A rare Phaeodactylum tricornutum cruciform morphotype: culture conditions, transformation and unique fatty acid characteristics

Abstract

A rare Phaeodactylum tricornutum cruciform morphotype was obtained and stabilized with a proportion of more than 31.3% in L1 medium and is reported for the first time. Long-term culture and observation showed that the cruciform morphotype was capable of transforming to the oval form following the degeneration of arms by two processes. After three months of culture, four morphotypes existed in a relatively stable proportion in culture for six months (10.5% for oval, 11.3% for fusiform, 37.2% for triradiate and 41.0% for cruciform). Low temperature was particularly beneficial for cruciform cell formation. As the culture temperature decreased from 25°C to 10°C, the percentage of the cruciform morphotype increased from 39.1% to 55.3% approximately. The abundant cruciform cells endowed this strain with unique fatty acid characteristics. The strain cultured at 15°C showed both maximum content of neutral lipid in a single cell and total yield. The maximum content of fatty acid methyl esters was C16:1 for Phaeodactylum tricornutum cultured at four temperatures (43.82% to 50.82%), followed by C16:0 (20.47% to 22.65%). Unique fatty acid composition endowed this strain with excellent quality for biodiesel production.

Figure 1. Proportions of the four morphotypes during long-term culture.

Data are averages of duplicate measurements. Error bars represent standard deviation.

Figure 2. Transformation from cruciform, triradiate and fusiform to oval morphotype.

This transformation involves four stages. In stage 1, the three morphotypes show the characteristics of a thin form indicating that the content of intracellular substances is poor. In stage2, the cells are plump indication many intracellular substances. In stage 3, the overall outline of the cells begins to change. In stage 4, the arms of the three morphotypes gradually disappear and the oval form is observed. Scale bars = 10 μm.

Figure 3. Transformations between the four morphotypes.

Transformations 1–6 are study results of previous researchers. Pathway 7 observed in this study is a new discovery.

Figure 4. Intermediate forms in the culture.

A–E are DIC photos. Scale bars = 10 μm. F is SEM result and the white arrows direct to the “conjunct arm”.

Figure 5. Proportion of the three morphotypes at different temperatures.

Data are averages of duplicate measurements. Error bars represent standard deviation.

Figure 6. Biomass of Phaeodactylum tricornutum cultured at different temperatures.

Data are averages of duplicate measurements. Error bars represent standard deviation.

Figure 7. Images of Phaeodactylum tricornutum cells stained with BODIPY 505/515.

a1, a2, a3, a4 and a5 are images at white light, while b1, b2, b3, b4 and b5 are corresponding images at the exciting light. Scale bars = 10 μm.