Reserve lipids of microalgae are promising for biodiesel production. However, optimization of cultivation conditions for both biomass yield and lipid production of microalgae is a contradictory problem because required conditions for both targets are different. In this study, a twostage cultivation strategy is proposed to enhance lipid production of the microalga Nannochloropsis oculata. Biomass growth and lipid production were carried out in two separate and non-interacting stages. In first-stage cultivation, microalgae were cultivated in optimal conditions for cell growth. Then, microalgae were harvested and transferred into a growth-limited environment, thus enhancing lipid production of microalgae. Here, optimization of the lipid production stage (second stage) with respect to different levels of inoculum concentration, salinity of culture broth, and intensity of irradiance was performed. The results show that irradiance exhibits a significant influence on lipid production. The highest lipid productivity of 0.324 g L −1 day −1 was obtained with an inoculum concentration of 2.3 g L −1 , a salinity of 35 g L −1 , and an irradiance of 500 μmol photons m −2 s −1 . The final yield of lipid obtained from the two-stage process was 2.82-times higher than that from traditional single-stage batch cultivation systems.

Microbial hyaluronic acid (HA), commonly produced by pathogenic Streptococcus, was made possible to be produced by a generally recognized as safe Lactococcus lactis by coexpressing HA synthase and uridine diphosphate-glucose dehydrogenase (UDP-GlcDH) of Streptococcus equi subsp. zooepidemicus in a nisin-controlled expression (NICE) system. With scarce expressed HA synthase alone, the constructed recombinant L. lactis (LL-NA) strain could produce HA with a concentration about 0.08 g/l in the M17 medium supplemented with 1% (w/v) glucose. In contrast to HA synthase, UDP-GlcDH of Streptococcus could be overexpressed in the NICE system. With coexpression of heterologous UDP-GlcDH with HA synthase, the constructed LL-NAB strain grew slightly slower to a concentration about 10% lower that of the LL-NA strain. However, the HA concentration produced was enhanced about eightfold to 0.65 g/l.

Bacillus subtilis strains that can produce hyaluronic acid (HA) were constructed by integrating the HA synthase gene (hasA) and the UDP-glucose dehydrogenase gene of group C Streptococcus (hasB) or of B. subtilis itself (tauD) into the amyE locus of the B. subtilis chromosome. All of the inserted genes were under the control of a strong constitutive vegII promoter of B. subtilis. Although HA production could be achieved by expressing hasA alone, coexpressing hasB or tauD with hasA could enhance HA production at least 2-fold. To replenish the energy consumed for HA biosynthesis, Vitreoscilla hemoglobin (VHb) was coexpressed with the HA-expressing genes. With the expression of VHb, not only the cell concentration was enhanced 25%, but also HA production was further increased by 100%. About 1.8 g/L of HA was obtained by the recombinant strain B. subtilis carrying VHb, hasA, and tauD genes in the expression cassette after 30 h cultivation.

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