Ecophysiological Effects Of Iron On Bloom-Forming Cyanobacteria

Cyanobacterial blooms due to the increasing eutrophication have been a worldwide serious environmental problem in water. Severe cyanobacterial blooms reduced the availability of water resource, causing serious ecological damage and gigantic economic losing, further more, the cyanotoxins biosynthesized from bloom-forming cyanobacteria may pose the major health concerns to humans, as well as to the wildlife. Efforts for cyanobacterial bloom control have been made since people are clearly aware of these serious problems. For eutrophication control, most of the past work has been focused on reducing macronutrients, such as nitrogen, phosphorus, little have been done about the effects of micronutrient on the formation and disappearance of cyanobacterial blooms. According to a hypothesis of that,"regarding the micronutrient enrichment, or deficiency, as a measure or an assistant method in the formation of cyanobacterial blooms", this paper reported the results from experiments in different scales, they were, 6.01 km² experimental area in Lake Dianchi, 100 m² enclosures near Lake Dianchi and 100-500 mL conical flasks in laboratory. Ecophysiological effects of iron on the bloom-forming cyanobacteria were studied. The main results are as followings:1. Experiments were conducted in the northeastern part of the shallow, hypertrophic lake Dianchi from March 2003 to February 2004. The experimental area was 6.01 km², in a relatively large scale. Iron concentrations were measured for three size fractions: particulate iron (φ>0.22μm), colloidal iron (φ: 0.025-0.22μm) and soluble iron (φ<0.025μm), and the main environmental factors were also synchronously analyzed. Results showed that size-fractionated iron, related physico- chemical factors and cyanobacterial species composition all varied with seasonal changes; colloidal iron accounted for only 5⁹%, while particulate iron and soluble iron accounted for 40⁵0% of total iron respectively; size-fractionated iron can transform into each other, thus, it could satisfy the growth requirements of phyto- plankton and aquatic plants. Significant linear correlations were found among the size-fractioned iron, and significant correlations were also obtained between chlorophyll a and environmental factors, such as TN, TP and secchi depth data; but no obvious correlation was found between iron and chlorophyll a in this larger scale experimental area. In addition, cyanobacterial species composition was decided by many factors; results of CCA analysis suggest, the abundance and dominance of M. aeruginosa were influenced by TP, NO₃^--N, NO₂^--N, DO and WT, and the abundance and dominance of A. flos-aquae are influenced by NH₄⁺-N.2. Secondly, experiments were done in 100 m2 enclosures closely nearby lake Dianchi from June to October 2003, scale was much smaller than the above-mentioned experiment area. Results showed that, under the conditions of pH 7⁹ and water temperature 17.5²0.5 oC, photoplankton thrived and absorbed ferrous iron, so the concentrations of ferrous iron decreased highly; DO, phosphate and dissolved total phosphate had no strong influences on the concentrations of different forms of iron; ferrous has significant correlations with population density and chlorophyll a content of phytoplankton. Under serious cyanobacterial bloom, total iron is not a limiting factor in eutrophic freshwater lakes, but iron bioavailability played an important role in waterbloom formation and disappearrance.3. With smaller scale further, experiments were carried out in 100⁵00 mL conical flasks in laboratory under certain conditions. Physiological results in these cases would be helpful to elucidate some ecological phenomena of cyanobacterial bloom formation.(1) After M. aeruginosa and M. wesenbergii, the two dominant species in lake Dianchi, were treated with different iron (III) concentrations, variances of growth rate, chlorophyll a, PSII photochemical efficiency and siderophore production were observed and determined. Results showed that, these algae grew only in a certain range of iron (III) (0.01¹00μM [Fe³⁺]); under iron-limited condition, growths was inhibited, pigments and photosynthetic apparatuses were damaged, so as to caused the yield, ETRmax and Ik all declined sharply. By the way, the algae produced some protective substances under stress conditions, such as siderophore et al., and showed small differences of Iron requirement, as M. aeruginosa required higher iron concentration as compared with M. wesenbergii.(2) Changes in growth rate, chlorophyll a, activities of ACP/ALP, NR, and Mg²⁺, Ca²⁺, H⁺-ATPase on thylakoid membrane of M. wesenbergi were studied under iron limitation and iron addition conditions. Results showed that all these parameters decreased under iron-limited condition and increased markedly after iron enrichments.(3) Complex effects of temperature, light, different nitrogen sources and iron on growth, PSII photochemical efficiency and phosphorus uptake of M. aeruginosa were studied. Results showed that under optimal temperature, light and nitrogen, iron limitation inhibited the growth and decreased the PSII efficiency of M. aeruginosa. Under an iron-replete condition, the optimal growth conditions were temperature 30 oC, light intensity 30μmol quanta·m^-2·s^-1 and nitrate-nitrogen, In this case, the phosphorus uptake rates were highly fast.