Community composition and functional genes explain different ecological roles of heterotrophic bacteria attached to two bloom-forming cyanobacterial genera.

Eutrophication leads to frequent outbreaks of cyanobacterial blooms, however, the effect of heterotrophic bacteria attached to cyanobacterial cells is unclear. Field investigations were carried out to gain a deeper understanding of the community composition and functional role of heterotrophic bacteria attached to Dolichospermum and Microcystins cells. The significantly positive relationships between Dolichospermum density and total nitrogen (TN) and between Microcystins density and particle nitrogen (PN) indicated the strong nitrogen (N) demand of these two species. The lack of functional genes that mediate the nitrification process in bacteria attached to both Microcystins and Dolichospermum cells indicated that these two genera preferred ammonium (NH4+-N). Dolichospermum cells obtained more available N through N2 fixation, which was expressed by high nitrogenase gene abundance. Bacteria attached to Microcystins cells showed a higher activity of leucine aminopeptidase and a significantly higher abundance of functional genes that mediate dissimilatory nitrate reduction to ammonium (DNRA) than those attached to Dolichospermum cells. The significantly higher abundance of carbon degradation genes and ß-glucosidase activity of bacteria attached to Microcystins cells compared with those of bacteria attached to Dolichospermum cells suggested that abundant organic carbon was bound to Microcystins cells, which is a prerequisite for DNRA. In addition, Microcystins cells exhibited a great advantage in soluble reactive phosphorus (SRP) production through high levels of organic phosphorus (P) hydrolysis associated with high levels of phosphatase genes of attached bacteria. In conclusion, bacteria attached to Microcystins cells performed more important functions on NH4+-N and SRP production through ammonification and DNRA, as well as phosphatase hydrolysis respectively, compared to those attached to Dolichospermum. Thus, algal growth is the result of different variables such as nutrient concentration, their ratio and the microbial ability.

An enzymatic mechanism for balancing the stoichiometry of nitrogen and phosphorus in a shallow Chinese eutrophic lake.

The over-enrichment of lake waters with nitrogen (N) and phosphorus (P) has become a serious environmental problem, but modes of change in stoichiometry and enzymatic regeneration along trophic gradients are largely unknown. Seasonal variations in the kinetics of extracellular aminopeptidase (LAP) and alkaline phosphatase (AP), together with the composition of phytoplankton and concentrations of N and P, were examined from Jun 2013 to September 2014 in a Chinese shallow lake in which two basins had contrasting trophic states. The turbid basin had a significantly higher concentration of chlorophyll a and lower ratios of N to P. In parallel, the turnover time of organic N mediated by LAP (LAPT) was significantly shorter, and its maximum velocity (Vmax) was significantly higher compared to those in the clear basin. Considering data from both basins, there were linear decreases in N/P and the ratios between dissolved inorganic N and total N with an increasing trophic state index, coupled with a significantly positive relationship between N/P and LAPT. Additionally, with decreasing TN/TP, the Michaelis constant (Km) of the AP increased linearly, reducing the efficiency of P regeneration. In contrast, the Km value of LAP decreased, and Vmax increased, which enhanced N mineralization by simultaneously increasing the reaction velocity and improving the affinity for substrate. Additionally, the Km value of LAP was significantly related to that of AP and the ammonium concentration. Thus, substrate affinity acted as a key factor modifying the pathways of enzymatic degradation of organic N and P according to their stoichiometry in the water column. Phytoplankton composition was directly linked to LAPT. Overall, this study seemed to be the first to connect a stoichiometric shift of N and P with kinetics of extracellular enzymes responsible for their regeneration along trophic gradients, presenting an additional pathway to overcome nitrogen deficiency in eutrophic lakes.

The phosphorus release pathways and their mechanisms driven by organic carbon and nitrogen in sediments of eutrophic shallow lakes.

To reveal phosphorus (P) release pathways from sediment and their mechanisms induced by organic matter enrichment, 116 sampling sites (including surface water and sediment) in 29 shallow lakes with different eutrophic degrees in Wuhan city, China, were investigated from July 2011 to November 2011. Empirical relationship and structural equation model indicated that the decomposition of total organic matter (TOM), including proteins (PRT), carbo-hydrates (CHO) and lipids (especially PRT) mediated by extracellular enzymes, accelerated the formation of anaerobic status. On the other hand, coupled nitrification-denitrification caused by ammonium (NH4+-N) accumulation due to PRT decomposition further aggravated anaerobic status and nitrate removal in terms of the increase of dehydrogenase activity (DHA). As a consequence, ferric iron was reduced to ferrous iron and soluble reactive phosphorus (SRP) was released from iron-bound phosphorus (Fe(OOH)~P) in sediments. In addition, extracellular alkaline phosphatase can be induced by organic carbon and nitrogen on condition that the input of nitrogen (N) and carbon (C) exceeded by far that of P. Taken together, enrichment of N and C can result in P release through the formation of anaerobic status and alkaline phosphatase production. Hence, we indicated that a close coupling existed among C, N and P cycles.