Phototrophs in High-Iron-Concentration Microbial Mats: Physiological Ecology of Phototrophs in an Iron-Depositing Hot Spring

Abstract

At Chocolate Pots Hot Springs in Yellowstone National Park the source waters have a pH near neutral, contain high concentrations of reduced iron, and lack sulfide. An iron formation that is associated with cyanobacterial mats is actively deposited. The uptake of [14C]bicarbonate was used to assess the impact of ferrous iron on photosynthesis in this environment. Photoautotrophy in some of the mats was stimulated by ferrous iron (1.0 mM). Microelectrodes were used to determine the impact of photosynthetic activity on the oxygen content and the pH in the mat and sediment microenvironments. Photosynthesis increased the oxygen concentration to 200% of air saturation levels in the top millimeter of the mats. The oxygen concentration decreased with depth and in the dark. Light-dependent increases in pH were observed. The penetration of light in the mats and in the sediments was determined. Visible radiation was rapidly attenuated in the top 2 mm of the iron-rich mats. Near-infrared radiation penetrated deeper. Iron was totally oxidized in the top few millimeters, but reduced iron was detected at greater depths. By increasing the pH and the oxygen concentration in the surface sediments, the cyanobacteria could potentially increase the rate of iron oxidation in situ. This high-iron-content hot spring provides a suitable model for studying the interactions of microbial photosynthesis and iron deposition and the role of photosynthesis in microbial iron cycling. This model may help clarify the potential role of photosynthesis in the deposition of Precambrian banded iron formations.

Didymosphenia geminata: Algal blooms in oligotrophic streams and rivers

“…As the mats develop, new stalks are produced at the surface and older stalks with bound Fe and P are displaced to the inner regions of the mat. In Rapid Creek, thick (2-4 cm) mats develop a redox gradient, in which the surface is oxidized as a result of photoautotrophy, while the inner and bottom regions of the mats are reduced, a feature observed in other microbial mats [Pierson et al, 1999;Roden et al, 2004;Baumgartner et al, 2006]. In the absence of oxygen, microbes can utilize other electron acceptors such as oxidized Fe (Fe 3+ ) and SO 4 2− for respiration, resulting in the generation of reduced Fe (Fe 2+ ) and soluble sulfides (S 2− ), respectively.…”

Physiology of phototrophic iron(II)-oxidizing bacteria: implications for modern and ancient environments

“…Indeed, a range of pH fitting the limits for the phototrophic Fe(II)oxidizers described here occur in naturally forming microbial mats (e.g. Pierson et al, 1999).…”

“…In modern microbial iron-rich mats, light penetrates on average about 2-3 mm and thus phototrophic bacteria will live in the upper anoxic millimeter of such an environment (e.g. Pierson et al, 1999). The amount of light the phototrophic Fe(II)-oxidizing strains need for maximum Fe(II) oxidation follows the general observation that green-sulfur bacteria (such as strain KoFox) have a very low light saturation, while purple sulfur and purple nonsulfur bacteria need more light (in our case represented by strains F4 and SW2, respectively) (Overmann & Garcia-Pichel, 2000).…”

Formation of biogenic sheath‐like Fe oxyhydroxides in a near‐neutral pH hot spring: Implications for the origin of microfossils in high‐temperature, Fe‐rich environments

“…Under anoxic conditions, the formation of biogenic Fe oxyhydroxides can be related to the activity of nitrate-reducers and photoautotrophic bacteria using Fe(II) as an electron donor [Straub et al, 1996;Kappler and Newman, 2004]. Cyanobacteria are also considered to play an important role in the formation of biogenic Fe oxyhydroxides, due to the production of O 2 during their metabolism, which can oxidize the dissolved reduced Fe and lead to subsequent precipitation of Fe oxyhydroxides [Cloud, 1973;Pierson et al, 1999;Pierson and Parenteau, 2000;Parenteau and Cady, 2010]. Despite these metabolic activities, passive Fe sorption and nucleation may also occur on many bacterial cell walls and lead to Fe oxyhydroxides formation in high-Fe, near-neutral pH environments [Ferris et al, 1989a[Ferris et al, , 1989bMcLean et al, 1992].…”