Planktonic protists are an essential component of marine pelagic ecosystems where they mediate important trophic and biogeochemical functions. Although these functions are largely influenced by their taxonomic affiliation, the composition and spatial variability of planktonic protist communities remain poorly characterized in vast areas of the ocean. Here, we investigated the diversity of these communities in contrasting oceanographic conditions of the southwest Pacific (33–58 °S) using DNA metabarcoding of the 18S rRNA gene. Seawater samples collected during twelve cruises ($n$ = 482, 0–3100 m) conducted east of New Zealand were used to characterize protist communities in Subtropical (STW) and Subantarctic (SAW) surface water masses and the Subtropical Front (STF) that separates them. Diversity decreased with increasing latitude and increasing temperature but tended to be lowest in the STF. Sample ordination resulting from the abundance of amplicon single variants (ASVs) corresponded to the different water masses. Overall, Dinoflagellata (Syndiniales, 27%; Dinophyceae, 24% of standardized number of reads) dominated the euphotic zone followed by Chlorophyta (20%), but their relative abundance and composition at class and lower taxonomic levels varied consistently between water masses. Among Chlorophyta, several picoplanktonic algae species of the Mamiellophyceae class including Ostreococcus lucimarinus dominated in STW, while the Chloropicophyceae species Chloroparvula pacifica was most abundant in SAW. Bacillariophyta (5%), Prymnesiophyceae (5%), and Pelagophyceae (2%) classes were less abundant but showed analogous water mass specificity at class and finer taxonomic levels. Protist community composition in the STF had mixed characteristics and showed regional differences with the southern STF (50 °S) having more resemblance with subantarctic communities than the STF over the Chatham Rise region (42–44 °S). Below the euphotic zone, Syndiniales sequences (40%) dominated the dataset followed by Radiolaria (31%), Dinophyceae (14%) and other heterotrophic groups like Marine Stramenopiles and ciliates (1–1.5%). Among Radiolaria, several unidentified ASVs assigned to Spumellaria were most abundant, but showed significantly different distributions between STW and SAW highlighting the need to further investigate the taxonomy and ecology of this group. The present study represents a significant step forward towards characterizing protistan communities composition in relation to major physical oceanographic features in the southwest Pacific providing new insights about the biogeography and ecological preferences of different planktonic protist taxa from class to species and genotypic level.

浮游原生生物是海洋中上层生态系统的重要组成部分，它们在其中调节重要的营养和生物地球化学功能。尽管这些功能在很大程度上受其分类隶属关系的影响，但在大片海洋中，浮游原生生物群落的组成和空间变异性仍然很差。在这里，我们使用 18S rRNA 基因的 DNA 元条形码研究了这些群落在西南太平洋 (33-58°S) 对比海洋条件下的多样性。在十二次航行中收集的海水样本（$n$= 482, 0–3100 m) 在新西兰东部进行，用于表征亚热带 (STW) 和亚南极 (SAW) 地表水团以及将它们分开的副热带锋 (STF) 中的原生生物群落。多样性随着纬度的增加和温度的升高而降低，但在 STF 中趋于最低。由丰富的扩增子单一变体 (ASV) 产生的样本排序对应于不同的水团。总体而言，Dinoflagellata（Syndiniales，27%；Dinophyceae，占标准化读数的 24%）在透光区占主导地位，其次是 Chlorophyta（20%），但它们在类和较低分类水平上的相对丰度和组成在水团之间始终存在差异。在 Chlorophyta 中，有几种微型浮游藻类包括Ostreococcus lucimarinus在内的Mamiellophyceae类在 STW 中占主导地位，而Chloropicophyceae种Chloroparvula pacifica在 SAW 中最丰富。Bacillariophyta (5%)、Prymnesiophyceae (5%) 和Pelagophyceae (2%) 类较少，但在类和更精细的分类水平上表现出类似的水质量特异性。STF中的原生生物群落组成具有混合特征，并显示出区域差异，南部STF（50°S）与Chatham Rise地区（42-44°S）的STF相比与亚南极群落更相似。在透光区下方，Syndiniales 序列（40%）主导数据集，其次是放射虫（31%），恐龙纲(14%) 和其他异养类群，如海洋原生藻和纤毛虫 (1-1.5%)。在放射虫中，分配给Spumellaria的几种身份不明的 ASV 最为丰富，但在 STW 和 SAW 之间显示出显着不同的分布，突出了进一步研究该组的分类学和生态学的必要性。本研究代表了朝着描述与西南太平洋主要物理海洋学特征相关的原生生物群落组成迈出的重要一步，为从类别到物种和基因型水平的不同浮游原生生物分类群的生物地理学和生态偏好提供了新的见解。