Abstract
Located 20 km south of Puertecitos on the Baja California Peninsula, Mexico, is a salt-crusted lagoon with a surface area of approximately 265,000 m2 that is isolated from the adjacent Upper Gulf of California by a 50-m wide berm. The berm rises 2 m above mean sea level extending for 530 m across the lagoon’s seaward front to bar replenishment by normal seawater except possibly by seepage. On another side of the lagoon an extinct Pliocene volcano, Volcán Prieto, marks an equally abrupt boundary delineated by basalt flows. The lagoon’s well-constrained physical geography represents a high-salinity environment under conditions of extreme aridity, flooded only during rare events associated with subtropical storms. The Volcán Prieto Lagoon (so named herein) formed through distinct stages in developmental geomorphology outlined in this study. A duplicate set of sediment cores (17 cm in length) were retrieved from the lagoon and sampled for biological associations that record high-diversity colonization and stratification of microbial mats dominated by bacteria. Small subunit ribosomal RNA gene sequencing of different horizons revealed at least 25 major named bacterial phyla and 8 major named archaeal phyla as well as several unnamed candidate taxa from miscellaneous groups. Lipid biomarker analyses of the same horizons revealed that cyanobacteria contributed significantly to biomass production only at shallow depth, whereas the lipids of anoxygenic phototrophic bacteria persisted to a depth of 15 cm, although with decreasing contents. The lipid patterns also showed that sulfate-reducing bacteria became more abundant with depth, whereas the contents of archaeal lipids increased from 1 to 5 cm depth but remained relatively constant below. Closed lagoons on the Gulf of California are widely distributed over the length of the Baja California Peninsula, but detailed taxonomic studies regarding the diverse microbial communities that colonized these extreme habitats have only begun to shed light on complex colonization patterns.
 Key words: coastal processes, closed lagoons, Gulf of California, microbial assemblages, lipid biomarkers.
Highlights
Located 20 km south of Puertecitos on the Baja California Peninsula, Mexico, is a salt-crusted lagoon with a surface area of approximately 265,000 m2 that is isolated from the adjacent Upper Gulf of California by a 50-m wide berm
Lipid biomarker analyses of the same horizons revealed that cyanobacteria contributed significantly to biomass production only at shallow depth, whereas the lipids of anoxygenic phototrophic bacteria persisted to a depth of 15 cm, with decreasing contents
Closed lagoons on the Gulf of California are widely distributed over the length of the Baja California Peninsula, but detailed taxonomic studies regarding the diverse microbial communities that colonized these extreme habitats have only begun to shed light on complex colonization patterns
Summary
Microbial mats from lagoons on the outer Pacific coast of the Baja California Peninsula (Mexico) have long been studied (i.e., Horodyski and Vonder Haar 1975, Stolz 1983, Spear et al 2003, Omoregie et al 2004). In a recent overview of coastal ecosystems in the Gulf of California, 5 closed lagoons in the Bahía de Los Ángeles region were identified as habitats for active microbial mats (Johnson and Ledesma-Vázquez 2016). Los mantos microbianos de las lagunas sobre la costa externa del Pacífico de la península de Baja California, México, han sido estudiados desde hace mucho tiempo (i.e., Horodyski y Vonder Haar 1975, Stolz 1983, Spear et al 2003, Omoregie et al 2004). El rasgo más distintivo, una estructura volcánica con forma cónica llamada volcán Prieto, se eleva a 275 m sobre el nivel del mar. The aims of this contribution are 2-fold: (1) to provide an overview of the physical geography of a closed lagoon and the stages through which it developed during post-Pliocene time, and (2) to offer a state-of-the-art case study on the rich biodiversity of microbial life, including both bacteria and archaea, in such an extreme habitat
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