School of BioSciences, University of Melbourne, Parkville, Victoria, 3010, Australia.
Westmead Institute for Medical Research, The University of Sydney, Westmead, New South Wales, 2145, Australia.
J Phycol. 2019 Dec;55(6):1210-1225. doi: 10.1111/jpy.12909. Epub 2019 Oct 9.
Prasinophytes (Chlorophyta) are a diverse, paraphyletic group of planktonic microalgae for which benthic species are largely unknown. Here, we report a sand-dwelling, marine prasinophyte with several novel features observed in clonal cultures established from numerous locations around Australia. The new genus and species, which we name Microrhizoidea pickettheapsiorum (Mamiellophyceae), alternates between a benthic palmelloid colony, where cell division occurs, and a planktonic flagellate. Flagellates are short lived, settle and quickly resorb their flagella, the basal bodies then nucleate novel tubular appendages, termed "microrhizoids", that lack an axoneme and function to anchor benthic cells to the substratum. To our knowledge, microrhizoids have not been observed in any other green alga or protist, are slightly smaller in diameter than flagella, generally contain nine microtubules, are long (3-5 times the length of flagella) and are not encased in scales. Following settlement, cell divisions result in a loose, palmelloid colony, each cell connected to the substratum by two microrhizoids. Flagellates are round to bean-shaped with two long, slightly uneven flagella. Both benthic cells and flagellates, along with their flagella, are encased in thin scales. Phylogenies based on the complete chloroplast genome of Microrhizoidea show that it is clearly a member of the Mamiellophyceae, most closely related to Dolichomastix tenuilepsis. More taxon-rich phylogenetic analyses of the 18S rRNA gene, including metabarcodes from the Tara Oceans and Ocean Sampling Day projects, confidently show the distinctive nature of Microrhizoidea, and that the described biodiversity of the Mamiellophyceae is a fraction of its real biodiversity. The discovery of a largely benthic prasinophyte changes our perspective on this group of algae and, along with the observation of other potential benthic lineages in environmental sequences, illustrates that benthic habitats can be a rich ground for algal biodiscovery.
黄藻门（绿藻）是一个多样化的、并系的浮游微藻群体，其底栖物种在很大程度上是未知的。在这里，我们报道了一种沙生、海洋黄藻，它具有几个新的特征，这些特征是从澳大利亚各地的众多地点建立的克隆培养物中观察到的。这个新的属和种，我们命名为Microrhizoidea pickettheapsiorum（Mamiellophyceae），在一个底栖的盘状群体和一个浮游鞭毛体之间交替，在盘状群体中发生细胞分裂。鞭毛体的寿命很短，沉降后很快重新吸收鞭毛，基底体然后核生出新的管状附属物，称为“微根”，这些微根没有轴丝，用于将底栖细胞固定在基质上。据我们所知，微根在任何其他绿藻或原生动物中都没有观察到，直径略小于鞭毛，通常含有 9 个微管，长度较长（是鞭毛的 3-5 倍），并且不被鳞片包裹。沉降后，细胞分裂导致松散的盘状群体，每个细胞通过两个微根与基质相连。鞭毛体呈圆形至豆形，有两条长而略微不均匀的鞭毛。底栖细胞和鞭毛体以及它们的鞭毛都被薄鳞片包裹。基于 Microrhizoidea 的完整叶绿体基因组的系统发育分析表明，它显然是 Mamiellophyceae 的一个成员，与 Dolichomastix tenuilepsis 关系最密切。对包括 Tara Oceans 和 Ocean Sampling Day 项目 metabarcodes 的 18S rRNA 基因进行的更多分类群丰富的系统发育分析，有信心地表明了 Microrhizoidea 的独特性质，以及描述的 Mamiellophyceae 的生物多样性只是其真实生物多样性的一部分。发现一种主要的底栖黄藻改变了我们对该藻类群体的看法，并且，与环境序列中其他潜在底栖谱系的观察一起，说明了底栖生境可以成为藻类生物多样性发现的丰富之地。
