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The antialgal activity of newly synthesized bacillamides against several cyanobacteria and microalgae isolates was screened using a rapid 96-well microplate bioassay. Cultures were exposed to serial dilutions of each bacillamide derivative (0-160 μg mL −1 ) in the microplate wells and daily optical measurements were used to estimate growth over a 216 h period. Inhibition values (%) were calculated from the estimated growth curves and inhibitory concentrations (IC 50 -216 h) were obtained from the sigmoidal inhibition curves fitted by regression analysis. The effects of bacillamides on cell morphology and ultrastructure were also analysed by light and transmission electron microscopy. In general, the toxic cyanobacteria Microcystis aeruginosa, Aphanizomenon gracile, Anabaena circinalis and Anabaenopsis circularis were much more sensitive to bacillamides then the chlorophytes Ankistrodesmus falcatus and Scenedesmus obliquus. However, clear signs of morphological and ultrastructural changes induced by bacillamide were observed on both cyanobacteria and chlorophytes. Other cyanobacteria, namely the nostocalean Nodularia spumigena and the oscillatorialeans Leptolyngbya sp. and Planktothrix rubescens, exhibit higher tolerances to bacillamides, similar to that shown by different eukaryotic microalgae. Diatoms, on the other hand, proved to be quite as sensitive to most bacillamides as the most affected cyanobacteria. The properties of 5-iodo-Bacillamide (algicide or algistatic) were further investigated. This compound acted as an algistactic agent against eukaryotic algae and, depending on its concentration, acted as either an algicide or algistactic agent against most of the cyanobacteria tested. Although bacillamides cannot be considered as broad spectrum cyanobacterial algicides, different bacillamides might be of use in selectively controlling the growth of particular species of cyanobacteria.

cyanobacteria are ubiquitous organisms with a relevant contribution to primary production in all range of habitats. cyanobacteria are well known for their part in worldwide occurrence of aquatic blooms while producing a myriad of natural compounds, some with toxic potential, but others of high economical impact, as geosmin. We performed an environmental survey of cyanobacterial soil colonies to identify interesting metabolic pathways and adaptation strategies used by these microorganisms and isolated, sequenced and assembled the genome of a cyanobacterium that displayed a distinctive earthy/musty smell, typical of geosmin, confirmed by GC-MS analysis of the culture's volatile extract. Morphological studies pointed to a new Oscillatoriales soil ecotype confirmed by phylogenetic analysis, which we named Microcoleus asticus sp. nov. our studies of geosmin gene presence in Bacteria, revealed a scattered distribution among Cyanobacteria, Actinobacteria, Delta and Gammaproteobacteria, covering different niches. Careful analysis of the bacterial geosmin gene and gene tree suggests an ancient bacterial origin of the gene, that was probably successively lost in different time frames. The high sequence similarities in the cyanobacterial geosmin gene amidst freshwater and soil strains, reinforce the idea of an evolutionary history of geosmin, that is intimately connected to niche adaptation. Cyanobacteria can be found in marine, brackish and freshwaters exhibiting both planktonic and benthic lifestyles, covering almost all ranges of temperatures and salinities, in symbiosis with a variety of organisms, in soil, caves, deserts, and buildings, and are among the oldest life forms that evolved throughout time 1,2. It represents a prokaryotic group with huge diversity; in morphology presenting from unicellular to filamentous forms with specialized cells; chemo-diverse since they produce a panoply of bioactive compounds: antifungal 3,4 , anticancer 3,5 , antibacterial 3 , antiviral 5 , algicidal 6 , and antioxidant 7 , as well as toxic compounds noxious to the health of humans and wildlife 8,9. Two of the most common volatile organic compounds produced by Cyanobacteria are 2-Methylisoborneol (MIB, PubChem CID:16913) and Geosmin (PubChem CID:29746) 10-12. These compounds are frequently described in fresh and brackish waters Cyanobacteria, as they cause taste and odor problems in drinking water, fish and shellfish 11,13,14. Although non-toxic to humans, these compounds change the perception of safety by the public, causing major economic losses in both sectors 10,11,14. Geosmin is greatly associated with terrestrial environments. It is assumed that the organisms responsible for the gross production of geosmin in soil are Bacteria belonging to actinomycetes-Norcardia cummidelens; N. fluminea; Streptomyces Luridiscabiei; S. albidoflavus-since they are typical soil inhabitants 15,16. Other known producers of geosmin are fungi-Penicillium discolour, P. crustosum, P. expansum, Botrytis cinerea, and Chaetomium sp.-that are also present i...