Electron microscopy of calcified microaggregates occurring within Cladophora sp. mats.

FERNANDEZ, H.; CORREA, M.; MARTINEZ, L.; LEYRIA, N.; ALBARRACÍN V. H.

Bariloche

Congreso; 4to CONGRESO ARGENTINO DE MICROSCOPÍA ? SAMIC; 2016

Increased solar exposure, high temperatures and lack of precipitation, conditions common at the end ofwinter and the early spring in subtropical Andean rivers, favor the production of river algae bloomssuch as Cladophora sp [1]. Cladophora is considered the most important of the attached benthic algaeas they tend to alter the conditions of their microhabitats by modifying current, blocking sunlight,absorbing dissolved oxygen, collecting detritus and providing refuge for invertebrates [2]. Cladophorais also an important substrate for epiphytic growth due to its ability to withstand the shear stress typicalin the benthic regions of rivers. In marine systems Cladophora has been associated with lithifiedcarbonate structures, where it has been found to form mixed communities with cyanobacteria while, infreshwater streams mats have also been described in association with carbonate layers. During a fifteenyear research project in the microhabitat produced by Cladophora?s mats in the Lules River (Tucumán,Argentina) annual cycles of exceptionally prolonged drought, in conjunction with naturally occurringdiffuse organic pollution, generated the unique conditions for the precipitation of lithified carbonatemicroaggregates (microbialite-like). This work presents an initial characterization of calcifiedmicrostructures attached to Cladophora sp. mats using scanning electron microscopy coupled toenergy-dispersive X-ray spectroscopy (EDS). Samples taken from Cladophora?s mats of the LulesRiver were fixed in the field with 4% formaldehyde. The fixed samples were washed three times withphosphate buffer and CaCl2 for 10 min, and fixed with osmium tetroxide (2% v/v) over night.Afterwards, the samples were washed twice with ethanol (30% v/v) for 10 min, dried at a critical point,and sputtered with gold. Specimens were observed under high vacuum using a JEOL 35CF and a ZeissSupra 55VP (Carl Zeiss NTS GmbH, Germany) scanning electron microscope coupled with an energydispersive X-ray detector (INCA Oxford, EDS detector). Abundant microaggregates forming aroundthe filaments from Cladophora mats were found (Fig. 1A). They varied in both size and weight (mean2 mm diameter, 0.37 g weight). Electron microscopy allowed a closer analysis of the collectedmicroaggregates; numerous diatomeas and bacteria together with typical crystals of calcite agglutinatedsurrounding the main filaments of the algae, (Fig. 1B). Small quantities of quartz and albite were alsorecorded. The mineral presence was confirmed by X-ray diffractometry and also EDS (Fig. 2). Wehypothesize that the calcificated microaggregates observed around the filaments of Cladophora sp. areproduced by the release of extracellular polysaccharides of some diatomea such as Gomphonema sp., aspreviously noted by Winsborough and Golubic [3]. These microaggregates represent incipientformation of microbialite-like structures which are organosedimentary deposits formed from theinteraction between benthic microbial communities and detrital or chemical sediments. As such, theyconstitute interesting models to further explore microbe-mineral interaction processes in subtropicalfreshwater habitats.