The results of eight experiments on the effects of soil extract and its micro-elements as well as of the different concentrations of vitamin B₁₂and cobalt on thegrowth of Nitzschia closterium, in natural and artificial sea water, are presented.The influences of different temperatures and light intensities are also studied. 1. The growth of Nitzschia closterium in natural sea water, enriched with N(NO₃) 4 p.p.m., P (PO₄) 0.4 p.p.m. and Fe (citrate) 0.04 p.p.m., is greatlyimproved (Table 1, 2nd column) by the addition of asbed soil extract) and furtherimproved by the addition of the organic contents of soil extract as well (Table.1,lst column). This shows that the natural sea water used is deficient in microelementsand organic substances, both of which can be supplemented by proper soil extract. 2. The addition of vitamin B₁₂ 5mμg/L [Table 3, (1)] to enriched naturalsea water gives better growths than the addition of ashed soil extract [Table 3, (2)].In different B₁₂ concentrations in enriched natural sea water, best growth occurs inB₁₂ 5mμg/L (Fig. 1 2), noticeable deficiency appears in lower concentrationsand inhibitory effect increased in higher concentrations. 3. Optimum growth is obtained in cobalt concentrations 0.5-5μg/L (Fig. 3-5)in enriched natural sea water, while deficiency is observed in lower concentrationsand inhibitory effect in higher concentrations. 4. In artificial sea water without Al, Li, Ba, Cu, Mo Zn, only slightgrowth can be obtained in various cobalt concentrations and growth is comparativelybetter in Co 500μg/L, being less in lower as well as in higher concentrations [Fig.5 6. (A)]. The addition of 0. 1% soil extract makes a remarkable difference[Fig. 5 6, cf. (A) with (B)], and here best growth occurs in Co 0.5-5μg/L(Fig. 5, A; Fig. 6, A B). 5. Inhibitory effect is very remarkable in cobalt concentrations 2,500-5,000μg/L (Fig. 7 8); but with the addition of microelements (Al, Li, Ba, Cu, Mo Zn) the inhibitory effect is greatly lessened (cf. Fig 5 6, A B with Fig.7 8, A, B, C). and may disappear under certain conditions (Fig. 8, A). Thusthe presence of various microelements, such as in artificial sea water "A", may havean effect of neutralizing the inhibitory or harmful effects due to high concentrationsof cobalt, or of other metals as well.At the same time, the presence of various micro-elements in sufficient quantitics may also relief, in one way or another, the demandor the effect of deficiency of Co (Fig. 7-10), or of some other metals as well. 6. At the presence of sufficient various microelements (such as in artificialsea water "A"), the optimum concentration of B₁₂, 5mμg/L (Fig. 1) may have aninhibitory effect (cf. Fig. 11, A with Fig. 9, A) on growths in all The cobaltconcentrations tested (Fig. 11) and the inhibitory effect is greater in higher cobaltconcentrations (5-500μg/L). 7. In natural sea water, the eontents of B₁₂ and Co vary from very deficientquantities to within the optimum range shown by these experiments. The presenceof various microelements and organic substances in various quantities may havevarious influences on the effect of B₁₂, Co or some other substances on the growthof phytoplanktonic organisms. All these substances are continuously enriched innatural sea water, in the form of natural soil extract washed to rivers and hence tothe sea, as land drainage. They are also continuously produced in the sea by livingorganisms and chemical activities using substances contained in this natural soilextract. Thus the mechanism controlling the growth of marine phytoplankton is verycomplicated and further research in this field is urgently needed.