Microcoleus chthonoplastes and Phormidium cerium were isolated from microbial mats covering all sediments along the Arabian Gulf coasts. These isolates could consume and oxidise n-alkanes. The establishment of axenic cultures faced the problem that with progressive axenity the cyanobacterial growth seemed to cease. The associated organotrophic bacteria, Rhodococcus rhodochrous, Arthrobacter nicotianae, Pseudomonas sp. and Bacillus sp., could utilize n-alkanes. The total number of these organotrophs was about 2x10(6) cells g(-1) fresh culture, and R. rhodochrous was the most dominant. In order to test the potential of cyanobacteria for n-alkane consumption, experiments were constructed to rule out the role of the associated organotrophic bacteria. Aliquots, 0.5 g fresh cyanobacterial samples, each containing about I x 10(6) organotrophic bacterial cells (= 0.001 mg fresh bacteria) were incubated in inorganic medium aliquots supplied with an n-alkane. The same was repeated using 1.0 x 10(6) cells each of the four organotrophic bacteria instead of the cyanobacterial samples. The nonaxenic cyanobacterial samples consumed up to 60% of the available alkane, whereas no detectable consumption was measured in any of the pure organotrophic bacterial cultures. For all organotrophic bacteria, the numbers had to be increased ten-thousand times in order that detectable alkane consumption might become measurable. The fatty acids resulting from the n-alkane oxidation were found incorporated in cell lipid classes characteristic of cyanobacteria, namely in galactolipids and sulfolipids. These results may imply that the two test cyanobacteria contribute directly to n-alkane uptake and oxidation.
