Benthic eukaryotic diversity in the Guaymas Basin hydrothermal vent environment

Abstract

Molecular microbial ecology studies have revealed remarkable prokaryotic diversity in extreme hydrothermal marine environments. There are no comparable reports of culture-independent surveys of eukaryotic life in warm, anoxic marine sediments. By using sequence comparisons of PCR-amplified small subunit ribosomal RNAs, we characterized eukaryotic diversity in hydrothermal vent environments of Guaymas Basin in the Gulf of California. Many sequences from these anoxic sediments and the overlaying seawater represent previously uncharacterized protists, including early branching eukaryotic lineages or extended diversity within described taxa. At least two mechanisms, with overlapping consequences, account for the eukaryotic community structure of this environment. The adaptation to anoxic environments is evidenced by specific affinity of environmental sequences to aerotolerant anaerobic species in molecular trees. This pattern is superimposed against a background of widely distributed aerophilic and aerotolerant protists, some of which may migrate into and survive in the sediment whereas others (e.g., phototrophs) are simply deposited by sedimentary processes. In contrast, bacterial populations in these sediments are primarily characteristic of anoxic, reduced, hydrocarbon-rich sedimentary habitats. P hylogenetic studies of ribosomal RNAs from cultured species imply that large evolutionary distances separate major groups of protists. Certain aerotolerant anaerobic parasites seem to represent early diverging lineages in the eukaryotic line of descent (1). Because they do not require atmospheric levels of oxygen for growth, these organisms lack mitochondria and peroxisomes. The absence of these features in the most divergent eukaryotic lineages and the thermophilic phenotype of the deepest prokaryotic branches suggest that warm, anoxic environments surrounding deep-sea hydrothermal vents might support novel and diverse eukaryotic communities. Little information about the phenotypic and evolutionary diversity of free-living protists from anoxic marine environments is available. Most studies of protist diversity rely upon morphological characters to differentiate between genera. However, culturebased studies and microscopical criteria cannot provide quantitative measures of genetic diversity. More significantly, these methods frequently do not provide comprehensive profiles of community composition. Cultivation-independent molecular surveys of eukaryotic microbial diversity based on comparisons of PCR amplicons of small subunit (SSU) rRNA genes from marine pelagic environments reveal a microbial world with few taxonomically assignable protists (2, 3). There are no analogous molecular surveys of eukaryotic microbial diversity in anoxic, deep-sea sediments.Here, we report the results of a molecular survey of eukaryotic community SSU rRNA gene composition in sediments and the seawater interface proximal to a deep-sea hydrothermal vent. The study site is Guaymas Basin, Gulf of California, which is a hydrothermally active env...

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