Chloromonas rubrosalmonea sp. nov. (Chlorophyta) causes blooms of salmon-red snow due to high astaxanthin and low chlorophyll content

Melting snowfields support microbial communities, including blooms of phototrophic psychrophiles. “Watermelon snow” and its flagship species Sanguina nivaloides are mostly found at exposed, alpine or polar sites. Here, we investigated an alga that is morphologically similar but likely less common, thriving in exposed or semi-exposed snowpacks at lower altitudes frequently close to the tree line, and causing characteristic, intensely salmon-red snow surface spots in Central European and Norwegian mountains. Our aim was to illustrate the taxonomic heterogeneity of red snow by performing a cellular and molecular characterization of this species. The phylogenetic analysis placed the alga into the genus Chloromonas, clade “group C”. Single-cell Sanger sequencing enabled us to link the cell morphotype to its genotype. In addition to light microscopy of the snow samples, applying 18S rRNA gene and ITS2 metabarcoding confirmed that this alga dominated the bloom communities. The cysts had multi-layered smooth cell walls, and had a broadly oval to fully spherical shape, in striking contrast to most other Chloromonas species found on snow surfaces. Another remarkable feature was a pronounced astaxanthin accumulation in relation to a low proportion of chlorophylls, occasionally accompanied by a significant reduction of thylakoid membranes. This likely caused the photosynthetic dormancy of mature cysts found in some samples. Based on these data, we describe the snow alga as Chloromonas rubrosalmonea, sp. nov.融雪生境支撑着各类微生物群落，其中包含光合嗜冷微生物（phototrophic psychrophiles）的藻华。"西瓜雪"及其代表物种雪生血球藻（Sanguina nivaloides）多分布于裸露的高山或极地生境。本研究针对一种形态相似但较为少见的藻类展开探究：该藻类栖息于常紧邻林线的低海拔裸露或半裸露积雪层中，并在中欧与挪威的山地形成特征鲜明、呈深鲑鱼红色的雪面斑块。本研究旨在通过对该物种进行细胞与分子特征分析，阐明红雪的分类学异质性。系统发育分析显示，该藻类隶属于绿囊藻属（Chloromonas）的"C群"演化支。单细胞桑格测序（Sanger sequencing）帮助我们将细胞形态型与其基因型建立关联。除对雪样开展光学显微镜观察外，通过18S rRNA基因及ITS2元条形码（metabarcoding）测序分析证实，该藻类为藻华群落的优势类群。该藻类的孢囊具有多层光滑细胞壁，形态从宽椭圆形至近完美球形不等，这与多数已报道的雪生绿囊藻属物种存在显著差异。另一显著特征是其虾青素（astaxanthin）积累量极高，而叶绿素（chlorophylls）占比极低，有时还伴随类囊体膜（thylakoid membranes）的显著减少，这一特征可能导致部分样本中成熟孢囊进入光合休眠状态。基于上述研究结果，我们将该雪生藻类正式描述为新种红鲑色绿囊藻（Chloromonas rubrosalmonea, sp. nov.）。