Does metabolic water control the phosphate oxygen isotopes of microbial cells?

The oxygen isotopes ratio (δ18O) of microbial cell water strongly controls the δ18O of cell phosphate and of other oxygen-carrying moieties. Recently it was suggested that the isotopic ratio in cell water is controlled by metabolic water, which is the water produced by cellular respiration. This potentially has important implications for paleoclimate reconstruction, and for measuring microbial carbon use efficiency with the 18O-water method. Carbon use efficiency strongly controls soil organic matter preservation. Here, we directly tested the effect of metabolic water on microbial cells, by conducting experiments with varying the δ18O of headspace O2 and the medium water, and by measuring the δ18O of cell phosphate. The latter is usually assumed to be in isotopic equilibrium with the cell's water. Our results showed no correlation between the δ18O of O2 and that of the cell phosphate, contradicting the hypothesis that metabolic water is an important driver of δ18O of microbial cell water. However, our labeled 18O water experiments indicated that only 43% of the oxygen in the cell's phosphate is derived from equilibration with the medium water, during late-log to early-stationary growing phase. This could be explained by the isotopic effects of intra-and extra-cellular hydrolysis of organic compounds containing phosphate.

X-ray Spectroscopic Quantification of Phosphorus Transformation in Saharan Dust during Trans-Atlantic Dust Transport.

Saharan dust is an important phosphorus (P) supply to remote and oligotrophic parts of the oceans and American lowland tropical rainforests. Phosphorus speciation in aeolian dust ultimately controls the release and bioavailability of P after dust deposition, but the speciation in Saharan dust and its change during the trans-Atlantic transport remains unclear. Using P K-edge X-ray absorption near edge structure (XANES) spectroscopy, we showed that with increasing dust traveling distance from the Sahara Desert to Cape Verde and to Puerto Rico, about 570 and 4000 km, respectively, the proportion of Ca-bound P (Ca-P), including both apatite and non-apatite forms, decreased from 68-73% to 50-71% and to 21-37%. The changes were accompanied by increased iron/aluminum-bound P proportion from 14-25% to 23-46% and to 44-73%, correspondingly. Laboratory simulation experiments suggest that the changes in P speciation can be ascribed to increasing degrees of particle sorting and atmospheric acidification during dust transport. The presence of relatively soluble non-apatite Ca-P in the Cape Verde dust but not in the Puerto Rico dust is consistent with the higher P water solubility of the former than the latter. Our findings provide insights into the controls of atmospheric processes on P speciation, solubility, and stability in Saharan dust.

Tropospheric carbonyl sulfide mass balance based on direct measurements of sulfur isotopes.

Robust estimates for the rates and trends in terrestrial gross primary production (GPP; plant CO2 uptake) are needed. Carbonyl sulfide (COS) is the major long-lived sulfur-bearing gas in the atmosphere and a promising proxy for GPP. Large uncertainties in estimating the relative magnitude of the COS sources and sinks limit this approach. Sulfur isotope measurements (34S/32S; δ34S) have been suggested as a useful tool to constrain COS sources. Yet such measurements are currently scarce for the atmosphere and absent for the marine source and the plant sink, which are two main fluxes. Here we present sulfur isotopes measurements of marine and atmospheric COS, and of plant-uptake fractionation experiments. These measurements resulted in a complete data-based tropospheric COS isotopic mass balance, which allows improved partition of the sources. We found an isotopic (δ34S ± SE) value of 13.9 ± 0.1‰ for the troposphere, with an isotopic seasonal cycle driven by plant uptake. This seasonality agrees with a fractionation of -1.9 ± 0.3‰ which we measured in plant-chamber experiments. Air samples with strong anthropogenic influence indicated an anthropogenic COS isotopic value of 8 ± 1‰. Samples of seawater-equilibrated-air indicate that the marine COS source has an isotopic value of 14.7 ± 1‰. Using our data-based mass balance, we constrained the relative contribution of the two main tropospheric COS sources resulting in 40 ± 17% for the anthropogenic source and 60 ± 20% for the oceanic source. This constraint is important for a better understanding of the global COS budget and its improved use for GPP determination.

