Evidence for protection of nitrogenase from O(2) by colony structure in the aerobic diazotroph Gluconacetobacter diazotrophicus.

Gluconacetobacter diazotrophicus is an endophytic diazotroph of sugarcane which exhibits nitrogenase activity when growing in colonies on solid media. Nitrogenase activity of G. diazotrophicus colonies can adapt to changes in atmospheric partial pressure of oxygen (pO(2)). This paper investigates whether colony structure and the position of G. diazotrophicus cells in the colonies are components of the bacterium's ability to maintain nitrogenase activity at a variety of atmospheric pO(2) values. Colonies of G. diazotrophicus were grown on solid medium at atmospheric pO(2) of 2 and 20 kPa. Imaging of live, intact colonies by confocal laser scanning microscopy and of fixed, sectioned colonies by light microscopy revealed that at 2 kPa O(2) the uppermost bacteria in the colony were very near the upper surface of the colony, while the uppermost bacteria of colonies cultured at 20 kPa O(2) were positioned deeper in the mucilaginous matrix of the colony. Disruption of colony structure by physical manipulation or due to 'slumping' associated with colony development resulted in significant declines in nitrogenase activity. These results support the hypothesis that G. diazotrophicus utilizes the path-length of colony mucilage between the atmosphere and the bacteria to achieve a flux of O(2) that maintains aerobic respiration while not inhibiting nitrogenase activity.

Constraints in cropping heavy-metal contaminated fluvial sediments.

Growth and heavy-metal uptake of various food crops and grass cultivated on harbour dredge spoils were studied, and health aspects in consuming the marketable products were discussed. Vegetables (potato, carrot, radish, endive, lettuce) and grass (English ryegrass) performed well on dredge spoils, but small grains (wheat, barley) were affected by manganese deficiency. As compared with crops grown on uncontaminated reference soils, there was a net accumulation of As and heavy metals, especially so Cd, Zn and Cu, and a reduced uptake of Mn. Mainly because of the elevated Cd concentrations of the edible parts, exceeding the guideline of 0.1 mg/kg in fresh matter, the harbour dredge spoils investigated are considered unfit for the production of food crops, but may be used as grassland for dairy cattle. Highest Cd concentrations were attained in leafy vegetables and wheat (grain) and lowest in potato (tuber).