Drivers of cyanobacterial diversity and community composition in mangrove soils in south-east Brazil Janaina Rigonato, 1 Angela D. Kent, 3 Danillo O. Alvarenga, 1 Fernando D. Andreote, 2 Raphael M. Beirigo, 2 Pablo Vidal-Torrado 2 and Marli F. Fiore 1 * 1 Center for Nuclear Energy in Agriculture, and 2 Department of Soil Science, ‘Luiz de Queiroz’ College of Agriculture, University of São Paulo, Piracicaba, Brazil. 3 Department of Natural Resources and Environmental Sciences, University of Illinois at Urbana-Champaign, IL, USA. Summary Cyanobacteria act as primary producers of carbon and nitrogen in nutrient-poor ecosystems such as mangroves. This important group of microorganisms plays a critical role in sustaining the productivity of mangrove ecosystems, but the structure and function of cyanobacteria assemblages can be perturbed by anthropogenic influences. The aim of this work was to assess the community structure and ecological drivers that influence the cyanobacterial community harboured in two Brazilian mangrove soils, and examine the long-term effects of oil contamination on these keystone species. Community fingerprinting results showed that, although cyanobacterial commu- nities are distinct between the two mangroves, the structure and diversity of the assemblages exhibit similar responses to environmental gradients. In each ecosystem, cyanobacteria occupying near- shore areas were similar in composition, indicating importance of marine influences for structuring the community. Analysis of 16S rRNA sequences revealed the presence of diverse cyanobacterial com- munities in mangrove sediments, with clear differ- ences among mangrove habitats along a transect from shore to forest. While near-shore sites in both mangroves were mainly occupied by Prochlorococ- cus and Synechococcus genera, sequences retrieved from other mangrove niches were mainly affiliated with uncultured cyanobacterial 16S rRNA. The most intriguing finding was the large number of potentially novel cyanobacteria 16S rRNA sequences obtained from a previously oil-contaminated site. The abun- dance of cyanobacterial 16S rRNA sequences observed in sites with a history of oil contamination was significantly lower than in the unimpacted areas. This study emphasized the role of environmental drivers in determining the structure of cyanobacterial communities in mangrove soils, and suggests that anthropogenic impacts may also act as ecological filters that select cyanobacterial taxa. These results are an important contribution to our understanding of the composition and relative abundance of previously poorly described cyanobacterial assemblages in mangrove ecosystems. Introduction Mangroves are marine coastal ecosystems that consti- tute a transitional forest between coast and mainland. In Brazil, mangroves extend from the extreme north to Santa Catarina State, at the south of the country (Schaeffer-Novelli et al., 1990a). These productive eco- systems are important habitats for a diversity of plants and animals, and are the major source of organic matter in Brazilian coastal waters (Dittmar et al., 2001). Despite their ecological importance, mangrove ecosystems and the biodiversity they sustain are endangered by impacts from social and economic pressures (Ferreira et al., 2010). Mangroves are dynamic ecosystems, subject to con- stant variations in environmental factors like salinity, tides, temperature and UV radiation (Kathiresan and Bingham, 2001). Although they can include a range of soil bio- geochemistry and vegetation (Lacerda et al., 1995), man- grove soils commonly exhibit low oxygen availability and are especially deficient in nitrogen and phosphorus (Fer- reira et al., 2007). Mangrove soils provide niches suitable for the development of functionally and taxonomically diverse assemblages of microorganisms (Dias et al., 2009; 2010; Sahoo and Dhal, 2009). Bacteria can inhabit a wide range of niches and play an important ecological role in supporting other organisms in nutrient-poor and extreme environments, since they control biogeochemical processes that are fundamental to the function, produc- tion, conservation and rehabilitation of mangrove forests Received 21 March, 2012; revised 22 June, 2012; accepted 26 June, 2012. *For correspondence. E-mail fiore@cena.usp.br; Tel. (+55) 19 3429 4657; Fax (+55) 19 3429 4610. Environmental Microbiology (2013) 15(4), 1103–1114 doi:10.1111/j.1462-2920.2012.02830.x © 2012 Society for Applied Microbiology and Blackwell Publishing Ltd