Dynamics of Microbial Community During Nitrification Biofilter Acclimation with Low and High Ammonia.

The acclimation of a nitrifying biofilter is a crucial and time-consuming task for setting up a recirculating aquaculture system (RAS). Gaining a better understanding of the dynamics of the microbial community during the acclimation period in the system could be useful for the development of mature nitrifying biofilters. In this study, high-throughput DNA sequencing was applied to monitor the microbial communities on a biofilter during the acclimation period (7 weeks) in high (100 mg N/L) and low (5 mg N/L) total ammonia nitrogen (TAN) treatments. Both treatments were successful for developing a mature nitrifying biofilter, dominated by Proteobacteria, Bacteroidetes, and Nitrospirae. Complete nitrification was found after 7 days of biofilter acclimation as indicated by decreasing TAN concentration, increasing nitrate concentration, and high abundances of the nitrifying bacteria, Nitrosomonadaceae and Nitrospiraceae. The beta diversity analysis of microbial communities showed different clustering of the samples between high and low TAN treatment groups. A greater abundance of nitrifying bacteria was found in the high TAN treatments (27-51%) than in the low TAN treatment (15-29%). The bacterial diversity in biofilters acclimated at high TAN concentration (Shannon's index 5.40-6.15) were lower than those found at low TAN treatment levels (Shannon's index 6.40-7.01). The higher diversity in biofilters acclimated at low TAN concentrations, consisting of Planctomycetes and Archaea, might benefit the nutrient recycling in the system. Although nitrification activity was observed from the first week of the acclimation period, the acclimation period should be taken as at least 6 weeks for full development of nitrifying biofilm. Moreover, the reduction of potentially pathogenic Vibrio on biofilters was found at that period.

Na+-stimulated phosphate uptake system in Synechocystis sp. PCC 6803 with Pst1 as a main transporter.

BACKGROUND: Most living cells uptake phosphate, an indispensable nutrient for growth from their natural environment. In Synechocystis sp. PCC 6803, the cells lack phosphate-inorganic transport (Pit) system but contain two phosphate-specific transport (Pst) systems, Pst1 and Pst2. We investigated the kinetics of Pi uptake of these two Pst systems by constructing the two mutants, ΔPst1 and ΔPst2, and comparing their kinetic properties with those of the wild-type cells under both Pi-sufficient and deficient conditions. The effects of pH and Na+ on the uptake of phosphate in Synechocystis were also studied. RESULTS: Growth rates of the two mutants and wild type were similar either under phosphate-sufficient or deficient condition. The Km for phosphate uptake was 6.09 µM in wild type and this was reduced to 0.13 µM in ΔPst1 cells and 5.16 µM in the ΔPst2 strain. The Vmax values of 2.48, 0.22, and 2.17 µmol • (min • mg of chlorophyll a)-1 were obtained for wild type, the ΔPst1 and ΔPst2 strains, respectively. A monophasic phosphate uptake was observed in wild-type cells. The uptake of phosphate was energy and pH-dependent with a broad pH optimum between pH 7-10. Osmolality imposed by NaCl stimulated phosphate uptake whereas that imposed by sorbitol decreased uptake, suggesting stimulation of uptake was dependent upon ionic effects. CONCLUSION: The data demonstrate that Pst2 system of Synechocystis has higher affinity toward phosphate with lower Vmax than Pst1 system. The Pst1 system had similar Km and Vmax values to those of the wild type suggesting that Pst1 is the main phosphate transporter in Synechocystis sp. PCC 6803. The Km of Pst1 of Synechocystis is closer to that of Pit system than to that of the Pst system of E. coli, suggesting that Synechocystis Pst1 is rather a medium/low affinity transporter whereas Pst2 is a high affinity transporter.

The extended N-terminal region of SphS is required for detection of external phosphate levels in Synechocystis sp. PCC 6803.

A novel 47 amino acid extension at the N-terminus of the SphS histidine kinase has been identified in the cyanobacterium Synechocystis sp. PCC 6803. Here, we demonstrate this region is required for activation of the SphS-SphR phosphate-sensing two-component system under phosphate-limiting conditions and mutants lacking this extension do not show constitutive alkaline phosphatase activity when the negative regulator SphU is inactivated. We have also identified a putative membrane-associated domain within this region involved in control of the Pho regulon. In addition, there are two high-affinity ABC-type phosphate uptake systems in this organism. Our results demonstrate that the Pst1 system, but not the Pst2 system, is required for suppression of the Pho regulon under phosphate-sufficient conditions. Deletion of the pst1 operon and disruption of the membrane-spanning domain may both target the same control mechanism since constitutive alkaline phosphatase activity is similar in the double and single mutants.