C/¹²C) and nitrogen (¹⁵N/¹⁴N) as tracers. During the research cruises of this study, 16 species of invertebrates and 18 species of fishes were collected, as were various food sources (phytoplankton, macroalgae, and sediment). The carbon and nitrogen isotope ratios of all collected samples were measured and used for trophic level analyses. The results showed that the grazers Oregonia gracilis, Notoacmea schrenckii, Chlorostoma rustica, and Chelon affinis, preferentially consumed Zostera marina (p < 0.01), whereas Anthocidaris crassispina preferred Ulva conglobata (p < 0.01). Two trophic models based on nitrogen isotope ratios were built to identify the trophic level of each species. The model that combined food sources was more appropriate than the model that used a single primary producer to identify the relative trophic positions of primary consumers. Scombermorus niphonius, an important fishery resource, was at the top trophic level. The seastar Asterias amurensis was at the highest level among the invertebrates and directly threatened production of shellfish. Based on the trophic level and food source relationships identified in this study, we gave some advice for ecological restoration, such as optimizing the structure of food source distribution, limiting seastar numbers, and improving the applicability of habitat for fishery species.     

准噶尔盆地南缘荒漠区土壤碳分布及其稳定同位素变化

以亚洲中部干旱区准噶尔盆地南缘荒漠区为研究区,根据荒漠距离绿洲的距离,分别在荒漠边缘、中部和腹地设置3条样带,并采集2 m深的土壤剖面样品,研究土壤有机碳(SOC)、无机碳(SIC)含量及其稳定碳同位素的分布,探讨土壤碳变化与距绿洲距离的关系.结果表明: SOC含量随剖面土层深度增加而减少.受距绿洲距离的影响,SOC含量表现为荒漠边缘>荒漠中部>荒漠腹地.荒漠边缘SOC的δ¹³C值范围为-21.92‰～-17.41‰,且随深度增加而递减;荒漠中部和荒漠腹地的δ¹³C值范围为-25.20‰～-19.30‰,且随深度增加先增后减,由此推断准噶尔盆地南缘荒漠中部和腹地地表植被以C3植物为主,而绿洲边缘经历了从C3植物为主到C4植物为主的演替过程.荒漠边缘SIC平均含量为38.98 g·kg^-1,是荒漠腹地的6.01倍,表明0～2 m深度内大量SIC在荒漠边缘呈聚集趋势.SIC的δ¹³C值随深度增加先减后增,底层富集,主要受原生碳酸盐含量和剖面土壤CO₂的影响.     

Identification of a conserved 8 aa insert in the PIP5K protein in the Saccharomycetaceae family of fungi and the molecular dynamics simulations and structural analysis to investigate its potential functional role

Homologs of the phosphatidylinositol‐4‐phosphate‐5‐kinase (PIP5K), which controls a multitude of essential cellular functions, contain a 8 aa insert in a conserved region that is specific for the Saccharomycetaceae family of fungi. Using structures of human PIP4K proteins as templates, structural models were generated of the Saccharomyces cerevisiae and human PIP5K proteins. In the modeled S. cerevisiae PIP5K, the 8 aa insert forms a surface exposed loop, present on the same face of the protein as the activation loop of the kinase domain. Electrostatic potential analysis indicates that the residues from 8 aa conserved loop form a highly positively charged surface patch, which through electrostatic interaction with the anionic portions of phospholipid head groups, is expected to play a role in the membrane interaction of the yeast PIP5K. To unravel this prediction, molecular dynamics (MD) simulations were carried out to examine the binding interaction of PIP5K, either containing or lacking the conserved signature insert, with two different membrane lipid bilayers. The results from MD studies provide insights concerning the mechanistic of interaction of PIP5K with lipid bilayer, and support the contention that the identified 8 aa conserved insert in fungal PIP5K plays an important role in the binding of this protein with membrane surface. Proteins 2017; 85:1454–1467. © 2017 Wiley Periodicals, Inc.     

Feeding ecology of the sand smelt <Emphasis Type="Italic">Atherina boyeri</Emphasis> (Risso 1810) (Osteichthyes,Atherinidae) in the western Mediterranean: evidence for spatial variability based on stable carbon and nitrogen isotopes

Synopsis The feeding ecology of the sand smelt Atherina boyeri Risso 1810 (Osteichthyes, Atherinidae) was investigated in a Mediterranean coastal basin (Stagnone di Marsala, Italy) by means of carbon and nitrogen stable isotope ratios. Sampling was carried out seasonally in 1999 in two locations characterised by different depth and vegetal coverage. Throughout the year the sand smelt showed enriched ¹³C and ¹⁵N values in both sampling locations (¹³C about –12 and ¹⁵N about 12). This result suggests the overall importance of the benthic pathway in the food web leading to A. boyeri. Our isotopic picture is only in partial agreement with stomach content data from the literature. Stomach contents highlighted that, while A. boyeri is benthivore in the location with high vegetal coverage, it is able to shift its diet toward zooplankton in the location with higher depth and lower complexity. In our opinion, this discrepancy can be a consequence of the different temporal scale which stomach contents and stable isotopes are referred to. Contrarily to stomach contents, stable isotope analysis provides time-integrated information on the real diet averaged over some months or more. We suggest that long-term and short-term diets of the sand smelt differ. Zooplankton, due to its low biomass in the study site seems to be exploited scarcely by A. boyeri and its trophic role is detectable only considering the short term diet with stomach contents, while the time-integrated diet of A. boyeri seems to be based on benthic prey.