Data Sheet 4_Mass development of a filamentous and likely nitrophilous aerophytic green alga on tree bark: Apatococcus ammoniophilus sp. nov. (Chlorophyta, Trebouxiophyceae).xlsx

IntroductionA filamentous green alga forming significant biomass on twigs and needles was observed to have increased invasively in Denmark in recent decades. It was particularly abundant in coniferous plantations in western parts of Denmark that experience the highest modelled concentration of atmospheric nitrogen deposition. However, its species identity and taxonomy remained unknown.
Material and methodsSelected algal samples from various substrates were analyzed for their ribosomal DNA sequences, metagenomic, and biochemical compounds [polyols and mycosporine-like amino acids (MAAs)].
ResultsPhylogenetic analyses revealed the alga’s position within the Trebouxiophyceae (Chlorophyta), forming an independent lineage within Apatococcus. Though it was associated with various other Trebouxiophyceae species, the metagenome showed exceptionally high coverage of the Apatococcus contigs, proving its predominance, consistent with the amplicon-based approach. The low molecular weight carbohydrates, arabitol, erythritol, and trehalose – with erythritol displaying the highest concentrations—were recovered. The presence of erythritol provided chemotaxonomic support for the classification in Apatococcus. Additionally, a unique UV-absorbing mycosporine amino acid (MAA), likely new for the Trebouxiophyceae, was found. The species is described here as A. ammoniophilus, and the observed morphological features leave no doubt that it has been recorded from Denmark more than a 100 years ago. Morphological features are shared with its closer relatives, such as the presence of a ring of particles surrounding the nucleus and the formation of two-celled units.
DiscussionThe presence of low molecular weight carbohydrates and the unique MAA in A. ammoniophilus well explain the biochemical basis for its aeroterrestrial lifestyle, as these organic compounds protect against desiccation and UV-radiation, respectively. Even though the genotype of A. ammoniophilus has also been found in inconspicuous biofilms devoid of filamentous stages on various substrates with presumably low ammonia deposition, the very invasive colonization in recent decades in western Denmark is assumed to be due to ammonia deposition. Consequently, A. ammoniophilus is suggested to be a potential biological indicator of air borne nitrogen deposition. A possible connection between filamentous growth and nitrogen accumulation needs further investigation, including culture experiments.

引言
近数十年来，丹麦境内一种在小枝与针叶表面形成显著生物量的丝状绿藻出现了入侵性扩张。该藻类在丹麦西部的针叶人工林中分布尤为密集，而该区域的大气氮沉降模拟浓度居全国最高。然而，其物种归属与分类地位长期以来不明。

材料与方法
从不同基质中选取藻类样本，对其核糖体DNA序列、宏基因组及生化组分（多元醇与类菌孢素氨基酸（mycosporine-like amino acids, MAAs））开展分析。

结果
系统发育分析结果表明，该藻类隶属于绿藻门（Chlorophyta）共球藻纲（Trebouxiophyceae），在平藻属（Apatococcus）内形成独立演化支系。尽管该藻类与多种其他共球藻纲物种存在关联，宏基因组测序结果显示平藻属重叠群的覆盖度极高，证实该藻类为优势类群，这一结果与扩增子测序方法的分析结果一致。研究检测到三类低分子量碳水化合物：阿拉伯醇（arabitol）、赤藓糖醇（erythritol）与海藻糖（trehalose），其中赤藓糖醇的浓度最高。赤藓糖醇的存在为该藻类归属于平藻属提供了化学分类学依据。此外还发现一种独特的紫外吸收类菌孢素氨基酸（MAA），该物质可能是共球藻纲中尚未被记录的新类群。本研究将该物种命名为氨生平藻（Apatococcus ammoniophilus，缩写A. ammoniophilus），且形态学证据确凿表明，该物种早在100多年前就已在丹麦被记录。其形态特征与近缘类群高度一致，例如存在围绕细胞核的颗粒环以及两细胞单元的形成。

讨论
氨生平藻所含的低分子量碳水化合物与独特的类菌孢素氨基酸，分别可抵御干燥胁迫与紫外辐射，这很好地解释了其气生陆生生活方式的生化基础。尽管在氨沉降水平较低的各类基质上形成的无丝状阶段的不显眼生物膜中，也曾发现氨生平藻的基因型，但丹麦西部近数十年来出现的大规模入侵性定殖，被认为与氨沉降密切相关。因此，氨生平藻被提议为大气氮沉降的潜在生物指示剂。丝状生长与氮积累之间的潜在关联，尚需包括培养实验在内的进一步研究加以验证。