Sulfur isotopes ratio of atmospheric carbonyl sulfide constrains its sources.

Carbonyl sulfide (COS) is the major long-lived sulfur bearing gas in the atmosphere, and is used to estimate the rates of regional and global (both past and current) photosynthesis. Sulfur isotope measurements (34S/32S ratio, δ34S) of COS may offer a way for improved determinations of atmospheric COS sources. However, measuring the COS δ34S at the atmospheric concentrations of ~0.5 ppb is challenging. Here we present high-accuracy δ34S measurements of atmospheric COS done by gas chromatograph (GC) connected to a multicollector inductively coupled plasma mass spectrometer (MC-ICPMS), after pre-concentrating from 2-liters of air. We showed that the precision of COS δ34S measurement for gas standards is ≤0.2‰, and that N2 and CO2 in the gas standard mixture had no effect on the measured δ34S. Natural air samples were collected in Israel and in the Canary Islands. The COS δ34S values in both locations were found to be 13.2 ± 0.6‰, and are believed to represent the background tropospheric value. This δ34S value is markedly different from the previously reported value of 4.9‰. We estimate the expected isotopic signature of COS sources and sinks, and use the δ34S value of atmospheric COS we measured to estimate that ~48% of it originates from the ocean.

Use of 13 C- and phosphate 18 O-labeled substrate for studying phosphorus and carbon cycling in soils: a proof of concept.

RATIONALE: Stable isotope tracers are commonly used to track the transformations of organic carbon (C) and nitrogen (N) in soils but they have not been used to follow coupled cycles of phosphorus (P) and C because P has only one stable isotope. METHODS: A novel dually labeled substrate, composed of a 13 C-labeled glucose backbone attached to a phosphate group with known δ18 OP value, was used to follow the fate of C and P derived from the same compound. The substrate was amended to soils from two natural oak forests, differing in their P levels, and the δ13 C values of respired CO2 and the δ18 OP values of soil bioavailable P were measured. RESULTS: The δ18 OP values of the phosphate accumulated in the soil deviated from the expected value of P that is released from phosphomonoesters by enzymatic reactions (3.7 ‰) and were driven towards isotopic equilibrium with soil water (21.1 ‰), a process which is produced as part of microbial metabolism. This suggests that the substrate's P passed through the microbial biomass before accumulating in the soil, reflecting the rapid microbial turnover rate of the added P. A higher fraction of the substrate-C was respired in the higher P soil by the end of the incubation (20.3 % in comparison with 9.1 % in the lower P soil), indicating stronger energy limitation in the higher P soils. The higher 13 C glucose respiration in the higher P soil also triggered a stronger priming effect than in the lower P soil. CONCLUSIONS: Our approach demonstrates that 13 C- and P-18 O-dually labeled tracers provide invaluable data on the fate of organic P and C in soils. We encourage biogeochemists and soil scientists to develop similar tracers of this kind. Copyright © 2017 John Wiley & Sons, Ltd.

Measuring the ratio of CO2 efflux to O2 influx in tree stem respiration.

In recent studies, the ratio of tree stem CO2 efflux to O2 influx has been defined as the apparent respiratory quotient (ARQ). The metabolism of carbohydrates, the putative respiratory substrate in trees, is expected to yield an ARQ of 1.0. However, previous studies have reported ARQ values ranging between 0.23 and 0.90. These interesting results may indicate internal transport of respired CO2 within stems; yet no simple field applicable methods for ARQ measurement have been available. Here, we report on the assembly of a closed circulating system called 'Hampadah', which uses CO2 and O2 analyzers to measure air samples from stem chambers. We tested the performance of the Hampadah with samples from 36 trees (Tetragastris panamensis (Engl.) Kuntze). Additionally, we showed the feasibility of measuring ARQ directly from stem chambers, using portable CO2 and O2 sensors, in both discrete and continuous modes of operation. The Hampadah measurement proved to be consistent with CO2 gas standards (R2 = 0.999) and with O2 determined by O2/Ar measurements with a mass spectrometer (R2 = 0.998). The Hampadah gave highly reproducible results for ARQ determination of field samples (±0.01 for duplicates). The portable sensors measurement showed good correlation with the Hampadah in measuring CO2, O2 and ARQ (n = 5, R2 = 0.97, 0.98 and 0.91, respectively). We have demonstrated here that the Hampadah and the sensors' methods enable accurate ARQ measurements for both laboratory and field research.

