The circadian rhythm ensures that this occurs even when the organism is growing in continuous light or continuous darkness. In the dark, the cyanobacteria act as heterotrophs, getting their energy and carbon from the medium. "Cyanothece" has the genes for the use of a variety of sugar molecules; although glycerol is the only one that has been used successfully to grow "Cyanothece" in the dark. Many of the genes that are unique to the genera have homologs in anaerobic bacteria, including those responsible for formate production through mixed-acid fermentation and also fermentative lactate production. Some "Cyanothece" species also are capable of tryptophan degradation, methionine salvage, conversion of stored lipids into carbohydrates, alkane and higher alcohol synthesis, and phosphonate metabolism. They can switch between a photoautotrophic and photoheterotrophic metabolism depending on the environmental conditions that maximize their growth, employing the pathways that use the least amount of energy. The genomes of many of "Cyanothece" species have been sequenced, ranging from 4.79 to 7.84 Mbp. Between 4367 and 6642 coding sequences are an amalgamation of genes encoding capabilities for fermentation and aerobic nitrogen fixation (like filamentous cyanobacteria). Unusually, the genes for nitrogen fixation are in a large, contiguous cluster (under the control of multiple promoters), including genes for the uptake hydrogenase, regulators, and transporters. The organism's robust circadian rhythm is apparent in the co-ordination of transcription of correlated processes. Using microarrays, about 30% of 5000 genes tested exhibited diurnal oscillations in 12-hour light/dark conditions, while 10% continued the behavior in continuous light. About 1,705 of the gene groups are >99.5% homologous with other cyanobacteria genera, largely "Microcystis" and filamentous, nitrogen-fixing strains. Typical GC content is about 40%. "Cyanothece" species also have three to six plasmids ranging between 10 and 330 kb. Unique to some species of this genus is one to three linear pieces of DNA. The linear DNA encodes enzymes for glucose and pyruvate metabolism (recall that glycerol is the only organic carbon source on which "Cyanothece" has grown successfully), lactate fermentation, transposons, and CRISPR proteins. "Cyanothece" species do not typically use homologous recombination, which greatly hinders genetic manipulation; an exception is "Cyanothece" sp. PCC 7822 in which gene knock-outs can be generated.