So, the cyano was not 'self sustaining' after all, nothing is. That was some woolly thinking by me. No matter how much I removed of the slime, how much I diluted it in the water, how low I knocked it down with antibiotics, it would grow back. The conditions in my tank were obviously ideal for its growth. I'm not saying that controlling cyano by phosphate limitation (achieved by stimulating plant and algal growth) doesn't work. I'm just saying it didn't work for me. Maybe I didn't have enough plants/the right types of plants or add enough nutrients. Maybe it works if you have more lights. I note from re-reading the cyano section of The Krib archive that great success has apparently been had using phosphate limitation while adding CO2. But surely it should be possible to have a cyano free tank without injecting CO2! From what I have read the strategy may be flawed anyway. It seems to me to be based on the assumption that plants and algae, the so called 'higher' lifeforms (eukaryotes), are more efficient at phosphate uptake than cyano (cyanobacteria are prokaryotes). I can find no evidence to support this idea, but I can find people who say otherwise.
"Phosphate uptake in cyanobacteria follows saturation-type kinetics with kinetics parameters (K1/2 and Vmax) comparable to those of eukaryotic algae"
Maria del Carmen Avendano and Eduardo Fernandez Valiente. Effect of Sodium on Phosphate Uptake in Unicellular and Filamentous Cyanobacteria. Plant Cell Physiol. 35(7): 1097-1101 (1994)
These scientists have measured rates of phosphate uptake in the lab and basically found cyano and algae take it up at the same rate and with the same efficiency so when phosphate levels are low, neither should have any particular advantage.
"Phosphorus storage in cyanobacteria appears to be much larger than in other species and this capacity was reported to give them a competitive advantage over diatoms and chlorophytes when P was supplied in pulses"
Santosh Kumar Singh, Vandana Pandey, and Kapil Deo Pandey. Phosphate uptake kinetics and its regulation in N2-fixing cyanobacterium Anabaena oryzae Fritsch under salt stress. African Journal of Biotechnology. 6 (20):2363-2368 (2007)
This work suggests that when phosphate is low cyano may even out-compete the algae (diatoms and chlorophytes are algae) because they can store it better. Anyway, this type of speculation is pointless because cyanobacteria are a very diverse group. If you're interested read The Ecology of Cyanobacteria: Their Diversity in Time and Space it's on google books. They're so diverse that even if you knew which species you had in your tank, you might not be able to predict how they would behave because they vary so much within species. There is a suggestion on The Krib that cyano plagues in fish tanks are "probably Oscillatoria or Lyngbya". I haven't found out if anything is known about the relative efficiency of phosphate uptake in these species, but not knowing what species my cyano is makes it a moot point. It was time for a change of tactic.
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