Laboratory Studies (cont)

Re-Distributed by http://www.tpub.com

National Renewable Energy Laboratory

This has implications for media costs in a commercial production facility,

because silicate is a relatively expensive chemical. On the other hand, Si

deficiency is known to promote storage lipid accumulation in diatoms, and

thus could provide a controllable means to induce lipid synthesis in a two-

stage production process.

Another characteristic of diatoms that

distinguishes them from most other algal groups is that they are diploid

(having two copies of each chromosome) during vegetative growth; most

algae are haploid (with one copy of each chromosome) except for brief

periods when the cells are reproducing sexually. The main ramification of

this from a strain development perspective is that it makes producing

improved strains via classical mutagenesis and selection/screening

substantially more difficult. As a consequence, diatom strain development

programs must rely heavily on genetic engineering approaches.

Green Algae. Green algae, often referred to as chlorophytes, are also

abundant; approximately 8,000 species are estimated to be in existence.

This group has chlorophyll a and chlorophyll b. These algae use starch as

their primary storage component. However, N-deficiency promotes the

accumulation of lipids in certain species. Green algae are the evolutionary

progenitors of higher plants, and, as such, have received more attention than

other groups of algae. A member of this group, Chlamydomonas reinhardtii

(and closely related species) has been studied very extensively, in part

because of its ability to control sexual reproduction, thus allowing detailed

genetic analysis. Indeed, Chlamydomonas was the first alga to be

genetically transformed. However, it does not accumulate lipids, and thus

was not considered for use in the ASP. Another common genus that has

been studied fairly extensively is Chlorella.

Golden-Brown Algae. This group of algae, commonly referred to as

chrysophytes, is similar to diatoms with respect to pigments and biochemical

composition. Approximately 1,000 species are known, which are found

primarily in freshwater habitats. Lipids and chrysolaminarin are considered

to be the major carbon storage form in this group. Some chrysophytes have

lightly silicified cell walls.

Prymnesiophytes. This group of algae, also known as the haptophytes,

consists of approximately 500 species. They are primarily marine

organisms, and can account for a substantial proportion of the primary

productivity of tropical oceans. As with the diatoms and chrysophytes,

fucoxanthin imparts a brown color to the cells, and lipids and

chrysolaminarin are the major storage products. This group includes the

coccolithophorids, which are distinguished by calcareous scales surrounding

the cell wall.

A Look Back at the Aquatic Species Program--Technical Review