The Guerrero Negro (GN) hypersaline microbial mats have become one focus for biogeochemical studies of stratified ecosystems. The GN mats are found beneath several of a series of ponds of increasing salinity that make up a solar saltern fed from Pacific Ocean water pumped from the Laguna Ojo de Liebre near GN, Baja California Sur, Mexico. Molecular surveys of the laminated photosynthetic microbial mat below the fourth pond in the series identified an enormous diversity of bacteria in the mat, but archaea have received little attention. To determine the bulk contribution of archaeal phylotypes to the pond 4 study site, we determined the phylogenetic distribution of archaeal rRNA gene sequences in PCR libraries based on nominally universal primers. The ratios of bacterial/archaeal/eukaryotic rRNA genes, 90%/9%/1%, suggest that the archaeal contribution to the metabolic activities of the mat may be significant. To explore the distribution of archaea in the mat, sequences derived using archaeon-specific PCR primers were surveyed in 10 strata of the 6-cm-thick mat. The diversity of archaea overall was substantial albeit less than the diversity observed previously for bacteria. Archaeal diversity, mainly euryarchaeotes, was highest in the uppermost 2 to 3 mm of the mat and decreased rapidly with depth, where crenarchaeotes dominated. Only 3% of the sequences were specifically related to known organisms including methanogens. While some mat archaeal clades corresponded with known chemical gradients, others did not, which is likely explained by heretofore-unrecognized gradients. Some clades did not segregate by depth in the mat, indicating broad metabolic repertoires, undersampling, or both.Photosynthetic microbial mats occur worldwide and serve as models for microbial community interactions. Fossil microbial mats in the form of stromatolites are one of the earliest distinguishable life forms in the rock record (3.4 billion years) (53) and are often studied to gain insights into the development of life on Earth (3, 11) and the influence that early life may have had on the development of the planet's atmosphere, geosphere, and hydrosphere (20).A large contemporary microbial mat system that has received the attention of microbiological studies is found in the solar saltern operated by the Exportadora de Sal SA in Guerrero Negro (GN), Mexico. The GN saltern, fed from Pacific Ocean seawater pumped from the Laguna Ojo de Liebre, occupies ϳ100 km 2 and consists of ϳ12 precrystallizer ponds with increasing salinity due to evaporation. Many of the ponds in this saltern contain persistent microbial mats. More than a million metric tons of biomass (ϳ17 km 2 by ϳ6 cm by 1.2 gm/cm 3 ) covers the floor of hypersaline pond 4 in the form of a laminated photosynthetic microbial mat that is 4 to 6 cm thick. The subjective, macroscopic appearance of the pond 4 mat is stable from year to year, and the mat has been a subject of numerous microbiological and biogeochemical studies (3,8,15,19,24,29,30,38,47,49,56). Chemical measurements of th...