Biomineralization of Fe- and Mn-rich silicates by cyanobacteria under oxic and alkaline conditions

Abstract

Abstract. Iron and manganese are poorly soluble elements in oxic and alkaline solutions, whereas they are much more soluble under anoxic conditions. As a result, the formation of authigenic mineral phases rich in Fe and/or Mn has traditionally been viewed as diagnostic of global or local anoxic conditions. Here we reveal that some specific picocyanobacteria can biomineralize abundant, authigenic Fe(III)- and Mn(IV)-rich amorphous silicates under oxic conditions in an alkaline lake in Mexico. The resulting biominerals cluster as small globules arranged as rings around the division septum of cyanobacterial cells. These rings are enveloped within an organic, likely polysaccharidic envelope, and are preserved upon sedimentation. Based on their 16S rDNA sequence, these cyanobacteria were affiliated to the Synechococcales order. The high Fe and Mn enrichment of the silicates questions the systematic inference of anoxic conditions based on their detection. Moreover, this process scavenges iron from the water column, an overlooked biological contribution to the Fe cycle. Finally, it reveals a new case of controlled biomineralization of silicates by bacteria.