藻类对磷酸盐形成和沉积作用影响的模拟实验

A SIMULATED TEST OF ALGAL INFLUENCE ON FORMATION AND DEPOSITION OF PHOSPHATE

本文介绍现代蓝藻钝顶螺旋藻(Spirulina platensis)的模拟培养成矿实验。该实验采用微机数据采集系统记录数据。从藻类改变微环境促进磷酸盐形成并沉积两方面来研究藻类对磷聚集作用的影响。结果表明,伴随藻类的对数生长,培养液的pH值上升,可达10左右,氧化还原电位下降100—200mV,使培养液向弱还原性转变,从而产生明显的碳酸钠钙和磷酸镁钙的白色沉淀物,为生物成矿理论提供了证据。

This paper deals with a simulated test of biological mineralization made with a blue-green alga, Spirulina platensis. The hydrochemical data and the OD value which reflects the algal growth were determined and recorded by a Sharp PC-1500 A computer. Spirulina platensis could grow in a culture liquor that had the PO_4^(3-) concentra. tion up to 17.25g/L. It grew very well in the culture liquor with 9.825g/L PO_4^(3-). The pH value of the algal liquor gradually increased to 10 and its redox potential (Eh) decreased by 100—200 mV during its logarithmic growth stage. At the same time, the concentration of PO_4^(3-) progressively reduced. Some white deposits accumulated on the floor of the culture containers at the end of the logarithmic growth stage. Examination by X-ray diffractometer showed that the deposits are calciummagnesium phosphate [Ca_2Mg(PO_4)_2·2H_2O] and calcium-sodium carbonate [Na_2Ca(CO_3)·5H_2O]. These experimental results indicate that life-activities of the algae could change the balance of the CO_2-HCO_3^(2-)CO_3^(2-)-and Ca^(2+)-Mg^(2+)-Na^+-PO_(?)^(3-) systems in the culture liquor because the algae absorbed CO_2 and some other ions and gave out O_2 by photosynthesis and other physiologic actions. Thus a change to a weakly reducing condition took place in the culture liquor. Such processes could promote the formation and deposition of phosphate, which could be transformed into phosphorite during the diagenesis of the relevant deposits. Marine phosphorites are mainly distributed on ancient continental margins, shelfs and slopes as well as platforms and shoals, where algae were most flourishing. Abundant phosphatic stromatolites, cryptalgal structures and algal fossils have been found in phosphorite deposits. To sum up, the authors consider that the algae have played an important role in the formation and deposition of phosphate in natural water bodies containing abundant phosphorus.