Genetic modification of algae

This section provides a short overview of the state of the art on transgenic research on algae, the algal strains that have been used as hosts for genetic modification and the DNA delivery methods. It then presents the targets of genetic modification of algae.

Genetically modified algal strains and their stability: DNA delivery methods

A first prerequisite transformation of the cyanobacterium Synechocystis was already reported in 1970 (Shestakov and Khyen, 1970). Successful transformation of the green alga Chlamydomonas reinhardtii was reported in 1989 (Harris, 2009). C. reinhardtii has become the model species in molecular biology of (eukaryotic) algae and is therefore the best described one (Harris, 2009). Since then, successful genetic transformation of approximately 30 algal species has been demonstrated (Hallmann, 2007; Radakovits et al., 2010). Table 4.1 provides an overview of genetically transformed algal species.

Table 4.1. Overview of genetically transformed algal species

Species

Stability of transformation1

Species

Stability of transformation1

Chlorophyta

Heterokontophyta

Chlamydomonas reinhardtii

Stable

Laminaria japonica

Stable

Chlamydomonas reinhardtii

Stable (chloroplast)

Undaria pinnatifida

Stable

Volvox carteri

Stable

Phaeodactylum tricornutum

Stable

Dunaliella salina

Stable

Navicula saprophila (Fistulifera saprophila)

Stable

Dunaliella viridis

Stable

Cylindrotheca fusiformis

Stable

Haematococcus

Pluvialis

Stable

Cyclotella cryptic

Stable

Chlorella sorokiniana;

Stable

Thalassiosira weissflogii

Transient

Chlorella kessleri (Parachlorella kessleri)

Stable

Nannochloropsis sp.

Stable

Chlorella ellipsoidea

Stable

Dinoflagellates

Chlorella vulgaris

Transient

Amphidinium sp.

Stable

Ulva lactuca

Transient

Symbiodinium

microadriaticum

Stable

Ostreococcus tauri

Stable

Rhodophyta

Cyanobacteria

Cyanidioschyzon Merolae

Stable

Spirulina platensis (Arthrospira platensis)

Porphyra yezoensis

Stable/transient

Anabaena sp

Porphyra miniata

Transient

Synechocystis sp.

Kappaphycus alvarezii

Transient

Synechococcus

Gracilaria chang'd

Transient

Nosctoc muscorum

Porphyridium sp

Stable (chloroplast)

Porphyridium sp

Stable

Euglenids

Gracilaria

Stable

Euglena gracilis

Stable (chloroplast)

Note: 1. Nuclear transformation unless indicated otherwise.

Methods used for DNA delivery into eukaryotic algae are micro-particle bombardment (or biolistic), cell agitation with micro- or macroparticles (e.g. glass beads), protoplast transformation with polyethylene glycol or protoplast or whole cell transformation by means of electroporation, and finally Agrobacterium mediated transformation (Coll, 2006), i.e. methods that are also used for DNA delivery into plants.

Selectable traits used include resistance against antibiotics, chemical agents such as herbicides and genes that rescue mutations such as auxotrophies; marker genes allowing election of transformants include Gus and GFP genes (Leon-Banares, 2004; Technopolis, 2012).

The promoters used to drive gene expression in transgenic algae are either homologous promoters, e.g. the Rubisco small subunit (RbcS2) or the ubiquitin (Ubi1) promoter or the heterologous promoters CaMV35S and SV40. CaMV35S, the cauliflower mosaic virus promoter, a typical promoter for strong expression in higher plants, works well in several algal strains while the SV40, the simian virus 40 promoter a polyomavirus promoter, has been shown to work in H. pluvialis and in C. reinhardtii (Coll, 2006).

Nuclear transformation of algae generally results in random integration of transgenes. In C. reinhardtii and C. merolae and Ostreococcus homologous recombination has been achieved but the frequency is low (Radakovits et al., 2010). Recently one alga, the oil-producing algae Nannochloropsis sp., was shown to have a high frequency of homologous recombination after transformation and selection (Kilian et al., 2011). In contrast, chloroplast transformation often results in homologous recombination (Lapidot, 2002; Purton, 2007).

 
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