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.

中文翻译：

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探索高效生物钙化剂的生态位：真核微藻 Oocardium 地层 Nägeli 1849 在东阿尔卑斯山的泉水中的钙化

微生物介导的钙化是碳酸盐生产的主要过程，但对真核微藻钙化剂知之甚少。我们描述了高山泉流中单细胞微藻 Oocardium 层的钙化和传播。泉水流出温度为 8-11 °C 的 Ca-Mg-HCO3 水。生物群以O.地层和硅藻为主；苔藓、蓝藻和丝状真核藻类是附件。O. 地层全年在溪流内的各种基质中定殖。定植时，单个细胞附着在粘液上，然后诱导方解石边缘沉淀，并进行第一次分裂。O.层的成熟克隆通常由单个细胞组成，每个细胞都位于由微藻沉淀的方解石管内。细胞分裂时，管子也会分支，在保留方解石的光学取向的情况下。持续生长、细胞分裂和钙化导致 Oocardium 方解石 (OC) 的薄片与基质形状一致。O. 地层加速但似乎不能控制方解石沉淀。记录到弹簧下游约 25 m 处的最大垂直钙化率为 5 mm/a。“晶体-骨架”OC 的特点是方解石过饱和度低，而较高的过饱和度与菱面体 OC 相对应。无论初始晶体习性如何，向下生长的方解石管上端的非生物沉淀导致致密的晶石晶体。在 OC 上繁衍生息的硅藻受益于巨大的差异化栖息地。我们的研究证实了之前的工作，这些工作将 O. 地层而非蓝藻确定为某些硬水泉中的主要生物钙化剂。