Diversity of Microscopic Green Algae (Chlorophyta) in Calcifying Biofilms of Two Karstic Streams in Germany

For the first time the diversity of microscopic green algae (Chlorophyta) from calcified biofilms of karstic streams was analyzed using a combined approach based on pure cultures, i.e., 18S rRNA gene sequencing and microscopic analyses. Our study focused on two creeks in Germany. A considerable diversity of 34 species of green microalgae comprising three classes, the Trebouxiophyceae, Chlorophyceae and Ulvophyceae, was recovered. The biofilms of both streams were rather different in their species compositions which may reflect that they are exposed to differed hydrochemical conditions. The shallow Westerhöfer creek harbored predominantly Trebouxiophyceae and exhibited higher Mg²⁺ and SO₄²⁻ concentrations. In contrast, the deeper, longer and spatially more heterogeneous Deinschwanger creek harbored numerous species of Chlorophyceae. A lower number of species from the Ulvophyceae were spread on both studied streams. The closest relatives of the identified species were from other freshwater habitats, but mostly from phytoplankton. However, also several species we recovered from freshwater for the first time; so far they have been known from terrestrial habitats only. Less than half of the recovered species could be identified with names at the species level based on high sequence identities with available sequences from named reference strains. Most recovered species could not be identified with names to species level, i.e., they may represent species for which no 18S rRNA gene sequence have become available yet or which have been retrieved for the first time. The genus <i>Gongrosira</i> Kützing, often reported from freshwater tufa-stromatolites, was found to represent most likely a collective morphotype formed by several genera nested within the Ulvophyceae.

本研究首次采用纯培养结合的联合分析策略——即18S rRNA基因测序与显微形态观察，对喀斯特溪流钙化生物膜内的微型绿藻（绿藻门Chlorophyta）多样性展开分析。本研究的研究对象为德国境内的两条溪流。研究共分离得到隶属于3个纲的34种微型绿藻，涵盖共球藻纲（Trebouxiophyceae）、绿藻纲（Chlorophyceae）与石莼纲（Ulvophyceae）。两条溪流的生物膜物种组成差异显著，这或反映二者所处的水文化学环境各不相同。浅水环境的韦斯特赫弗溪以共球藻纲物种为优势类群，其水体中Mg²+与SO₄²−浓度更高。相较而言，更深、更长且空间异质性更强的戴施万格溪（Deinschwanger creek）则拥有大量绿藻纲物种。石莼纲物种在两条研究溪流中均有分布，但物种丰度较低。所鉴定物种的近缘类群多来自其他淡水生境，且以浮游藻类为主。此外，本研究还首次在淡水生境中发现了若干物种——此前这些物种仅被记录于陆生生境中。基于与已命名参考菌株的高序列同源性比对，仅有不到半数的分离物种可鉴定到物种水平的学名。多数分离物种无法鉴定到物种水平的学名，这意味着它们可能是尚无18S rRNA基因序列公开的物种，或是首次被采集记录的新类群。通常被报道见于淡水钙华叠层石中的孔丝藻属（Gongrosira Kützing），经本研究证实极有可能是隶属于石莼纲的多个嵌套属共同形成的复合形态类群。