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Anaerobic iron cycling in a neoarchean ocean analogue
Auteur(s)
Walter, Xavier Alexis
Editeur(s)
Date de parution
2011
Résumé
The deposition of Banded Iron Formation (BIF) during the Archean and Paleoproterozoic is conventionally attributed to the precipitation of iron-oxides resulting from the abiotic reaction of ferrous iron (Fe(II)) with oxygen (Cloud, 1968). Oxygenic photosynthesis, however, appeared only around 2.7 Ga (Des Marais, 2000; Kump, 2008; Godfrey & Falkowski, 2009 ), thus raising questions as to what may have caused BIF precipitation before that time. The discovery of anoxygenic phototrophs thriving through the oxidation of Fe(II) (Widdel et al., 1993; Heisinget al., 1999) has provided support for a bacterial origin for early BIFs (Konhauseret al., 2002; Kappler et al., 20051). Despite reports of anoxygenic phototrophs that may oxidise Fe(II) in the environment (Crowe et al., 2008), a model ecosystem where photoferrotrophs are demonstrably active is still lacking (Svermann & Anbar, 2009; Johnston et al., 2009). Therefore, the ferruginous meromictic lake La Cruz (Spain) that sustains dense populations of purple and green anoxygenic phototrophic bacteria despite low sulfate and sulfide concentrations was investigated. First, the system was characterized by a physico-chemical analysis of its water column and sediments. Then we focuses on the chemocline compartment where iron oxides were found to be produced. On the one hand, We performed <i>in situ</i> <sup>14</sup>C-bicarbonate incubations to detect any stimulation of autotrophy by Fe(II) addition, while on the other hand <i>ex situ</i> incubations were carried out with the same natural sample to detect any Fe(II) oxidation by Lake La Cruz microbiota. In parallel, we have done enrichment cultures targeting anaerobic iron oxidizing metabolisms. <br> In the second chapter, we show direct evidences of a photoferrotrophic activity in the ferruginous meromictic lake La Cruz (Spain) that sustains dense populations of purple and green anoxygenic phototrophic bacteria despite low sulfate and sulfide concentrations. We observed in situ photoferrotrophic activity through stimulation of phototrophic carbon uptake in the presence of Fe(II), and quantified light-dependent Fe(II)-oxidation by the natural chemocline microbiota to assess their potential quantitative contribution to ancient BIF formation. In addition, a green photoferrotrophic bacterial consortium was enriched for the first time from a ferruginous water column. This new model ecosystem will allow testing current concepts on ancient primary productivity and its interactions with the iron- and sulphur cycles and may help to refine paleoenvironmental proxies. <br> In the third chapter, our results indicate, for the first time, that nitrate-dependent chemoautotrophic iron-oxidation occurred within Lake La Cruz chemocline. The organisms responsible for this Fe(II)-oxidation demonstrated, in optimized conditions, that their Fe(II)-oxidation rate was sufficiant to oxidize the totality of the dissolved Fe(II) arriving in the chemocline compartment (21.6 - 38.4 μmol Fe(II) I<sup>-1</sup> d<sup>-1</sup> and 0.174 - 1.393 μmol Fe(II) <sup>-1</sup> d<sup>-1</sup> respectively). MPN counts for anaerobic nitrate-dependent iron-oxidizers in other stratified water columns suggest that this metabolism is more widespread than previously thought. In addition, those results support a possible participation of such metabolism to BIFs formation during the Neoarchean, once nitrates were made available after the apparition of oxygenic photosynthesis. <br> In the forth chapter, the results from this study combined with those of previous studies allowed us to establish a biogeochemical model, including all the metabolisms thought to have existed in the Late Archean Ocean. Thus, Lake La Cruz illustrates how those microorganisms could have driven and shaped biogeochemical cycles during this period. Accordingly, the water column of Lake La Cruz may represent an ecotone between the two main Neoarchean Ocean compartments and, consequently, be a good model system, or samples source, for studying metabolic activity interactions in experimental conditions that reflect theoretical models of the Archean Ocean.
Notes
Thèse de doctorat : Université de Neuchâtel, 2011 ; 2214
Identifiants
Type de publication
doctoral thesis
