Oncoids from the river Alz (southern Germany): Tiny ecosystems in a phosphorus-limited environment

Spectacular modern freshwater oncoids from the river Alz (Bavaria, Southern Germany) grow in-situ, without being regularly overturned. In order to understand growth processes, properties of Alz river waters such as nutrient balance and pH were analysed and monitored, and the biotic composition of the oncoids was examined. Key features for the understanding of the growth of the Alz oncoids include (1) the presence of subconcentric calcareous laminae; (2) the existence of extracellular polymeric substances (EPS) within the enveloping biofilm that are produced by cyanobacteria and diatoms and cover the entire oncoid except for the contact area with riverbed sediments; (3) a stratified, ‘telescoping’ pattern of cyanobacteria living both intertwined within the soft outer biofilm and in radial arrangement within the partially calcified cortex below; (4) the calcification pattern of individual cyanobacterial colonies; (5) the presence of an oncoidal food web; (6) the occurrence of destructive processes such as boring by endolithic cyanobacteria and EPS-burrowing of animals; (7) a lack of multicellular algae and higher plants owing to pronounced phosphate limitation.Organismic interactions important in the extant Alz oncoids are believed to be important for a more complete understanding of fossil oncoids.