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.

Figure 1

Phylogenetic tree of partial SSU rRNA genes obtained from Guaymas sediment cores. Sequences in purple were obtained in the current study. Color-coded numbers to the right of sequence names indicate counts of similar sequences (< 0.06 substitutions per site in a distance analysis of complete sequences) represented from specific core samples. Tree topology was obtained by a heuristic search of 10 random-addition starting trees under a Tamura Nei minimum evolution distance model using 1,021 aligned positions. Distance bootstrap values over 70% from an analysis of 100 bootstrap replicates are given at respective nodes. (Bar = 0.05 substitutions per site.) *Marine mammal rRNA sequences corresponding to the region amplified in this study are not available in public databases.

Figure 2

Sampling saturation profile. Numbers of discrete groups are plotted as a function of numbers of clones sampled. Clone sequences were randomly resampled to completion without replacement to quantify coverage of phylotype diversity. Discrete groups are defined to encompass clones that shared less than 0.06 substitutions per site in a distance analysis based on an alignment of 1,021 nucleotide positions.