The nature of electronic excitations created by photon absorption in the cyclic B850 aggregates of 18 bacteriochlorophyll molecules of LH2 antenna complexes of photosynthetic bacteria is studied over a broad temperature range using absorption, fluorescence, and fluorescence anisotropy spectra. The latter technique has been proved to be suitable for revealing the hidden structure of excitons in inhomogeneously broadened spectra of cyclic aggregates. A theoretical model that accounts for differences of absorbing excitons in undeformed and emitting exciton polarons in deformed antenna lattices is also developed. Only a slight decrease of the exciton bandwidth and exciton coupling energy with temperature is observed. Survival of excitons in the whole temperature span from cryogenic to nearly ambient temperatures strongly suggests that collective, coherent electronic excitations might play a role in the functional light-harvesting process taking place at physiological temperatures.

Silicification was a major mode of fossilization in Proterozoic peritidal environments, but marine silica concentrations and the chemical and biological mechanisms that drove microbial silicification and formation of early diagenetic chert in these environments remain poorly constrained. Here, we use taphonomy experiments to demonstrate that photosynthetically active cyanobacteria that are morphologically analogous to the oldest cyanobacterial fossil, Eoentophysalis, mediate the formation of magnesium-rich amorphous silica in seawater that is undersaturated with respect to silica. These results show that microbes in Proterozoic tidal environments may have mediated their own silicification at lower silica concentrations than previously assumed.

Oxygenic photosynthesis supplies organic carbon to the modern biosphere, but it is uncertain when this metabolism originated. Based on the inferred presence of manganese oxides in the sediments as old as 3 billion years, it has been proposed that photosynthetic reaction centers capable of splitting water arose by that time. However, this assumes that manganese oxides can only be produced in the presence of molecular oxygen 1 , reactive oxygen species 2,3 or by high-potential photosynthetic reaction centers 4,5 . Here we show that anoxygenic photosynthetic microbial communities biomineralize manganese oxides under strictly anaerobic conditions and in the absence of high-potential photosynthetic reaction centers. This light-dependent process can produce manganese oxide minerals and stimulate the redox cycling of carbon, sulfur, nitrogen and other elements in the photic zones of modern anoxic water bodies and sediments. Microbial oxidation of Mn(II) in the absence of molecular oxygen during the Archean Eon would have produced geochemical signals identical to those used to date the evolution of oxygenic photosynthesis before the Great Oxidation Event (GOE) 6,7 . Manganese (Mn) and more than 30 of its described oxides and hydroxides mediate the cycling of various trace metals and nutrients in the environment. The microbial ability to oxidize Mn(II) anaerobically is also hypothesized to have been a critical step in the evolution of oxygenic photosynthesis on the early Earth 4 . However, modern microbes are not known to anaerobically oxidize manganese. Here, we demonstrate this activity in active microbial cultures that grow in the presence of nanomolar oxygen concentrations relevant for the Archean Earth.Inoculum for the enrichment cultures of strictly anaerobic, photosynthetic biofilms came from the meromictic Fayetteville Green Lake (FGL), NY. The anaerobic photic zone of the lake contains 20 nM to 61 µM Mn(II) and 0-0.04 mM of H 2 S [8], and the most abundant phototroph there is the green sulfur bacterium Chlorobium sp. 9 . This microbe uses sulfide, hypothesized to be the oldest electron donor for photosynthesis 10 , as an electron donor. Photosynthetic biofilms of this organism and other strict anaerobes (Fig. 1a) were enriched in a minimal medium amended with 20-50 µM Na 2 S and 1 mM MnCl 2 and equilibrated with an anaerobic atmosphere of 80% N 2 and 20% CO 2 at pH 7. The concentration of O 2 in the medium was lower than 2 nM during the course of the experiment and the maximum total inflow of O 2 over two weeks was lower than 300 nmol (see Methods and Extended Data Fig. 1). These experimental concentrations match the upper estimates for the Archean Earth 11 . The anaerobic medium also lacked other potential oxidants for Mn(II) such as nitrite, nitrate and H 2 O 2 and these species were not produced in sterile controls (Extended Data Section 5). References 1 Tebo, B. M. et al. Biogenic manganese oxides: properties and mechanisms of formation.

Microbial fossils preserved by early diagenetic chert provide a window into the Proterozoic biosphere, but seawater chemistry, microbial processes, and the interactions between microbes and the environment that contributed to this preservation are not well constrained. Here, we use fossilization experiments to explore the processes that preserve marine cyanobacterial biofilms by the precipitation of amorphous silica in a seawater medium that is analogous to Proterozoic seawater. These experiments demonstrate that the exceptional silicification of benthic marine cyanobacteria analogous to the oldest diagnostic cyanobacterial fossils requires interactions among extracellular polymeric substances (EPS), photosynthetically induced pH changes, magnesium cations (Mg2+), and >70 ppm silica.