| Astaxanthin, as a good colorant additive and a bioactive substance, has great commercial potential. The limited output of astaxanthin nowadays can't meet the increasing demand worldwide. Therefore, much attention has been given to a unicellular green alga, Haematococcus pluvialis, in recent years due to its ability to accumulate large amount of astaxanthin. The possible effects of different traditional factors, like light (light regime and light intensity), temperature, nutrients (N, P, C, Fe, trace mental elements), vitamins, salinity and pH on the cell growth of this alga have been studied in a number of laboratories and countries with a lot fruitful results. Unfortunately, the cell growth rate is still poor, the cell density of the culture is relatively low and the cell cycle is not well regulated. High cell density cultivation is a useful method to solve the above problems. However, the algous physiological characteristics would change under such a high density due to severe mutual shading. up to now, few of such information are known to us.The aims of this thesis focus on the above issue. Ultrahigh cell-density was fulfilled in a 5cm light-path flat-photobioreactor artificially illuminated and the cultures were aerated. Our study showed that the traditional growth parameters of microalgae sometimes could not completely and precisely reflect the practical growth information in high cell density system. Two different adaptive modes, cooperative mode and competitive mode which are induced by severe mutual shading under highcell density condition are responsible for the problem. And the average available light per cell in high density culture dominates the two modes. Main results obtained are listed as below:1. After analyzing four most traditional growth parameters as cell-number, chlorophyll, cell-size and dry weight, we discovered that none of the above parameters could completely and precisely reflect the practical growth information in high cell density system..2. the reduction of .light per cell caused by severe mutual shading in high cell density culture turn out to be the key reason.3. During the acclimation of decreased light per cell, both cell size and cell chlorophyll were modified so as to optimizing light harvesting and light utilization.4. there existed two different high cell-density adaptive modes between microalgae community and individuals, cooperative mode and competitive mode.5. The chioce of the two modes is related to the proliferation behavior of Haematococcus induced by the light per cell. Relatively low light intensity make the Haematococcus cell lacking light energy sooner which is apt to use the second mode.6. Under the conditions of our experiments, no matter what high densities, Haematococcus cells which have an average light per cell between 2.4～ 4.8×109 photons s-1cell-1 will adopt the 2nd mode preferentially , whereas those between 4.8～6.0×109 photons s-1cell-1 will adopt the 1st mode. The critical point by light per cell is about 4.2～4.8×109 photons s-1cell-1. Individual cell who adopt the 1st mode needs an average light about 3.6～4.8×109 photons s-1cell-1, while adopt the 2nd mode needs 2.4～3.6×109 photons s-1cell-1. The average light per cell at turning point of the two modes is around 3.3～3.5×109 photons s-1cell-1.Inorganic nitrogen concentration in high cell density system decrease faster7. than that in relatively low density systems. The maintenance of motile cells needs a total inorganic nitrogen concentration of no less than 50μmol/L.
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