Data Sheet 3_Mass development of a filamentous and likely nitrophilous aerophytic green alga on tree bark: Apatococcus ammoniophilus sp. nov. (Chlorophyta, Trebouxiophyceae).docx

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，丹麦境内一种在嫩枝与针叶上形成可观生物量的丝状绿藻出现了入侵式增殖。该藻类在丹麦西部的针叶人工林中尤为繁盛，而该区域的大气氮沉降模拟浓度为全国最高。然而，其物种鉴定与分类学地位始终不明。

材料与方法
对取自多种基质的藻类样本进行了核糖体DNA序列、宏基因组及生化成分（多元醇与类菌孢素氨基酸（mycosporine-like amino acids，MAAs））分析。

结果
系统发育分析显示，该藻类隶属于绿藻门（Chlorophyta）共球藻纲（Trebouxiophyceae），在平藻属（Apatococcus）内形成一个独立演化支。尽管该藻类与多种其他共球藻纲物种存在关联，但宏基因组数据显示平藻属重叠群的覆盖度极高，证实其为优势类群，这与扩增子测序方法的结果一致。研究检出了阿拉伯醇、赤藓糖醇与海藻糖等低分子量碳水化合物，其中赤藓糖醇的浓度最高。赤藓糖醇的存在为其归入平藻属提供了化学分类学佐证。此外，研究还发现了一种独特的紫外吸收类菌孢素氨基酸，这可能是共球藻纲此前未被记录的新类群。本文将该物种描述为氨生平藻（A. ammoniophilus），其形态特征确凿表明，该物种早在100多年前就已在丹麦被记录。该物种的形态特征与近缘类群高度相似，例如细胞核周围存在颗粒环，以及可形成二细胞单元。

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，很好地解释了其气生陆生生活型的生化基础——这类有机化合物可分别抵御干燥脱水与紫外辐射。尽管在氨沉降水平推测较低的多种基质上的不显眼生物膜中，也发现了氨生平藻的基因型且未观察到丝状阶段，但丹麦西部近数十年来的入侵式定植被认为与氨沉降密切相关。因此，氨生平藻被提议作为大气氮沉降的潜在生物指示物。丝状生长与氮积累之间的潜在关联仍需进一步研究，包括培养实验。