Trace Metal and Phosphorus Enrichments in Cyanobacteria Cells and Cyanobacterial Precipitated Minerals

The enrichment of trace metals and other life-essential elements, like phosphorus, in carbonates may be a signature of microbial life. Enrichments of such elements in microbial carbonate facies in the rock record have been attributed to life in previous studies, but the biologic origin of these enrichments is contentious. We experimentally tested the hypothesis that enrichments of life-important trace elements occur in both cells and carbonate minerals that form as a result of cellular photosynthesis for the cyanobacteria Synechococcus PCC 8806. We grew Synechococcus PCC 8806 and measured the trace element concentrations of the cells and the minerals that precipitate with the cells, and we compared the results to abiotically precipitated mineral material from the same growth medium conditions. We found that for all the tested trace elements (B, P, K, Mn, Fe, Co, Cu, and Zn, chosen for their requirements in the growth medium of Synechococcus PCC 8806 and known uses in cellular machinery), nearly all the sample types were enriched relative to the medium concentrations. The dominant pattern for most elements was that cells were the most enriched, followed by biotic minerals, and then abiotic minerals. However, this pattern was complicated by varying concentrations of Mg in the mineral samples because the data were normalized to Mg (Mg was the dominant cation in the solution next to Na). Nonetheless, however the data are normalized, Fe was the most enriched element in the cells and both the biotic and abiotic minerals relative to the medium concentrations. Fe had the largest enrichment factor (E.F.) for all the sample types, with an E.F. of approximately 2800 in the biotic minerals, 1620 in the cells, and 230 in the abiotic minerals. Fe was followed by Zn (E.F. of ~329 in cells, 198 in biotic minerals, and 78 in abiotic minerals), Cu (E.F. of ~424 in cells, 171 in biotic minerals, and 50 in abiotic minerals), Mn (E.F. of ~200 in cells, 95 in biotic minerals, and 53 in abiotic minerals), and P (E.F. of ~149 in cells, 37 in biotic minerals, and 6 in abiotic minerals), suggesting that these elements can be useful as biosignatures when used in combination with other evidence.