Plants benefit extensively by harbouring endophytic microbes. They promote plant growth and confer enhanced resistance to various pathogens. However, the way the interactions among endophytes influence the plant productivity has not been explained. Present study experimentally showed that endophytes isolated from rice (Oryza sativa) used as the test plant produced two types of interactions; biofilms (bacteria attached to mycelia) and mixed cultures with no such attachments. Acidity, as measured by pH in cultures with biofilms was higher than that of fungi alone, bacteria alone or the mixed cultures. Production of indoleacetic acid like substances (IAAS) of biofilms was higher than that of mixed cultures, fungi or bacteria. Bacteria and fungi produced higher quantities of IAAS than mixed cultures. In mixed cultures, the potential of IAAS production of resident microbes was reduced considerably. There was a negative relationship between IAAS and pH of the biofilms, indicating that IAAS was the main contributor to the acidity. However, such a relationship was not observed in mixed cultures. Microbial acid production is important for suppressing plant pathogens. Thus the biofilm formation in endophytic environment seems to be very important for healthy and improved plant growth. However, it is unlikely that an interaction among endophytes takes place naturally in the endophytic environment, due to physical barriers of plant tissues. Further, critical cell density dependant quorum sensing that leads to biofilm formation may not occur in the endophytic environment as there is a limited space. As such in vitro production and application of beneficial biofilmed inocula of endophytes are important for improved plant production in any agro-ecosystem. The conventional practice of plant inoculation with monocultures or mixed cultures of effective microbes may not give the highest microbial effect, which may only be achieved by biofilm formation.

Cyanobacteria are among the earliest of inhabitants of Planet Earth and their existence can be traced back to 3.8 billion years. Their oxygenic photosynthesis led to the gradual conversion of the Earth"s atmosphere from an anaerobic to an aerobic one. This change enabled the advent of aerobic organisms that eventually underwent rapid evolution and became the dominant, highly diverse members of the current global biodiversity. Cyanobacteria are ubiquitous in their distribution and are found in all the latitudes from Arctic and Anatarctic regions to the Tropical deserts perhaps reflecting their pioneering habitation of the primitive earth. They are also unique in their ability to simultaneously perform oxygenic photosynthesis and oxygen labile nitrogen fixation. Through these processes they make significant contributions to the Carbon and Nitrogen bio-geochemical cycles, particularly in the deep oceans. The ability of these organisms to fix N 2 either independently or in symbiosis with other organisms not only contributes to natural ecosystems but is applied in certain countries particularly for rice cultivation. Their ability to grow in highly polluted environments is also used in the treatment of sewage and industrial effluents. Cyanobacteria are the most efficient among all living organisms in the harvesting of solar energy and are currently looked at as highly attractive candidates for biofuel production. A few species are being used for the production of highly nutritive food supplements. On the negative side, some cyanobacteria form massive growths called "blooms" in water bodies and many of them produce toxins harmful to fish, digastric animals and are suspected to be responsible for certain human ailments. Having reviewed most of these aspects of cyanobacteria, it is concluded that knowledge on these little known organisms would be invaluable not only for students, scientists and environmentalists but also for industrialists and policy makers.

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The course of heterocyst development in Anabacna cylindrica was studied in relation to the ability to fix nitrogen. When non-differentiated filaments, grown in the presence of ammonium-N, were transferred into a medium free from combined nitrogen and incubated under photosynthetic conditions, the cellular C:N ratio increased from 4.5:1 to 8:1 before the percentage heterocyst frequency and nitrogenase activity reached a steady value. The initial stages of differentiation were observed 24 h after transfer into nitrogen-free medium, but nitrogenase activity was only detected when the formation of the first heterocysts was completed. The transformation of a vegetative cell into a heterocyst is characterized by the dissolution of storage granules, the deposition of a multilayered envelope, the breakdown of photosynthetic thylakoids and the formation of new membraneous structures. The latter appear to develop by the coalescence of small newly formed vesicles arising in regions of pre-existing thylakoids. The course of heterocyst development was paralleled by that of nitrogenase activity both under aerobic and anaerobic conditions. Anaerobic incubation enhanced heterocyst production as well as nitrogenase activity. The results suggest that nitrogenase synthesis in Anabaena cylindrica is associated with heterocyst formation and that the primary factor which may regulate both processes is the cellular C:N balance of the alga.

The diversity of cyanobacteria in Sri Lanka was studied in different water reservoirs, paddy fields, brackish water and tsunami affected areas using light microcopy, 16S rRNA sequences, followed by phylogenetic analysis. Based on light microscopy, 24 genera were identified from environmental samples belonging to the orders Chroococcales, Oscillatoriales, Pleurocapsales and Nostocales. In cultures, 33 genera were identified from all five cyanobacterial orders, including Stigonematales. Based on 16S rRNA gene sequences and their morphology, two isolates were identified up to species level, 72 to genus level, one isolate up to family and 11 up to order level. Twelve isolates couldn't be assigned to any taxonomic level. The results of 16S rRNA gene sequences along with the phylogenetic analysis indicated that some cyanobacterial isolates could be accommodated to genus or order level. The 16S rRNA sequence analysis data in this study confirmed that order Nostocales and order Pleurocapsales cyanobacteria are monophyletic while orders Chroococcales, Oscillatoriales and Stigonematales cyanobacteria are polyphyletic. Polyphasic approach including the combination of light microscopy, cultures and the analysis of 16S rRNA gene sequences provide a promising approach to ascertain the diversity of cyanobacteria in different habitats.

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Abstract:Cyanobacteria are photosynthetic, prokaryotic microorganisms commonly found among the phytoplankton of stagnant waters. Some of them produce toxins and these have been implicated to contribute to environmental health problems including kidney disease of uncertain etiology. This paper is a review of published literature on freshwater cyanobacteria of Sri Lanka with special emphasis on toxigenic genera. It is evident that the freshwater cyanobacterial populations of Sri Lanka have undergone significant changes from the beginning of the 20 th century until recent times. While toxigenic genera such as Microcystis and Cylindrospermopsis have been observed occasionally during the early 20 th century they have now become dominant in most of our waters except in those whose catchment areas are protected forests devoid of human habitation. Eutrophication through nutrient loading and other sources of pollution by anthropogenic activities triggers off cyanobacterial bloom formation which is an environmental health hazard. Minimizing pollution of lentic water bodies by the reduction of the use of chemical fertilizer and other agro-chemicals, restoration of riparian vegetation, biomanipulation to control cyanobacterial populations and breakdown of cyanotoxins through chemical, physical and microbiological methods have been proposed for the amelioration of this hazard. Extensive research studies on such aspects are warranted to develop solutions to overcome this environmental problem.

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