Substantial dust loss of bioavailable phosphorus from agricultural soils.

Phosphorus (P) is an essential element in terrestrial ecosystems. Knowledge on the role of dust in the biogeochemical cycling of phosphorus is very limited with no quantitative information on aeolian (by wind) P fluxes from soils. The aim of this study is to focus on P cycling via dust emissions under common land-use practices in an arid environment by integration of sample analyses and aeolian experiments. The experiments indicate significant P fluxes by PM10 dust due to agricultural land use. Even in a single wind-dust event at moderate velocity (7.0 m s(-1)), P flux in conventional agricultural fields can reach 1.83 kg km(-2), that accumulates to a considerable amount per year at a regional scale. The results highlight a negative yearly balance in P content (up to hundreds kg km(-2)) in all agricultural soils, and thus more P nutrition is required to maintain efficient yield production. In grazing areas where no P nutrition is applied, the soil degradation process can lead to desertification. Emission of P from soil dust sources has significant implications for soil nutrient resources and management strategies in agricultural regions as well as for loading to the atmosphere and global biogeochemical cycles.

Carbon dioxide emitted from live stems of tropical trees is several years old.

Storage carbon (C) pools are often assumed to contribute to respiration and growth when assimilation is insufficient to meet the current C demand. However, little is known of the age of stored C and the degree to which it supports respiration in general. We used bomb radiocarbon ((14)C) measurements to determine the mean age of carbon in CO2 emitted from and within stems of three tropical tree species in Peru. Carbon pools fixed >1 year previously contributed to stem CO2 efflux in all trees investigated, in both dry and wet seasons. The average age, i.e., the time elapsed since original fixation of CO2 from the atmosphere by the plant to its loss from the stem, ranged from 0 to 6 years. The average age of CO2 sampled 5-cm deep within the stems ranged from 2 to 6 years for two of the three species, while CO2 in the stem of the third tree species was fixed from 14 to >20 years previously. Given the consistency of (14)C values observed for individuals within each species, it is unlikely that decomposition is the source of the older CO2. Our results are in accordance with other studies that have demonstrated the contribution of storage reserves to the construction of stem wood and root respiration in temperate and boreal forests. We postulate the high (14)C values observed in stem CO2 efflux and stem-internal CO2 result from respiration of storage C pools within the tree. The observed age differences between emitted and stem-internal CO2 indicate an age gradient for sources of CO2 within the tree: CO2 produced in the outer region of the stem is younger, originating from more recent assimilates, whereas the CO2 found deeper within the stem is older, fueled by several-year-old C pools. The CO2 emitted at the stem-atmosphere interface represents a mixture of young and old CO2. These observations were independent of season, even during a time of severe regional drought. Therefore, we postulate that the use of storage C for respiration occurs on a regular basis challenging the assumption that storage pools serve as substrates for respiration only during times of limited assimilation.

Increased root oxygen uptake in pea plants responding to non-self neighbors.

Recent studies have demonstrated that plants alter root growth and decrease competition with roots of the same individual (self); however, the physiological traits accompanying this response are still widely unknown. In this study, we investigated the effect of root identity on gas exchange in the model species pea (Pisum sativum L.). Split-root plants were planted so that each pot contained either two roots of the same plant (self) or of two different plants (non-self), and the responses of biomass, photosynthesis, and respiration were measured. The photosynthetic rate was not affected by the identity of the root neighbor. We found a reduction of leaf dark respiration by half, accompanied by an increase in nocturnal root respiration by 29 % in plants neighboring with non-self. The activity of the alternative oxidase (AOX) pathway increased when plants responded to non-self neighbors. The increased activity of AOX in plants responding to non-self indicates carbon imbalances in roots, possibly as a consequence of increased root exudation and communication between individuals. If such an effect occurs more widely, it may change the assumptions made for the quantity of respiration as used in carbon budget models.

