Structural heterogeneity in cyanobacterial mats is associated with geosmin production in rivers

Abstract

E. Vilalta and S. Sabater. 2005. Structural heterogeneity in cyanobacterial mats is associated with geosmin production in rivers. Phycologia 44: 678–684.

The structural characteristics and microprofiles of dissolved oxygen and redox potential were examined in benthic cyanobacterial mats, which developed in a nutrient-rich river. The mats were highly dynamic both in their structure and relative composition, and showed large spatial heterogeneity. The mats differed in the contribution of Oscillatoria limosa, which was higher in the free-floating than in the attached mats. Maximum dissolved oxygen concentration at saturating irradiances occurred within the first 2 mm in the attached mat, but was smaller and deeper in the free-floating mat. The free-floating mats showed heterogeneous profiles inside the same mat due to varying thickness in nearby zones of the mats. In these mats, some black micropatches (high proportion of Oscillatoria) showed anoxic conditions under low irradiances, whereas other brown micropatches (lower proportion of Oscillatoria) never became anoxic. After several hours in the dark, a negative redox potential (− 400 mV) was detected inside the mat in the black micropatch, but redox potential was always positive in the brownish micropatches. The dissolved oxygen and redox profiles indicate that potential conditions for the production of geosmin could occur in the black micropatches. There, diffusion constraints within the mat could be associated with resource depletion. High peptidase activities in the black micropatch indicated a high demand for inorganic nitrogen. These mats with prevalence of Oscillatoria functioned as “hot spots” of limited diffusion, which probably caused low nutrient availability, defining the appropriate conditions for the production of geosmin.

ACKNOWLEDGMENTS

This study was funded by the European Projects “BIO-FILMS” (EVK1-1999-00005) and “MODELKEY” (Contract no. 511237-2). Thanks are due to Enrique Navarro and Anna Romanífor help during the laboratory analyses. Aigües Ter Llobregat (ATLL) carried out the geosmin determination of the cyanobacterial extracts.