Microalgae are of great ecological importance as they represent a major source of global oxygen and contribute critically to carbon fixation [1, 2]. But also in biotechnical applications microalgae offer enormous potential and have been used in food and cosmetic industry already for many years now as certain species represent a natural source of omega-3-fatty acids, vitamins, pigments and anti-oxidants. Especially within the last decade microalgae came into focus of fuel industry as a renewable and beneficial source of lipid interesting for biodiesel production [3–5].
Another aspect of algal biotechnology is the idea of using microalgae as expression systems for recombinant proteins [6–9]. No matter if enzymes, hormones, antibodies or biotechnological relevant protein compounds – today there is a great demand for recombinant proteins especially in medical and industrial sectors . Classical expression systems like bacteria, yeast or mammalian cell cultures all depend on external carbon sources emerging as an important cost factor in large-scale expression. Microalgae combine various advantages of classical expression systems as they possess rapid growth rates, are very easy to handle, provide eukaryotic post-translational modifications and are no host to human pathogens. Additionally, microalgae are fueled by photosynthesis and work CO2-neutral making them very interesting as low-cost environment friendly protein factories [11–13].
Research in that field focused so far mainly on the green alga Chlamydomonas reinhardtii demonstrating that medical relevant proteins like antibodies, hormones and vaccines can be produced very efficiently in the chloroplast of the cells [14–18]. Recent work revealed that other species like the diatom Phaeodactylum tricornutum can express foreign proteins with high efficiency also from nuclear promoters having the advantage that even complex eukaryotic proteins can be synthesized, which need post-translational modifications and the assembly of multiple subunits. A fully-assembled and functional human IgG antibody against the Hepatitis B Virus surface protein (HBsAg) was shown to accumulate in P. tricornutum to 9% of total soluble protein . Furthermore, the introduction of the bacterial PHB-pathway led to efficient production of the bioplastic poly-3-hydroxybutyrate (PHB) demonstrating that algae might represent an production platform not only for proteins but also other bioproducts .
Efficient protein expression is an important issue, but before ending up with the final product time consuming and extensive processing steps such as cell harvesting, cell lysis followed by product purification are usually necessary. Hence, the ideal expression system does not only produce recombinant proteins with high efficiency but also secrets the proteins into the medium making many cost-intense purification steps dispensable. So far research on protein secretion in microalgae is very rare, but in cell wall deficient Chlamydomonas strains it was already shown that protein secretion of foreign proteins is basically possible even though efficiency seems to be rather low . In diatoms like P. tricornutum polysaccharides are known to be secreted depending on culture conditions and the morphotype , however, little is known about protein secretion [23–25].
Here we show for the first time that a microalgal system like the diatom P. tricornutum is able to secrete a fully assembled and functional human IgG antibody with high efficiency into the medium. Thus, this study highlights the great potential of these microalgae as novel protein factories secreting complex molecules, which remain functional within the medium for several days.