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Mn-oxidizing potential of two metal-tolerant bacterial strains - Halomonas meridiana and Marinobacter algicola isolated from the South West Indian Ridge waters were compared at varying concentrations of Mn (II), i.e., 1, 10, and 100 μmol and mmol L . Accompanying changes in their morphology and metabolism were also determined. At concentrations >1 mmol L Mn (II), Mn-oxidizing potential of M. algicola was 2-7 times greater than that of H. meridiana. Scanning electron microscopy revealed that exposure to elevated metal content prompted bacterial cells especially those of M. algicola to been enveloped in exopolymeric material and form aggregates. Energy dispersive spectrometric analysis showed that exopolymeric material acts as a nucleation site for Mn deposition and oxide formation which occurs in the form of microspherical aggregates. These features show striking resemblance to biogenically produced Fe-Mn oxide deposits from Lau Basin. Surprisingly, diffractograms of auto-oxidized and bacterially formed Mn-oxide showed similarities to the hydrothermal vein mineral Rhodochrosite indicating that it can also be produced biotically. Elongation of cells by up to 4× the original size and distortion in cell shape were evident at Mn (II) concentrations >100 μmol L . Marked differences in C-substrate utilization by the test strains were also observed in presence of Mn (II). A shift in use of substrates that are readily available in oceanic waters like N-acetyl-d-glucosamine to those that can be used under changing redox conditions (d-cellobiose) or in the presence of metal ions (d-arabinose, l-asparagine) were observed. These findings highlight the significant role of autochthonous bacteria in transforming reduced metal ions and aiding in the formation of metal oxides. Under natural or laboratory conditions, the mode of bacterially generated Mn-oxide tends to remain the same.

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Kuttanad agroecosystem, once the rice bowl of Kerala, is facing a serious threat from human activities. The present study assessed the nutrient status of Kuttanad paddy soils and the spatial distribution of Nutrients using GIS technique. 117 soil samples were collected from the six agroecological zones of Kuttanad. Out of the 117, 92 were from paddy fields and 25 were from canals. The soil samples were analyzed for pH, organic Carbon (%), available nitrogen (kg/ha), available phosphorous (kg/ha) and available potassium (kg/ha). The spatial distribution of nutrients showed that high available nitrogen content was observed at various parts of the Kuttanad agroecosystem except Vaikom Kari, northern part of North Kuttanad and western part of Kayal Lands. A cluster of area including southeast region of Lower Kuttanad, northern part of Purakkad Kari and north east regions of Upper Kuttanad as well as north east part of Vaikom Kari has showed the highest concentration of phosphorous. High intensity of potassium was observed in the central portion of Kayal Lands, North Kuttanad, northeast and southeast regions of Purakkad Kari and Lower Kuttanad respectively. It can conclude that monsoon flooding has a major role in the regulation of nutrients in the Kuttanad paddy fields. N and P are taken away from the soil by the inundated water whereas potassium is deposited in these areas. The high concentration of nutrients observed at the sites which are having more than one time cultivation and also watering and dewatering has a significant role in regulating the nutrient status of the Kuttanad agricultural wetland ecosystem.

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