Going nano: A new step toward understanding the processes governing freshwater ooid formation

Abstract

Marine and freshwater ooids were historically thought to form by purely physicochemical processes in turbulent environments. Recently, organomineralization has been identifi ed as a key process for the initiation of freshwater ooid cortex formation, but the exact biochemical mechanism(s) involved and subsequent contribution to the development of the growing cortex remain unknown. Here, we show that photosynthetic microbes not only enhance early carbonate precipitation around the ooid nucleus but also control the formation of the entire cortex in freshwater ooids from Lake Geneva, Switzerland. Microbial extracellular polymeric substances are fi rst permineralized as amorphous magnesium silicates ( am Mg-Si) before being calcifi ed. An ~5‰-6‰ depletion of 13 C in ooid cortices compared to both bulk values and carbonate nuclei supports this photosynthetic microbial mechanism and argues against contributions from sulfate-reducing bacteria or methanogens. These data have signifi cant implications for paleoenvironmental studies since photosynthetic microbes now provide an alternative to turbulent hydrodynamic conditions in the formation of freshwater ooids.

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 .…”

“…In contrast, our data indicate a primary origin for these am Mg-Si phases. Previous studies have established the role of cell walls and EPS as nucleation sites for am Mg-Si precipitates, leading to prokaryote fossilization 13,15 . Like EPS, the viral capsid is composed of proteins containing reactive carboxyl and amine groups 16,17 that are known to have a key role in cation binding 6 and thus, can act as a template for mineralization 18 .…”

“…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.…”

Biochemogenic origin of the ordovician chamosite in the Northern Urals

“…Êàê èçâåñòíî [20], â ñîâðå-ìåííûõ ãëèíèñòûõ ïîðîäàõ è èëàõ îáðàçîâàíèå ôðàìáîè-äàëüíîãî ïèðèòà îáóñëîâëåíî ñóùåñòâîâàíèåì ýâêñèííîé îáñòàíîâêè â ïðèäîííûõ âîäàõ è òðàêòóåòñÿ êàê ðåçóëüòàò ïèðèòèçàöèè íåïîñðåäñòâåííî íàíîáàêòåðèé. Èçó÷åíèå ñîâðåìåííûõ îêñèãèäðîêñèäîâ aeåëåçà è ôðàìáîèäàëüíî-ãî ïèðèòà íà ó÷àñòêàõ ìåòàíîâûõ âûñà÷èâàíèé [27] ïîêà-çàëî, ÷òî èìåííî àêòèâíîñòü ñóëüôàòðåäóöèðóþùèõ áàê-òåðèé â îáñòàíîâêàõ ñ íèçêîé êîíöåíòðàöèåé êèñëîðîäà îáåñïå÷èâàåò óñëîâèÿ äëÿ îáðàçîâàíèÿ ñóëüôèäîâ aeåëåçà.…”

“…Òàêîé ìèêðîýëåìåíòíûé ñîñòàâ â ìèíåðàëè-çîâàííûõ áèîïëåíêàõ ãîâîðèò î ñèëèêàòíîì ñîñòàâå ãëè-êîêàëèêñà, ÷òî ÿâëÿåòñÿ äîâîëüíî òèïè÷íûì, îñîáåííî â ìåçîçîéñêèõ òîëùàõ [29]. Äàaeå ïðè ôîðìèðîâàíèè èçâåñ-òêîâûõ îîèäîâ â ïðåñíîâîäíûõ áàññåéíàõ åùå äî êàëüöè-òèçàöèè ïðîèñõîäèò ìèíåðàëèçàöèÿ ãëèêîêàëèêñà ñ îá-ðàçîâàíèåì ðåíòãåíîàìîðôíîãî ìàãíåçèàëüíîãî ñèëèêà-òà [27]. Ïðîöåññû áèîèíäóöèðóåìîé ìèíåðàëèçàöèè ôåð-ðîëèòîâ ðàçëè÷àþòñÿ â çàâèñèìîñòè îò òèïà áàêòåðèé -aeåëåçîîêèñëÿþùèõ èëè aeåëåçîðåäóöèðóþùèõ.…”