Stephen A. Macko

Dependence of phytoplankton carbon isotopic composition on growth rate and [CO2)aq: Theoretical considerations and experimental results

The carbon isotopic composition of the marine diatom Phaeodactylum tricornutum (δ13Cp) was measured over a series of growth rates (μ) in a continuous culture system in which both δ133CCO2 and [CO2]aq were determined. In accord with theory, a linear relationship was found to exist between μ/ [CO2]aq and ɛp (≡1000(δ13CCO2 − δ13Cp)/1000 + d δ13Cp), the biological fractionation associated with carbon fixation. The range of [CO2]aq in the continuous culture system was 13–31 μmol kg−1. Measurements of δ13CCO2 and [CO2]aq in the mixed layer of the equatorial Pacific and estimates of δ13Cp obtained from the δ13C of chlorophyll a combined with the regression line fit to the P. tricornutum data give phytoplankton growth rates that are in excellent agreement with those estimated via other techniques. Measurement of ɛp and [CO2]aq in the field can provide an estimate of in situ phytoplankton growth rates without the potential artifacts associated with incubation methodologies. These findings also suggest that accurate estimations of ancient CO2(aq) concentrations will require knowledge of both ɛp and phytoplankton growth rate.

Isotopic fractionation of nitrogen and carbon in the synthesis of amino acids by microorganisms

Two classes of procaryotic organisms were cultured on specific inorganic and organic nitrogenous substrates. The organisms fractionated these substrates in characteristic ways during the synthesis of their cellular biochemical compounds. Blue-green algae, Anabaena sp., were raised on molecular nitrogen, nitrate and ammonia in the presence of excess carbon dioxide. The difference between δ15N of either nitrate or ammonium and the δ15N of the algae grown on either source was 13%o. The δ15N of Anabaena that fixed N2 was 2% lighter than the nitrogen gas supplied to the algae. These fractionations are associated with enzymatic incorporation of the nitrogen into the cell. A heterotrophic bacterium, Vibrio harveyi was grown on a single amino acid as its source of both nitrogen and carbon. Cells grown on glutamic acid were enriched in 15N relative to substrate, whereas those grown on alanine were depleted in 15N compared to source nitrogen. The bacterial cultures were enriched in 13C relative to the substrate. These cultures were then hydrolyzed and individual amino acids isolated and isotopically analyzed. The isotopic compositions of the amino acids have a wide range of values; most appear to have isotope fractionations associated with the metabolic pathways in their synthesis. These results and the application of the coupled separation-isotopic analysis of amino acids yield a better understanding of comparative biochemistry for these organisms. Such analyses offer valuable information for the tracing of biosynthesis and early diagenesis to help explain the fossil record.

Isotopic evidence for extraterrestrial non- racemic amino acids in the Murchison meteorite

Many amino acids contain an asymmetric centre, occurring as laevorotatory, L, or dextrorotatory, D, compounds. It is generally assumed that abiotic synthesis of amino acids on the early Earth resulted in racemic mixtures (L- and D-enantiomers in equal abundance). But the origin of life required, owing to conformational constraints, the almost exclusive selection of either L- or D-enantiomers, and the question of why living systems on the Earth consist of L-enantiomers rather than D-enantiomers is unresolved. A substantial fraction of the organic compounds on the early Earth may have been derived from comet and meteorite impacts. It has been reported previously that amino acids in the Murchison meteorite exhibit an excess of L-enantiomers, raising the possibility that a similar excess was present in the initial inventory of organic compounds on the Earth. The stable carbon isotope compositions of individual amino acids in Murchison support an extraterrestrial origin—rather than a terrestrial overprint of biological amino acids—although reservations have persisted (see, for example, ref. 9). Here we show that individual amino-acid enantiomers from Murchison are enriched in 15N relative to their terrestrial counterparts, so confirming an extraterrestrial source for an L-enantiomer excess in the Solar System that may predate the origin of life on the Earth.

Kinetic fractionation of stable nitrogen isotopes during amino acid transamination

Abstract This study evaluates a kinetic isotope effect involving 15N, during the transamination reactions catalyzed by glutamic oxalacetic transaminase. During the transfer of amino nitrogen from glutamic acid to oxaloacetate to form aspartic acid, 14NH2 reacted 1.0083 times faster than 14NH2. In the reverse reaction transferring NH2 from aspartic acid to α-ketoglutarate, 14NH2 was incorporated 1.0017 times faster than 15NH2. Knowledge of the magnitude and sign of these isotope effects will be useful in the interpretation of the distribution of 15N in biological and geochemical systems.

