Abstract

The sublittoral seafloor of the Humboldt Eastern Boundary Current ecosystem (HEBCE) is characterized by sediments rich in sulfur compounds and extensive and sometimes massive benthic bacterial mats. This sulfur-based sublittoral ecosystem is maintained by the very high input of organic matter from the productive surface waters and the concurrence of subsurface hypoxic to anoxic waters of the Peru-Chile Undercurrent (PCU). Generically here named the “Humboldt Sulfuretum” (HS), this benthic habitat extends latitudinally along the continental margins off Peru and Chile for over 3,500 km potentially covering around 100,000 km2 thus making it one of the largest reduced soft sediment regions in the world. Since its first recognition in 1962 the HS has consistently been found the home of massive bacterial mats which can attain standing-stocks in the order of 1,000 gm-2 (wet weight). Although hosting a complex and diverse community of prokaryotes, microbial eukaryotes, and meio- and occasional macrobenthic fauna, the dominant components, both in weight and volume, are large vacuolated species of megabacteria of the Beggiatoaceae family (Gammaproteobacteria). These mat-forming autotrophic/mixotrophic species singularly link the C, S, and N cycles (possibly also the P cycle) and constitute one of the largest macroscopic bacterial populations living in today’s naturally reduced world oceans, i.e., eastern boundary current systems, in fact the HS with its microbial communities could be considered modern analogs of Proterozoic benthic systems. This work intends to highlight the extensive geographic distribution, the huge, although variable, massiveness, and thus the potential biogeochemical significance of the giant mat-forming prokaryotes and their accompanying microbial community, as a contribution to a better understanding HEBCE’s functioning one of the most productive eastern boundary current systems of the world.