Microbialite Formation in Seawater of Increased Alkalinity, Satonda Crater Lake, Indonesia

Abstract

The crater lake of the small volcanic island Satonda, Indonesia, is unique for its red-algal microbial reefs thriving in marinederived water of increased alkalinity. The lake is a potential analogue for ancient oceans sustaining microbialites under open-marine conditions. Current reef surfaces are dominated by living red algae covered by non-calcified biofilms with scattered cyanobacteria and diatoms. Minor CaCO 3 precipitates are restricted to the seasonally flooded reef tops, which develop biofilms up to 500 m thick dominated by the cyanobacteria Pleurocapsa, Calothrix, Phormidium, and Hyella. Microcrystalline aragonite patches form within the biofilm mucilage, and fibrous aragonite cements grow in exopolymer-poor spaces such as the inside of dead, lysed green algal cells, and reef framework voids. Cementation of lysed hadromerid sponge resting bodies results in the formation of ''Wetheredella-like'' structures. Hydrochemistry data and model calculations indicate that CO 2 degassing after seasonal mixis can shift the carbonate equilibrium to cause CaCO 3 precipitation. Increased concentrations of dissolved inorganic carbon limit the ability of autotrophic biofilm microorganisms to shift the carbonate equilibrium. Therefore, photosynthesis-induced cyanobacterial calcification does not occur. Instead, passive, diffusioncontrolled EPS-mediated permineralization of biofilm mucus at contact with the considerably supersaturated open lake water takes place. In contrast to extreme soda lakes, the release of Ca 2؉ from aerobic degradation of extracellular polymeric substances does not support CaCO 3 precipitation in Satonda because the simultaneously released CO 2 is insufficiently buffered.Subfossil reef parts comprise green algal tufts encrusted by microstromatolites with layers of fibrous aragonite and an amorphous, unidentified Mg-Si phase. The microstromatolites probably formed when Lake Satonda evolved from seawater to Ca 2؉ -depleted raised-alkalinity conditions because of sulfate reduction in bottom sediments and pronounced seasonality with deep mixing events and strong CO 2 degassing. The latter effect caused rapid growth of fibrous aragonite, while Mg-Si layers replaced the initially Mg-calcite-impregnated biofilms. This could be explained by dissolution of siliceous diatoms and sponge spicules at high pH, followed by Mg-calcite dissolution and Mg-silica precipitation at low pH due to heterotrophic activity within the entombed biofilms.

Viruses as new agents of organomineralization in the geological record

“…Amorphous Mg-Si precipitates have previously been reported in carbonate environments, and are always associated with microbial activity [13][14][15] . It has also been demonstrated that am Mg-Si enhances fossilization, and is commonly found in the rock record 13 .…”

“…It has also been demonstrated that am Mg-Si enhances fossilization, and is commonly found in the rock record 13 . The authigenesis of am Mg-Si phases is, however, poorly understood, with previous work suggesting that am Mg-Si layers in microbialites are a replacement of a primary mineral phase 14 . In contrast, our data indicate a primary origin for these am Mg-Si phases.…”

“…Thus, we here propose to add viruses to the list of templates for am Mg-Si mineralization within microbial mats. With continued diagenesis in calcium carbonate-rich environments, progressive Ca incorporation and subsequent replacement of much of the am Mg-Si material by carbonate likely leads to the common geological occurrence of calcified bacterial cells, carbonate nanospheres and am Mg-Si phases [13][14][15] . This is indeed observed in both our diagenetic experiments and within the lithified layers in Lagoa Vermehla itself (Fig.…”

Specific carbonate–microbe interactions in the modern microbialites of Lake Alchichica (Mexico)

“…They can be laminated in the case of stromatolites. Although abundant in the Precambrian, microbialites are scarce today, being found only in restricted areas such as in the hypersaline Shark Bay in Australia (Logan, 1961) and in alkaline lakes such as Lake Van in Turkey (Kempe et al, 1991) or Lake Satonda in Indonesia (Arp et al, 2003). The stromatolites from the Bahamas are the only example of such structures developing in an open marine environment (Dravis, 1983;Dill et al, 1986;Reid and Browne, 1991;Reid et al, 2000Reid et al, , 2003.…”

Defining organominerals: Comment on ‘Defining biominerals and organominerals: Direct and indirect indicators of life’ by Perry et al. (2007, Sedimentary Geology, 201, 157–179)

“…Thereafter, examples of organominerals and organomineralization processes have been documented, and the former papers cited, in mainstream scientific literature, beginning with our own work on carbonate organomineral formation in modern and recent microbialites and ooids (Défarge et al, 1996;Reitner et al, 1997;Arp et al, 1999;Trichet et al, 2001;Arp et al, 2003;Gautret et al, 2004;Gautret and Trichet, 2005), ancient microbialite analogues (Camoin et al, 1999;Tribovillard et al, 1999Tribovillard et al, , 2000, mud mounds (Neuweiler et al, 1999), encrusting sponge cements (Russo et al, 2006), housing tubes of worms (Fischer et al, 2000), coral EPS (Reitner, 2005), decaying sponge tissues (Delecat et al, 2001), and during laboratory experiments with microbialite-, ooid-and meteorite-extracted organic matter (Reitner, 2004). Organominerals and organomineralization processes have also been brought up in a discussion, involving one of us, about the origin of lower Paleozoic carbonate mud (Pratt, 2001;Arp et al, 2002).…”