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. Using sequence comparisons of polymerase chain reaction (PCR)-amplified small subunit ribosomal ribonucleic acids (RNAs), we characterized eukaryotic diversity in hydrothermal vent environments of the Guaymas Basin in the Gulf of California. These anoxic sediments and the overlying seawater harbor a mixture of genetically diverse protists. Many sequence isolates represent novel 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 warm anoxic environments is evidenced by specific affinity of environmental sequences to microaerophilic species in molecular trees. This is superimposed against a background of widely distributed aerophilic protists, some of which may migrate into and survive in the sediment while others, e.g. phototrophs, are simply deposited by sedimentary processes.

During the Year 4 Reporting Period we completed the analysis of data on eukaryotic sequences from the upper 3 cm of two Guaymas cores, and published this work (Edgcomb et al. 2002).