A study on the ability of freshwater bacteria and ciliates to fix nitrogen and decompose Azolla for rice growth.

Results are presented on the following investigations: 1.The nitrogen fixation capacity of & methanogenic bacterium previously isolated from the anaerobic freshwater ciliate Metopus striatus McMurrich. 2.The conditions for isolation and growth of monocultures of two freshwater sapropelic ciliates: Metopus pulcher and Caenomorpha lata.
3.Rate of anaerobic decomposition of Azolla by sapropelic bacteria alone and sapropelic bacteria in combination with the above ciliates. 4.The influence of Azolla at different levels of decomposition on rice growth. The results of nitrogen fixation by Methanobacterium formicicum showed that pure cultures of the bacterium could utilize molecular nitrogen as the sole source of nitrogen for growth as monitored by methane production and culture turbidity measurements. The rate of methane production by the bacterium was correlated to the nitrogen gas concentrations. In the absence of any nitrogen gas or any other nitrogen source the bacteria completely stopped growing. It was also found that the presence of selenium and molybdenum in the culture medium was vital for growth of the bacterium under nitrogen fixing conditions. Two freshwater sapropelic ciliates, Metopus pulcher and Caenomorpha lata were isolated and monocultures of these could survive by feeding them with a mixed population of sapropelic bacteria and ground Azolla as the only source of organic material in the culture medium. Starting with 50 cells per ml of each ciliate type in 50 ml size carrel flasks, the monocultures grew to 700 and 280 cells per ml for M. pulcher and C. lata, respectively. Ways of ensuring a good monoculture of an anaerobic sapropelic ciliate are discussed. The cytoplasm of both ciliates contained large numbers of methanogenic endosymbionts and microbodies, as revealed by fluorescent and transmission electron microscopy. However, in M. pulcher the endosymbiont isolated was of the Methanobacterium type while in C. lata, there were two types of endosymbionts isolated viz., Methanosarcina and Methanobacterium. The biochemical pathways of these ciliates are discussed. The role of freshwater sapropelic bacteria and ciliates in the anaerobic decomposition and the release of fixed nitrogen of the freshwater floating fern Azolla was studied. The decomposition of Azolla was quantified in terms of methane production, changes in ammonium-N and decrease in fibres. The decomposition rate was faster when the Azolla was anaerobically decomposed by sapropelic bacteria and ciliates together rather than when only bacteria were present. Reasons for the faster Azolla decomposition in the presence of bacteria together with ciliates are discussed. In order to assess the importance of the decomposed Azolla on improving the soil fertility, an investigation was done on growth of rice plants by adding Azolla in different stages of decomposition. Dry weights and total nitrogen of rice plants grown without Azolla; with undecomposed Azolla; with Azolla decomposed by sapropelic bacteria alone; or with Azolla decomposed by sapropelic bacteria together with ciliates were measured. Significantly higher amounts of dry weight and total nitrogen were observed for rice plants grown with Azolla decomposed by sapropelic bacteria together with sapropelic ciliates Caenomorpha lata and Metopus pulcher. This was followed by rice plants grown with Azolla decomposed by bacteria only. Rice plants grown with undecomposed Azolla and no Azolla at all, had significantly the lowest amounts of dry matter and total nitrogen.