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Microbially mediated calcification is a major process of carbonate production, yet little is known about eukaryotic microalgal calcifiers. We describe calcification and propagation of the unicellular microalga Oocardium stratum in an Alpine spring stream. The spring sheds Ca–Mg–HCO3 water with a temperature of 8–11 °C. The biota is dominated by O. stratum and diatoms; mosses, cyanobacteria, and filamentous eukaryotic algae are accessories. O. stratum colonize various substrates within the stream throughout the year. When colonizing, single cells attached to mucilage, then induced precipitation of a rim of calcite, and underwent a first division. A mature clone of O. stratum typically consists of single cells each housed within a calcite tube precipitated by the microalga. Upon cell division, the tubes branch, too, under retention of the optical orientation of the calcite. Continued growth, cell division, and calcification result in laminae of Oocardium calcite (OC) concordant with substrate shape. O. stratum accelerates but seems not to control calcite precipitation. A maximum vertical calcification rate of 5 mm/a was documented for a site ~ 25 m downstream of the spring. ‘Crystal-skeletal’ OC characterized low calcite supersaturation, whereas higher supersaturation corresponded with rhombohedral OC. Abiotic precipitation downward of the upper tips of growing calcite tubes resulted in compact spar crystals, irrespective of initial crystal habit. Diatoms that thrived on OC benefit from a large differentiated habitat. Our study confirms previous works that identify O. stratum, not cyanobacteria, as major biocalcifiers in some hardwater springs. Diagenetically mature Oocardium tufa, however, may be confused with spring limestones of other origins.

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Exploring the niche of a highly effective biocalcifier: calcification of the eukaryotic microalga Oocardium stratum Nägeli 1849 in a spring stream of the Eastern Alps