Studies On The Light Adaptation Strategies In Freshwater Diatoms

The present study collected published data which correlated with environmental factors in aquatic ecosystems and analyzed the distribution of diatoms in rivers and lakes from the point of view of light intensity and nutrient. Simultaneously, we conducted indoor experiment to investigate the mechanism of light adaptation in diatoms and the effect of light intensity on nutrient uptake in diatoms. The main results of the present study as follows:1. The results from analysis of light intensity and diatom density in aquatic ecosystem showed that diatoms own excellent light adaptation. Diatoms from high-light ecosystems can become dominant population in high-light regimes and diatoms from low-light ecosystems can become dominant population in low-light regimes.2. The results from analysis of nutrient and diatom distribution in aquatic ecosystem showed that the proportion of diatom density and species can be influenced by the concentration of nitrogen(N) and phosphorus(P). The highest proportion of diatom species was found in the upper reaches of the Yangtze River and the lowest proportion of diatom density was found in the estuary of Yangtze River. Differences in the proportion of diatom density can be found in different regions.The proportion of diatom density in the main stream of Yangtze River reached the highest values(91%-100%), followed by the branches of Yangtze River and Songhuajiang River. Huaihe River and Zhujiang River which was in serious pollution showed the lowest proportion of diatom density(25%-30%).3. We confirmed the light adaptation of diatoms by conducting growth experiment using eight stains of diatoms isolated from high and low-light ecosystems. Diatoms which isolated from high-light ecosystem(high-light adapted, HLA diatoms) showed high-light adaptation. For example, HLA diatoms showed relatively high growth rates, high photosynthetic activity, high efficiency in dissipation of excess light energy, the relatively high maximum relative electron transport rate(rETRmax), high light saturation point of photosynthesis(Ik) and the relatively low photon capturing efficiency(α) at HL. HLA diatoms responded to low-light stress by increasing chlorophyll a(Chla) content and chloroplast volume to increase the capture of photons. In the first stage of low-light stimulation, the parameters of rapid light curve in HLA diatoms appeared as the similar trend with low-light adapted(LLA) diatoms(low Ik, low rETRmax and high α). Diatoms which isolated from low-light ecosystem(low-light adapted, LLA diatoms) showed low-light adaptation. For example, LLA diatoms showed relatively high growth rates, high photosynthetic activity, the relatively high α, low efficiency in dissipation of excess light energy, the relatively low rETRmax and low Ik at LL. LLA diatoms responded to high-light stress by increasing non-photochemical quenching(NPQ) to increase the dissipation of excess light energy. In the first stage of high-light stimulation, the parameters of rapid light curve in LLA diatoms appeared as the similar trend with HLA diatoms(high Ik, high rETRmax and low α).4. The results of nutrient uptake kinetics experiment showed that P and Si-uptake of diatoms can be influenced by light intensity. The HLA diatoms showed high P affinity, growth rate and photosynthetic activity at HL conditions while the LLA diatoms showed high P affinity, growth rate and photosynthetic activity at LL conditions. Si-uptake of eight diatoms can be affected by cell volume and the half saturation constant of Si(Km(Si)) in pennate diatoms were much higher than that in centric diatoms.The values of Km(Si) increased with the growing light intensity in pennate diatoms but decreased with the growing light intensity in centric diatoms. Our results imply that when we assess the development of diatom blooms and choose the suitable diatom control measures for local aquatic ecosystems, we should consider light intensity and diatom characteristics in addition to environmental nutrient concentration.