Gypsum and Halite Deposition on Cyanobacteria and Algae in Salt‐Affected Soils.

Soilsurfacecommunitiesaredifferentfromthoseofbulksoilduetothedevelopmentofphoto‐syntheticcommunitiescomprisingcyanobacteria,algae,andotherbacteriawhichcontributetothe formation of biological soil crusts. The structure of ecological niches forming biologicalcrustsinsalinesoilenvironmentsischaracterizedbycommunitiesdominatedbyextremophilesorganismslikearchea, halophilic bacteriaand cyanobacteria that can havea direct orindirectroleinshapingsoilproperties.Forexamplegypsumandhalitewerefoundbysomeauthorsas‐sociatedwiththecyanobacteriumEntophysalis (BraithwaiteandWhitton,1987);moreoverthecrystallization of gypsum and halite around filaments was observed in marine cyanobacterialmatsofaridregionsinthePersianGulf(Golubic,1973).Inallthesecasesnocausalrelationshiphasbeenestablishedbetweenthecrystallizationofgypsumorhaliteandcyanobacterialoralgalmetabolism, and no mention is made of specific sedimentary structures. The present accountdescribestheoccurrenceofgypsumandhaliteinasalinesoilcommunityandsuggestshypothe‐sis on how the organismsmay have exerted a physical control over the distinctive structuresproduced. The study site is a naturally salt‐affected soil located in an area, Piana del Signore,where some ecological variables acted as strong shaping factors in above and belowgroundcommunities distribution, driven by a spatial salinity gradient. Scanning electron microscopeobservationsandmicroanalysis(SEM‐EDS)coupledwithX‐Raydiffractionanalysis(XRD)werecarriedout toinvestigate thebiogeochemicalinterfacesinsoil,withadeeperoverviewon themorphology of theminerals occurringin the soil surface.Soilbulk samples and selected frag‐ments of the salted crust of the soil surfacewere analyzed. Threewell definedmineralswereidentified: halite, sulfates and gypsum. Energy Dispersive Spectroscopy confirmed the cubicmineralformtobehalite.Theresultsshowedaconspicuousdepositionofhaliteandgypsumontheverysuperficiallayersofthesoilareascharacterizedbya100%ofsaltcrustcover,andtheSEM‐EDSanalysis providedevidence of the presence of cyanobacteriaandalgae associated tohaliteandgypsumdeposits.Thelossofwaterfromthesoilsurfaceduetothenaturalevapora‐tioncaneasilyexplain thedepositionofhaliteand the formationofcharacteristicsulfatecrys‐tals. The inner association and stratification of these minerals along cyanobcateria and algaebodiesmightbeexplainedbyamorecomplexphenomenonthatinvolvetheC02depletionatthebiogeochemicalinterface,duetothealgalandbacterialphotosyntheticandmetabolicactivity.