| Iron is one of the most important trace metals for the photosynthetic organisms and is involved in the physiological processes of photosynthesis, respiration, nitrogen fixation, chlorophyll and phycobiliprotein chromophores and deoxyribonucleotide synthesis. So far, studies have showed that iron stress plays a role in plankton ecology, at least seasonally, in well over 30% and perhaps up to 60% of the global ocean surface area. Meanwhile, many studies have suggested that iron limitation on the phytoplankton community especially cyanobacteria can be also a general phenomenon in freshwater ecosystems. The reason maybe is that total concentrations of available iron for phytoplankton in lake water may be as low as open oceanic water and some cyanobacteria have significantly higher iron requirements than many eukaryotic algae because of their higher PSI/PSII ratio and N2 fixation. Thylakoid membranes are the dominant cellular sink with respect to iron and hence are highly vulnerable to iron limitation. Meanwhile, studies showed that photophysiological characteristics of phytoplankton in high-nutrient, low-chlorophyll (HNLC) waters respond rapidly to iron enrichment, these characteristics are physiologically unique under iron stress. Many studies have reported that some bloom-forming cyanobacteria were significantly inhibited in lakes by available iron concentration, but effects of iron availability on growth and photophysiology of bloom-forming cyanobacteria do not understand very well. To better understand the effects of iron availability on growth, photophysiology, competition ability, species distribution of bloom-forming cyanobacteria, in these studies, Fraquil medium which was developed specifically for studying trace-metal physiology of algae and the Metal Clean technique were used to rigorously control the trace metal contaminations all through our experiments. Meanwhile, some fluorescence techniques as well as Flow cytometry technique were used to study the effects of iron availability on competition between Microcystis and Chlorella or Pseudanabaena strains, and the effects of iron availability and light quality on growth and photophysiolgy of M. aeruginosa and Pseudanabaena sp., and the photophysiolgical responses of the petE mutant of Synechocystis sp. PCC 6803 under a light-dark photoperiod and continuous light. The mail results are as follows:1. Effects of iron availability on competition between microcystis and chlorella or pseudanabaena strains. Iron is essential for the photosynthetic organisms and participates in a wide range of physiological processes such as photosynthesis, respiration, and nitrogen fixation. Iron availability has been suggested to be one of key factors influencing the formation of Microcystis spp. bloom in eutrophic lakes during summer. However, the interspecies competition between Microcystis and other algae under iron limitation is still unknown. The growth, photosynthetic characteristics, and competitive ability of cyanobacterial strains M. aeruginosa, Pseudanabaena sp. and green microalga Chlorella pyrenoidosa were investigated under two different available iron concentrations (pFe 20.3 or 21.4). In mixed cultures, M. aeruginosa showed decreased percentages in both low and high iron media when competed with C. pyrenoidosa. In monoculture, low iron concentration caused significant decrease of pigment contents, PSI/PSII fluorescence ratio, and the maximal PSII quantum yield in M. aeruginosa but no change in C. pyrenoidosa. Interestingly, it was shown that low iron concentration induced the production of CAS-positive molecules by M. aeruginosa which may promote the growth of C. pyrenoidosa. M. aeruginosa showed decreased percentages under high iron medium but increased percentages under low iron medium when competed with Pseudanabaena sp.. In monoculture, low iron concentration caused less decrease of the specific growth rate, phycocyanin and allophycocyanin contents, PSI/PSII fluorescence ratio and other chlorophyll fluorescence parameters in M. aeruginosa than Pseudanabaena sp.. In summary, low iron concentration bring about different physiological changes in the three strains, as a result, M. aeruginosa possessed a competitive disadvantage compared to C. pyrenoidosa and advantage compared to Pseudanabaena sp. under iron limitation condition.2. Effects of iron availability and light quality on growth and photophysiology of microcystis and pseudanabaena strains. Many studies have suggested that iron limitation on the phytoplankton community especially cyanobacteria can be a general phenomenon in lakes. Meanwhile, the underwater light climate of natural fresh waters showed changes in the spectral distribution of light because of containing different depth, water molecules and dissolved salts, dissolved and colloidal substances of organic origin and the photosynthetic pigments of phytoplankton. As a result, the phytoplankton communities were all subject to the influences of iron availability and light quality in lake waters. In this study, the growth and photophysiology of M. aeruginosa and Pseudanabaena sp. were investigated under three different iron concentrations (pFe 20.3,21.4 or 22.3) and light qualities (white, red or blue light). Compared to high iron level (pFe 20.3), when M. aeruginosa cultured under lower iron level (pFe 21.4), the specific growth rate and pigment decreased most seriously in red light (RL) than in blue (BL) or white light (WL) while the PSI/PSII fluorescence ratio, nonphotochemical quenching (NPQ), the linear electron transport activity (rETRmax) and the reoxidation rate of QA decreased most seriously in WL than in BL or RL; when M. aeruginosa cultured under the lowest iron level (pFe 22.3), values of the specific growth rate, rETRmax and NPQ declined more seriously in RL than in BL. These results suggested that RL worsened while BL eased the stress of M. aeruginosa under low iron condition. Compared to high iron level (pFe 20.3), when Pseudanabaena sp. cultured under lower iron level (pFe 21.4), values of the specific growth rate, the maximal PSII quantum yield (Fv/Fm), NPQ, rETRmax declined most least in RL; then in BL and the most serious in WL. These results indicated that RL eased while WL worsened the stress of Pseudanabaena sp. under low iron condition. In summary, the specific growth rate of M. aeruginosa responded rapidly to low iron condition in RL and photosynthesis was susceptible to iron limitation in WL. Moreover, RL worsened and BL eased the stress of M. aeruginosa while RL eased and WL worsened the stress of Pseudanabaena sp. under low iron condition. As a result, iron availability and light quality may collectively affect species and the distribution of bloom-forming cyanobacteria in natural fresh waters.3. Inactivation of the petE gene for plastocyanin causes different photosynthetic responses in cyanobacterium Synechocystis sp. strain PCC 6803 under a light-dark photoperiod and continuous light. Plastocyanin, encoded by petE gene, can transfer electrons to photosystem I (PSI) and cytochrome c oxidase in both photosynthetic and respiratory metabolisms in cyanobacteria. When cultured under continuous light, the inactivation of petE accelerated the plastoquinone pool reoxidation of Synechocystis sp. strain PCC 6803, but slowed down the reoxidation rate of the primary quinone-type acceptor and decreased the connectivity factor between individual photosystem II (PSII) photosynthetic units. Meanwhile, the petE mutant showed a decrease of PSI/PSII fluorescence ratio and an increase of dark respiration rate compared to wild-type. When cultured under a light-dark photoperiod, the mutation of petE could further cause an evident increase of phycocyanin to chlorophyll ratio. As a result, the ApetE strain showed a darker blue than its wild-type. Moreover, the mutation of petE increased the efficiency of light capture, nonphotochemical quenching and the linear electron transport activity, and decreased the functional absorption cross-section of PSII. These results suggested that plastocyanin is involved in regulating the redox state of photosynthetic electron transfer chain and the mutation of petE could bring some special changes appeared just under a light-dark photoperiod. |
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