Autochthonous eukaryotic diversity in hydrothermal sediment and experimental microcolonizers at the Mid-Atlantic Ridge

Abstract

The diversity and mode of life of microbial eukaryotes in hydrothermal systems is very poorly known. We carried out a molecular survey based on 18S ribosomal RNA genes of eukaryotes present in different hydrothermal niches at the Mid-Atlantic Ridge. These included metal-rich and rare-earth-element-rich hydrothermal sediments of the Rainbow site, fluid-seawater mixing regions, and colonization devices (microcolonizers) containing organic, iron-rich, and porous mineral substrates that were exposed for 15 days to a fluid source. We identified considerable phylogenetic diversity, both at kingdom level and within kinetoplastids and alveolates. None of our sequences affiliates to photosynthesizing lineages, suggesting that we are targeting only autochthonous deep-sea communities. Although sediment harbored most phylogenetic diversity, microcolonizers predominantly contained bodonids and ciliates, indicating that these protists pioneer the colonization process. Given the large variety of divergent lineages detected within the alveolates in deep-sea plankton, hydrothermal sediments, and vents, alveolates seem to dominate the deep ocean in terms of diversity. Compared with data from the Pacific Guaymas basin, some protist lineages seem ubiquitous in hydrothermal areas, whereas others, notably kinetoplastid lineages, very abundant and diverse in our samples, so far have been detected only in Atlantic systems. Unexpectedly, although alvinellid polychaetes are considered endemic of Pacific vents, we detected alvinellid-related sequences at the fluid-seawater interface and in microcolonizers. This finding can boost further studies on deep-sea vent animal biology and biogeography.

Figure 1

Scanning electron microscopy photographs of Rainbow sediment showing coccoliths and foraminifer shells. [Bars = 1 μm (A and B), 10 μm (C), and 100 μm (D).]

Figure 2

Microcolonizers analyzed in this work being deposited during the ATOS 2001 cruise at a hydrothermal emission (Tour Eiffel chimney, Mid-Atlantic Ridge). Part of the articulated arm of the ROV Victor can be seen on top holding the floater attached to the microcolonizers. Just below the place where the microcolonizers lie, the bottom of the chimney is densely colonized by mussels (Bathymodiolus azoricus). The microcolonizer area appears fuzzy because of the mixing of hydrothermal fluid and seawater.

Figure 3

ML phylogenetic tree of alveolate 18S rDNA sequences. The tree was rooted by using three heterokont sequences (Hyphochytrium catenoides, Dictyocha speculum, and Nannochloropsis salina, not shown). Mid-Atlantic Ridge environmental clones are symbol-coded depending on the sample type. Sequences representing unknown alveolate groups are highlighted. The scale bar indicates the percentage of substitutions for a unit branch length. Numbers at nodes are bootstrap percentages.

Figure 4

Unrooted ML phylogenetic tree of major eukaryotic groups based on 18S rDNA sequences. Mid-Atlantic Ridge environmental clones are symbol-coded depending on the sample type. Clones representing possibly unknown kingdom-level groups are highlighted. The scale bar indicates the percentage of substitutions for a unit branch length. Numbers at nodes are bootstrap percentages.

Figure 5

ML phylogenetic tree of metazoan 18S rDNA sequences. The tree was rooted by using two Porifera (Hippospongia communis and Spongilla lacustris) and two Cnidaria (Dendrobrachia paucispina and Craterolophus convolvulus) sequences. Environmental clones are symbol-coded depending on the sample type. The unknown alvinellid-related sequences identified at the Mid-Atlantic Ridge are highlighted. The scale bar indicates the percentage of substitutions for a unit branch length. Numbers at nodes are bootstrap percentages.