Department of Earth, Environmental and Planetary Sciences, Brown University, Providence, Rhode Island.
The Josephine Bay Paul Center for Comparative Molecular Biology and Evolution, Marine Biological Laboratory, Woods Hole, Massachusetts.
Geobiology. 2019 May;17(3):272-280. doi: 10.1111/gbi.12330. Epub 2019 Feb 5.
Members of the order Isochrysidales are unique among haptophyte lineages in being the exclusive producers of alkenones, long-chain ketones that are commonly used for paleotemperature reconstructions. Alkenone-producing haptophytes are divided into three major groups based largely on molecular ecological data: Group I is found in freshwater lakes, Group II commonly occurs in brackish and coastal marine environments, and Group III consists of open ocean species. Each group has distinct alkenone distributions; however, only Groups II and III Isochrysidales currently have cultured representatives. The uncultured Group I Isochrysidales are distinguished geochemically by the presence of tri-unsaturated alkenone isomers (C Me, C Et, C Me, C Et) present in water column and sediment samples, yet their genetic diversity, morphology, and environmental controls are largely unknown. Using small-subunit (SSU) ribosomal RNA (rRNA) marker gene amplicon high-throughput sequencing of environmental water column and sediment samples, we show that Group I is monophyletic with high phylogenetic diversity and contains a well-supported clade separating the previously described "EV" clade from the "Greenland" clade. We infer the first partial large-subunit (LSU) rRNA gene Group I sequence phylogeny, which uncovered additional well-supported clades embedded within Group I. Relative to Group II, Group I revealed higher levels of genetic diversity despite conservation of alkenone signatures and a closer evolutionary relationship with Group III. In Group I, the presence of the tri-unsaturated alkenone isomers appears to be conserved, which is not the case for Group II. This suggests differing environmental influences on Group I and II and perhaps uncovers evolutionary constraints on alkenone biosynthesis.
盘形藻目成员是甲藻中唯一产生烯酮的类群，烯酮是一种长链酮，常用于古温度重建。产烯酮的甲藻类群主要基于分子生态学数据分为三大类群：I 组存在于淡水湖泊中，II 组通常存在于半咸水和沿海海洋环境中，III 组由开阔海洋物种组成。每个组都有独特的烯酮分布；然而，只有 II 组和 III 组的盘形藻目有培养的代表。未培养的 I 组盘形藻目在地球化学上的特征是水柱和沉积物样品中存在三不饱和烯酮异构体（C Me、C Et、C Me、C Et），但其遗传多样性、形态和环境控制因素在很大程度上是未知的。通过对环境水柱和沉积物样品中小亚基（SSU）核糖体 RNA（rRNA）标记基因扩增子高通量测序，我们表明 I 组具有单系性和高系统发育多样性，并包含一个支持良好的分支，将先前描述的“EV”分支与“格陵兰”分支分开。我们推断了第一个部分大亚基（LSU）rRNA 基因 I 组序列系统发育，其中揭示了 I 组内嵌入的其他支持良好的分支。与 II 组相比，尽管烯酮特征保守，与 III 组进化关系密切，但 I 组的遗传多样性水平更高。在 I 组中，三不饱和烯酮异构体的存在似乎是保守的，而在 II 组中则不是。这表明 I 组和 II 组受到不同的环境影响，或许揭示了烯酮生物合成的进化限制。