Use of phosphate oxygen isotopes for identifying atmospheric-P sources: a case study at Lake Kinneret.

The input of phosphorus (P) through atmospheric deposition can be a major source of P to fresh water bodies and may strongly affect their biogeochemistry. In Lake Kinneret (LK), northern Israel, dust deposition provides a significant fraction of the bioavailable P input. Here, we demonstrate that the oxygen isotopic composition of resin-extractable inorganic phosphate (δ(18)OP) in dust particles can be used to identify the phosphate source. Samples of soils with both natural vegetation and agricultural cover were collected upwind of LK and found to have distinct δ(18)OP value ranges (17.4-18.2‰ and 19.3-22.1‰, respectively). The δ(18)OP values for dust, collected continuously over LK during June 2011 to March 2012, were in the same range as agricultural soils. The dust concentration in the air decreased from the dry to the wet season and was correlated with a decrease in P concentration in air, yet no correlation was found between these parameters and dust δ(18)OP. Dust deposited during short-term desert dust events was characterized by a combination of high δ(18)OP values ranging from 22.2‰ to 22.7‰ and high concentrations of dust in the air. The data we present demonstrates a new application of δ(18)OP measurements for direct estimation of dust-P sources to lakes, as well as the potential for tracing dust-P on larger scales.

Soil phosphate stable oxygen isotopes across rainfall and bedrock gradients.

The stable oxygen isotope compositions of soil phosphate (δ(18)O(p)) were suggested recently to be a tracer of phosphorus cycling in soils and plants. Here we present a survey of bioavailable (resin-extractable or resin-P) inorganic phosphate δ(18)O(p) across natural and experimental rainfall gradients, and across soil formed on sedimentary and igneous bedrock. In addition, we analyzed the soil HCl-extractable inorganic δ(18)O(p), which mainly represents calcium-bound inorganic phosphate. The resin-P values were in the range 14.5-21.2‰. A similar range, 15.6-21.3‰, was found for the HCl-extractable inorganic δ(18)O(p), with the exception of samples from a soil of igneous origin that show lower values, 8.2-10.9‰, which indicate that a large fraction of the inorganic phosphate in this soil is still in the form of a primary mineral. The available-P δ(18)O(p) values are considerably higher than the values we calculated for extracellular hydrolysis of organic phosphate, based on the known fractionation from lab experiments. However, these values are close to the values expected for enzymatic-mediated phosphate equilibration with soil-water. The possible processes that can explain this observation are (1) extracellular equilibration of the inorganic phosphate in the soil; (2) fractionations in the soil are different than the ones measured at the lab; (3) effect of fractionation during uptake; and (4) a flux of intercellular-equilibrated inorganic phosphate from the soil microbiota, which is considerably larger than the flux of hydrolyzed organic-P.

Determining the relationship between tree-stem respiration and CO2 efflux by δO2/Ar measurements.

Respiration in forest tree stems is an important component of the global carbon cycle. This respiration is traditionally estimated by measurements of the CO(2) efflux from the stem. However, recent studies have suggested that movement of CO(2) in the transpiration stream causes large errors in the respiration estimated by the CO(2) efflux. Here we demonstrate a new approach for determining the ratio of respiration to CO(2) efflux, which is based on specially designed chambers, and combined CO(2) and O(2) measurements. The high accuracy O(2) measurement is performed by mass spectrometric measurement of the O(2)/Ar ratio. Testing the method gave repeatable results which point that in some conditions up to 40% of the respired CO(2) can be carried away from the site of respiration.

Increase in water-use efficiency and underlying processes in pine forests across a precipitation gradient in the dry Mediterranean region over the past 30 years.

