Part registries are lists and collections of standardized biological parts that can be strung together to build advanced and more complicated biological devices. Parts in this context mean DNA constructs that encode a given function. On the other hand, biological devices perform more complicated functions and are made from combining different parts. An example would be that a part encodes a specific enzyme in a pathway, while a device performs a complicated function like biological arsenic detection or bioplastics production. These registries are a list of DNA constructs (parts) and synthetic circuits (biological devices) that are ready for genetic insertion. The idea behind creating such registries is to list reliable and well tested biological parts that can be manufactured to a given standard and that serve as the building blocks of sophisticatedly engineered biological tools and devices (Canton et al. 2008). This standardization liberates the process of designing and fabricating complex biological devices from the daunting task of designing and fabricating each individual and necessary component. In addition, the fabricators of those devices can rely on specialized manufacturers of those standard parts to provide the components needed for fabricating the advanced biological devices. All in all, this has the effect of opening up the field and providing the impetus for more advanced applications (Baker 2006).

In biological part registries, standardization is of essence. Standardization in the context of biological parts refers to the ability of a part to assimilate into a larger structure without any complications. In other words, the part does not have any restriction sites that interfere with the process of assembling into larger devices. Another important aspect of standardization is with respect to function. Biological parts should serve a given function that is both well-defined and consistent. Perhaps the most widely known and used registry is the Registry for Standard Biological Parts (http://parts. igem. org/Main_Page) (Kahl and Endy 2013). The registry, like most other registries, relies on community contributions to expand its biological part offerings. The community accessible approach of the registry has expanded the registry’s offering to include thousands of parts serving numerous functions. The registry’s part types include promoters, ribosome binding sites, protein domains, protein coding sequences, translational units, terminators, plasmid backbones, primers and composite parts which are a composition of two or more simpler parts. To ensure openness, efficiency and consistency, parts offered by the registry comply with the BioBricks assembly standard (http://parts. igem. org/Help:Standards/Assembly). The current standard relies on defining a DNA prefix and suffix on a standard plasmid backbone. The part is then inserted into the plasmid backbone specifically between the prefix and suffix. The prefix and suffix also contain specific restriction sites, and this precise definition that is included in the standard also forbids the introduction of restriction sites that interfere with the part assembly and usage process. In general, those features allow the smooth and immediate use of the biological parts.

Algae-specific registries with the ambition of the registry for standard biological parts are yet to be fully developed. However many repositories are available that provide cell lines, several thousand algal strains, DNA constructs, and specific genetic engineering tools. One prominent example is the Chlamydomonas Resource Center, a repository of Chlamydomonas reinhardtii strains, plasmids, kits, and cDNA libraries among other things (http://chlamycollection. org/). While most of these registries are not registries of synthetic biological parts specifically, they still offer many tools and products of value to synthetic biology endeavors in algae. Also, the availability of standard and customized algal optimized plasmids prepared and sold by private companies, such as Life Technologies, (a brand of Thermo Fisher Scientific, Carlsbad, CA, USA), is another step forward into easing up synthetic biology applications with microalgae.

While the breadth of registries and repositories up and running is a call for optimism in the field of synthetic biology, certain challenges still permeate the part registry model. For example, characterization of many biological parts and the precise definition of their functions are still lacking. Furthermore, many parts display different behavior in different cells or organisms and in different laboratory conditions; this introduces a major challenge to the field with respect to repro­ducibility of function. Stability and reliability become even more daunting chal­lenges as the organism’s complexity increases. This means that extending biological parts and the registry model to algae, or organisms of higher complexity than simple microbes, becomes additively challenging quite quickly. Still, another challenge is in the long-term behavior of parts and their behavior as components of increasingly complex devices. Cell functions are prone to seemingly random behavior and noise which can complicate the ability of biological parts and complex devices to behave consistently for a significantly long period (Kwok 2010).