Natural iron (Fe) fertilization sustains phytoplankton blooms above the Kerguelen plateau (Indian sector of the Southern Ocean) within otherwise low productive off-plateau waters. In early spring and summer, these diatom-dominated blooms are associated with distinct heterotrophic prokaryotic communities, but whether a structuring effect extends to the post-bloom period has thus far not been investigated. To address this question, we carried out a detailed study of the prokaryotic community composition in the region of Kerguelen Island during late Austral summer (18 February to 27 March 2018; MOBYDICK project). Concentrations of chlorophyll a were seasonally low above the plateau (0.27–0.58 μg Chl a L⁻¹) and in a similar range to those at the 3 off-plateau sites investigated (0.14–0.34 μg Chl a L⁻¹), but we observed an accumulation of dissolved organic carbon and the build-up of heterotrophic prokaryotic biomass in Kerguelen plateau waters. Illumina sequencing of the 16S rRNA gene revealed that the total (DNA-based) and potentially active (RNA-based) prokaryotic communities were structured according to on- and off-plateau sites in the wind-mixed surface layer, in both the free-living (<0.8 μm size fraction) and particle-attached (>0.8 μm size fraction) fractions. The Amplicon Sequence Variants (ASV) with significantly higher relative abundances in on-plateau surface waters as compared to off-plateau waters belonged to Halieaceae OM60 group, several Flavobacteriaceae, such as the NS5 marine group, Aurantivirga and Ulvibacter, Rhodobacteraceae Loktanella, Saprospiraceae, and the Cryomorphaceae NS10 marine group. ASVs with higher relative abundances in off-plateau waters belonged to the Flavobacteriaceae Formosa, the Rhodobacteraceae Planktomarina and the SAR11 clade. We discuss the potential abiotic and biotic drivers of community composition in late Austral summer and the ecological roles of abundant prokaryotic taxa in Kerguelen plateau waters.

天然铁 (Fe) 施肥维持了凯尔盖朗高原（南大洋的印度部分）上方的浮游植物大量繁殖，否则高原外水域生产力较低。在早春和夏季，这些以硅藻为主的水华与不同的异养原核生物群落有关，但迄今为止尚未研究结构效应是否延伸到开花后时期。为了解决这个问题，我们对澳大利亚夏末（2018 年 2 月 18 日至 3 月 27 日；MOBYDICK 项目）期间凯尔盖朗岛地区的原核生物群落组成进行了详细研究。叶绿素a 的浓度在高原以上季节性较低（0.27–0.58 μg Chl a L ^-1) 并且与所研究的 3 个非高原地点的范围相似 (0.14–0.34 μg Chl a L ^-1 )，但我们观察到溶解有机碳的积累和凯尔盖朗高原水域中异养原核生物量的积累. Illumina 对 16S rRNA 基因的测序显示，总的（基于 DNA 的）和潜在活跃的（基于 RNA 的）原核生物群落是根据风混合表层的高原上和高原外位点构建的，在自由-活（<0.8 μm 大小分数）和颗粒附着（>0.8 μm 大小分数）分数。与高原外水相比，高原地表水中相对丰度显着更高的扩增子序列变体（ASV）属于海藻科OM60组，几个黄杆菌科，如 NS5 海洋组、Aurantivirga和Ulvibacter、红杆菌科 Loktanella、腐生菌科和Cryomorphaceae NS10 海洋组。在高原水域中相对丰度较高的ASVs属于黄杆菌科Formosa、红杆菌科Planktomarina和SAR11进化枝。我们讨论了澳大利亚夏末群落组成的潜在非生物和生物驱动因素，以及凯尔盖朗高原水域丰富的原核生物类群的生态作用。