Algae isotopic composition

The haptophyte microalga Emiliania huxleyi produces biomarkers in the form of long-chain (C37, C38, and C39) alkenones (Brassell, 1993). Alkenones are well preserved in marine sediments and their molecular distributions and isotopic composition have been used to infer paleo-temperatures (Brassell, 1993) and pC022 values (Jasper et ak, 1994), respectively. Unsaturation patterns in the alkenone series are related to the growth temperature of the haptophyte algae that produce these compounds (Brassell et ak, 1986 Prahl and Wakeham, 1987), and hold great promise as indicators of absolute ocean paleotemperature. 

Laws, E.A., B.N. Popp, R.R. Bidigare, U. Riesbesell, S. Burkhardt, and S.G. Wakeham. 2001. Controls on the molecular distribution and carbon isotopic composition of alkenones in certain haptophyte algae. Geochemistry Geophysics Geosystems January 25, 2001. 

Although the sediments in these systems accumulate Se over time, the small isotopic contrast suggests that dissimilatory reduction is not the dominant accumulation mechanism. If dissimilatory reduction of Se(VI) and/or Se(IV) to Se(0) by bacteria were the dominant mechanism, one would expect the accumulated Se(0) to be enriched in the lighter isotope. In the San Francisco Estuary case, this assumes that the isotopic fractionations measured by Ellis et al. (2003) can be extrapolated to much lower concentrations. Incorporation of Se into algae and macrophyte tissues, followed by decay of some material and conversion of its Se to Se(0), is more consistent with the observed Se isotope data. Notably, the mean Se isotope composition of the Se(0) in the sediments of the Herbel et al. (2002) study was identical to that of the macrophytes. 

Figure 16 Schematic of in-stream biogeochemical processes that affect the and of algae. Arrows indicate the general effect of each process on the isotopic composition of the resulting algae.

Pentecost, A. Spiro, B. (1990) Stable carbon and oxygen isotope composition of calcites associated with modern freshwater cyanobacteria and algae. Geomicro-biological Journal 8, 17-26. 

Similarly, Michalski et al. (2001) argued that the oxygen isotope composition of Antarctic soil nitrate indicates that those deposits are entirely due to atmospheric deposition. Arias (2003) also supported an atmospheric origin for the Atacama deposits, and suggested that the nitrates resulted from the decay of marine algae concentrations and inland transport of aerosols in sea-spray and fog. 

Figure 3. Idealized carbon isotope cycle in a small stratified lake. The isotopic composition of organic matter buried in sediments is determined by the proportions of different terrestrial and lacustrine organic mailer. Ihccartron isotopic composition of dissolved inorganic carbon (DlC), and the rates of primary production and respiration within the water column. Isotope enrichment factors (e), listed here as the difference between the product and the substrate, vary with the form of DlC that lake algae assimilate (e.g., C02(aq) or HCOy ). Inorganic carbonate (CaC03> typically forms in isotopic equilibrium with the dissolved inorganic carbon pool and. as such, is indirectly affected by organic matter sources and primary production and respiration rales.

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