Microbial alteration of stable nitrogen and carbon isotopic compositions of organic matter

Abstract An understanding of the interaction between microbes and organic matter can help elucidate the diagenesis of organic materials in sediments. Vibrio harveyi, a marine, aerobic, heterotrophic bacterium, was cultured on individual compounds each containing carbon and nitrogen, i.e. amino acids or amino sugars. When grown on different substrates, the bacteria fractionated the isotopes uniquely. These fractionations were related to the chemical nature of the substrate. Specifically, the C/N of the substrate, or the biosynthetic and metabolic pathways of these substances, or both, are important in determining the isotopic composition of the total organism. For example, bacterial nitrogen was enriched in 15N (up to 22%) when the bacterium was cultured on either glutamic or aspartic acid. These amino acids enter the metabolic pathways directly, and the remainder of cellular nitrogen is derived via transamination. When other amino acids, i.e. alanine and serine, enter the microbe, they are immediately deaminated, and free ammonia is then transferred to glutamic acid. Bacteria that have been cultured on alanine or serine are depleted of 15N by up to − 12%. The results are important in determining isotope effects in respiration and biosynthesis. Moreover, the results could be used to demonstrate that microbes have the potential to alter to a highly variable extent the isotopic composition of organic matter entering the sedimentary record.

Correlations between foliar δ15N and nitrogen concentrations may indicate plant-mycorrhizal interactions.

Abstract Nitrogen isotope measurements may provide insights into changing interactions among plants, mycorrhizal fungi, and soil processes across environmental gradients. Here, we report changes in δ15N signatures due to shifts in species composition and nitrogen (N) dynamics. These changes were assessed by measuring fine root biomass, net N mineralization, and N concentrations and δ15N of foliage, fine roots, soil, and mineral N across six sites representing different post-deglaciation ages at Glacier Bay, Alaska. Foliar δ15N varied widely, between 0 and –2‰ for nitrogen-fixing species, between 0 and –7‰ for deciduous non-fixing species, and between 0 and –11‰ for coniferous species. Relatively constant δ15N values for ammonium and generally low levels of soil nitrate suggested that differences in ammonium or nitrate use were not important influences on plant δ15N differences among species at individual sites. In fact, the largest variation among plant δ15N values were observed at the youngest and oldest sites, where soil nitrate concentrations were low. Low mineral N concentrations and low N mineralization at these sites indicated low N availability. The most plausible mechanism to explain low δ15N values in plant foliage was a large isotopic fractionation during transfer of nitrogen from mycorrhizal fungi to plants. Except for N-fixing plants, the foliar δ15N signatures of individual species were generally lower at sites of low N availability, suggesting either an increased fraction of N obtained from mycorrhizal uptake (f), or a reduced proportion of mycorrhizal N transferred to vegetation (Tr). Foliar and fine root nitrogen concentrations were also lower at these sites. Foliar N concentrations were significantly correlated with δ15N in foliage of Populus, Salix, Picea, and Tsuga heterophylla, and also in fine roots. The correlation between δ15N and N concentration may reflect strong underlying relationships among N availability, the relative allocation of carbon to mycorrhizal fungi, and shifts in either f or Tr.

Insights into nitrogen and carbon dynamics of ectomycorrhizal and saprotrophic fungi from isotopic evidence

Abstract The successful use of natural abundances of carbon (C) and nitrogen (N) isotopes in the study of ecosystem dynamics suggests that isotopic measurements could yield new insights into the role of fungi in nitrogen and carbon cycling. Sporocarps of mycorrhizal and saprotrophic fungi, vegetation, and soils were collected in young, deciduous-dominated sites and older, coniferous-dominated sites along a successional sequence at Glacier Bay National Park, Alaska. Mycorrhizal fungi had consistently higher δ15N and lower δ13C values than saprotrophic fungi. Foliar δ13C values were always isotopically depleted relative to both fungal types. Foliar δ15N values were usually, but not always, more depleted than those in saprotrophic fungi, and were consistently more depleted than in mycorrhizal fungi. We hypothesize that an apparent isotopic fractionation by mycorrhizal fungi during the transfer of nitrogen to plants may be attributed to enzymatic reactions within the fungi producing isotopically depleted amino acids, which are subsequently passed on to plant symbionts. An increasing difference between soil mineral nitrogen δ15N and foliar δ15N in later succession might therefore be a consequence of greater reliance on mycorrhizal symbionts for nitrogen supply under nitrogen-limited conditions. Carbon signatures of mycorrhizal fungi may be more enriched than those of foliage because the fungi use isotopically enriched photosynthate such as simple sugars, in contrast to the mixture of compounds present in leaves. In addition, some 13C fractionation may occur during transport processes from leaves to roots, and during fungal chitin biosynthesis. Stable isotopes have the potential to help clarify the role of fungi in ecosystem processes.