Motivated by persistent predictions of warming and drying in the entire Mediterranean and other regions, we have examined the interactions of intrinsic water-use efficiency (W(i)) with environmental conditions in Pinus halepensis. We used 30-year (1974-2003) tree-ring records of basal area increment (BAI) and cellulose (13)C and (18)O composition, complemented by short-term physiological measurements, from three sites across a precipitation (P) gradient (280-700 mm) in Israel. The results show a clear trend of increasing W(i) in both the earlywood (EW) and latewood (LW) that varied in magnitude depending on site and season, with the increase ranging from ca. 5 to 20% over the study period. These W(i) trends were better correlated with the increase in atmospheric CO(2) concentration, C(a), than with the local increase in temperature (~0.04°C year(-1)), whereas age, height and density variations had minor effects on the long-term isotope record. There were no trends in P over time, but W(i) from EW and BAI were dependent on the interannual variations in P. From reconstructed C(i) values, we demonstrate that contrasting gas-exchange responses at opposing ends of the hydrologic gradient underlie the variation in W(i) sensitivity to C(a) between sites and seasons. Under the mild water limitations typical of the main seasonal growth period, regulation was directed at increasing C(i)/C(a) towards a homeostatic set-point observed at the most mesic site, with a decrease in the W(i) response to C(i) with increasing aridity. With more extreme drought stress, as seen in the late season at the drier sites, the response was W(i) driven, and there was an increase in the W(i) sensitivity to C(a) with aridity and a decreasing sensitivity of C(i) to C(a). The apparent C(a)-driven increases in W(i) can help to identify the adjustments to drying conditions that forest ecosystems can make in the face of predicted atmospheric change.

A method for analyzing the δ18O of resin-extractable soil inorganic phosphate.

Improved tools for tracing phosphate transformations in soils are much needed, and can lead to a better understanding of the terrestrial phosphorus cycle. The oxygen stable isotopes in soil phosphate are still not exploited in this regard. Here we present a method for measuring the oxygen stable isotopes in a fraction of the soil phosphate which is rapidly available to plants, the resin-extractable P. This method is based on extracting available phosphate from the soil with anion-exchange membranes, soil organic matter removal by a resin, purification by precipitation as cerium phosphate, and finally precipitation as silver phosphate. The purified silver phosphate samples are then measured by a high-temperature elemental analyzer (HT-EA) coupled in continuous flow mode to an isotope ratio mass spectrometer. Testing the method with Mediterranean and semi-arid soils showed no artifacts, as well as good reproducibility in the same order as that of the HT-EA analytical uncertainty (0.3‰).

The changing carbon cycle at Mauna Loa Observatory.

The amplitude of the CO(2) seasonal cycle at the Mauna Loa Observatory (MLO) increased from the early 1970s to the early 1990s but decreased thereafter despite continued warming over northern continents. Because of its location relative to the large-scale atmospheric circulation, the MLO receives mainly Eurasian air masses in the northern hemisphere (NH) winter but relatively more North American air masses in NH summer. Consistent with this seasonal footprint, our findings indicate that the MLO amplitude registers North American net carbon uptake during the warm season and Eurasian net carbon release as well as anomalies in atmospheric circulation during the cold season. From the early 1970s to the early 1990s, our analysis was consistent with that of Keeling et al. [Keeling CD, Chin JFS, Whorf TP (1996) Nature 382:146-149], suggesting that the increase in the MLO CO(2) amplitude is dominated by enhanced photosynthetic drawdown in North America and enhanced respiration in Eurasia. In contrast, the recent decline in the CO(2) amplitude is attributed to reductions in carbon sequestration over North America associated with severe droughts from 1998 to 2003 and changes in atmospheric circulation leading to decreased influence of Eurasian air masses. With the return of rains to the U.S. in 2004, both the normalized difference vegetation index and the MLO amplitude sharply increased, suggesting a return of the North American carbon sink to more normal levels. These findings indicate that atmospheric CO(2) measurements at remote sites can continue to play an important role in documenting changes in land carbon flux, including those related to widespread drought, which may continue to worsen as a result of global warming.