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Astaxanthin (3,3′-dihydroxy-β,β-carotene-4,4′-dione), a high-value ketocarotenoid with a broad range of applications in food, feed, nutraceutical, and pharmaceutical industries, has been gaining great attention from science and the public in recent years. The green microalgae Haematococcus pluvialis and Chlorella zofingiensis represent the most promising producers of natural astaxanthin. Although H. pluvialis possesses the highest intracellular astaxanthin content and is now believed to be a good producer of astaxanthin, it has intrinsic shortcomings such as slow growth rate, low biomass yield, and a high light requirement. In contrast, C. zofingiensis grows fast phototrophically, heterotrophically and mixtrophically, is easy to be cultured and scaled up both indoors and outdoors, and can achieve ultrahigh cell densities. These robust biotechnological traits provide C. zofingiensis with high potential to be a better organism than H. pluvialis for mass astaxanthin production. This review aims to provide an overview of the biology and industrial potential of C. zofingiensis as an alternative astaxanthin producer. The path forward for further expansion of the astaxanthin production from C. zofingiensis with respect to both challenges and opportunities is also discussed.

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By assembling a ZnO nanowire (NW) array based nanogenerator (NG) that is transparent to UV light, we have investigated the performance of the NG by tuning its carrier density and the characteristics of the Schottky barrier at the interface between the metal electrode and the NW. The formation of a Schottky diode at the interface is a must for the effective operation of the NG. UV light not only increases the carrier density in ZnO but also reduces the barrier height. A reduced barrier height greatly weakens the function of the barrier for preserving the piezoelectric potential in the NW for an extended period of time, resulting in little output current. An increased carrier density speeds up the rate at which the piezoelectric charges are screened/neutralized, but a very low carrier density prevents the flow of current through the NWs. Therefore, there is an optimum conductance of the NW for maximizing the output of the NG. Our study provides solid evidence to further prove the mechanism proposed for the piezoelectric NG and piezotronics. The output current density of the NG has been improved to 8.3 µA/cm 2 .Developing nanomaterial-enabled technologies for energy harvesting has attracted a lot of interest recently. 1,2 Using aligned ZnO nanowire (NW) arrays, we have recently demonstrated a nanogenerator (NG) for converting mechanical energy into electricity. 3,4 The mechanism of the NG relies on the coupled semiconducting and piezoelectric properties and is composed of two processes. 5,6 When a clean ZnO NW or nanobelt is bent by an atomic force microscope (AFM) tip, an asymmetric strain is produced across the width of the NW. As a result of piezoelectricity, the stretched side of the NW has a positive potential and its compressed side has a negative potential. The contact between a Pt coated tip with ZnO is a Schottky diode. When the tip contacts the NW and bends it, the contact between the tip and the stretched side is a reversely biased Schottky diode. In such a case, a piezoelectric potential is created in the NW, but there is no charge flowing across the Schottky diode although there is a piezoelectric potential in the NW side, resulting in a charge creation and accumulation. This is the first process. When the tip scans in contact mode and reaches the compressed side of the NW, a forward biased Schottky diode is formed at the interface; thus, the external electrons can flow across the interface under the driving of the piezoelectric potential, resulting in an external current detected by the measurement meter. This is the current output process.The mechanism of the NG is based on two important physical quantities. One is the height of the Schottky barrier, which needs to be high enough to hold the charges from leaking. Second, the conductivity and carrier density of the ZnO NW are adequately low in the first step to preserve the piezoelectric potential distribution in the NW from being "neutralized" by the freely flowing charge carriers, which are electrons for n-type ZnO, but they need to be high ...