Sources of nitrogen in wet deposition to the Chesapeake Bay region

Nitrogen-containing compounds in wet deposition can provide significant sources of nutrients to phytoplankton and potentially contribute to eutrophication in estuaries and coastal waters. Quantifying both inorganic and organic forms of nitrogen in wet deposition as well as determining their sources is important for understanding how to control eutrophication. Stable nitrogen isotope data can provide information regarding what source processes produced nitrogen in precipitation and air mass trajectories can predict where the air mass which produced the precipitation was geographically located before the event occurred. In this study, the wet deposition concentrations, fluxes, and δ15N values of ammonium, nitrate, and dissolved organic nitrogen were determined for 60 precipitation events collected from May, 1993 to December 1994 at a site near the Chesapeake Bay, an estuary currently experiencing eutrophication. Grouping the concentration data according to season showed a peak in ammonium coupled with depleted δ15N values in the spring which were indicative of agricultural emissions. A peak in nitrate in the spring seemed to indicate greater soil emissions at that time, but concentrations were also high at other times of the year. No trend was observed for the dissolved organic nitrogen with season. Back trajectories were calculated for each precipitation event and grouped into five major transport patterns. Combining the flux and isotopic composition data with the air flow history revealed that dominant sources of ammonium in precipitation to the region are probably fertilizers, soil, and animal excreta emissions which have the highest fluxes in air masses originating from the southwest and west. The dominant source of nitrate to the region is probably fossil-fuel combustion and the highest fluxes originate from the northwest and west. Speculation on the dissolved organic nitrogen sources is probably premature, but its flux pattern is similar to the nitrate pattern, suggesting that their sources may be similar.

Abiogenic methanogenesis in crystalline rocks

Isotopically anomalous CH4-rich gas deposits are found in mining sites on both the Canadian and Fennoscandian shields. With δ 13CCH4 values from −22.4 to −48.5% and δDCH4 values from −133 to −372%, these methane deposits cannot be accounted for by conventional processes of bacterial or thermogenic methanogenesis. Compositionally the gases are similar to other CH4-rich gas occurrences found in Canadian and Fennoscandian shield rocks (Sherwood Lollar et al., 1993). However, the isotopically anomalous gases of this study are characterized by unexpectedly high concentrations of H2 gas, ranging from several volume percent up to 30 vol%. The H2 gases are consistently depleted in the heavy isotope, with δDH2 values of −619 to −659‰ 3H/4He ratios in the range of 0.4 × 10−8 to 4.4 sx 10−8 indicate that there is no resolvable component of mantle-derived He in these deposits. Based on these results a mantle-derived source for the C-bearing gases is unlikely. Several lines of evidence support an alternative abiogenic origin for the gases from Sudbury, Canada, and Juuka and Pori, Finland. The D-depleted H2 gas and calculated ΔD(CH4−H2) equilibration temperatures of 110–170°C at all three sites are in good agreement with results obtained for abiogenically produced CH4 and H2 in ophiolite sequences in Oman and the Philippines. Serpentinization and the hydration of ultramafic rocks are the proposed mechanisms for CH4 and H2 production in these ophiolite sequences. The widespread occurrence of serpentinized and altered ultramafic rocks at Juuka and at a number of other sites in both Canada and Finland implies that similar mechanisms may be involved in gas production at at least three sites on the shields. The origin of the gases at the remaining shield sites is discussed. Alternative hypothesis include (1) production of the rest of the shield gases by mixing between abiogenic endmembers and bacterially generated hydrocarbon gas such as identified elsewhere on the Canadian and Fennoscandian shields (Sherwood Lollar et al., 1993); and (2) production of a series of isotopically distinct abiogenic CH4 endmembers at each site due to variability in the isotopic composition of available carbon sources. We cannot conclusively distinguish between the alternative scenarios based on existing data. However, the evidence for an abiogenic CH4 endmember at at least three sites emphasizes the need for substantial revision of current theories of methanogenesis. Abiogenic processes of CH4 production may be considerably more widespread than previously anticipated.

The long-range transport of southern African aerosols to the tropical South Atlantic

Two episodes of long-range aerosol transport (4000 km) from southern Africa into the central tropical South Atlantic are documented. Stable nitrogen isotope analysis, multielemental analysis, and meteorological observations on local and regional scales are used to describe the observed surface aerosol chemistry during these transport episodes. The chemical, kinematic, and thermodynamic analyses suggest that for the central tropical South Atlantic, west Africa between 0° and 10°S is the primary air mass source region (over 50%) during austral spring. Over 70% of all air arriving in the lower and middle troposphere in the central tropical South Atlantic comes from a broad latitudinal band extending from 20°S to 10°N. Air coming from the east subsides and is trapped below the midlevel and trade wind inversion layers. Air from the west originates at higher levels (500 hPa) and contributes less than 30% of the air masses arriving in the central tropical South Atlantic. The source types of aerosols and precursor trace gases extend over a broad range of biomes from desert and savanna to the rain forest. During austral spring, over this broad region, processes include production from vegetation, soils, and biomass burning. The aerosol composition of air masses over and the atmospheric chemistry of the central South Atlantic is a function of the supply of biogenic, biomass burning, and aeolian emissions from tropical Africa. Rainfall is a common controlling factor for all three sources. Rain, in turn, is governed by the large-scale circulations which show pronounced interannual variability. The field measurements were taken in an extremely dry year and reflect the circulation and transport fields typical of these conditions.