Why Do Large Carbon Doses Suppress Cyano (in my system)?
A big part of this method is about using bacteria to digest the waste that fuels nuisance growth, and the other big part is about using large carbon dose to tweak bacterial behavior.
In the spirit of taking things apart, I wanted to take a closer look at the large carbon dose part, and why large doses of organic carbon suppress cyano growth in my system. It appears to disrupt cyano's ability to exploit accumulations of waste.
Usually in my tank, algae dying (from vibrant) results in debris material that supports cyano growth, but if I do a large daily carbon dose (while keeping vibrant), the cyano recedes despite the debris source still being produced.
So I turned cyano off/on/off to investigate.
(
left: while dosing carbon no cyano on dying algae,
middle: no carbon dosing and lots of red cyano,
right: heavy carbon dosing and re-disappearance of cyano. The carbon dosing here is 2/3 of the carbon dose recommended in the method.)
Bulk water parameters were not dramatically changed. PO4 stayed around ~0.10 and NO3 tests zero...if you don't look too closely. More on that in a bit.
So why is the cyano receding?
In the sump there's an interesting stringy cyano colony that goes through color shifts when I dose large amounts of carbon. It goes from deep emerald green -> light green -> yellow, and when carbon dosing stops, it reverses.
This sort of color transition in cyano has been known for decades to be associated with N depletion.
"...the alga [cyanobacteria] lost its characteristic blue-green color and took on the light brown to golden color of a diatom. Experimentation showed that it was not the wavelength of light, but rather the
depletion of nitrogen from the medium, and that alone, which was responsible for the color shift. In this and further work ..., it was determined that the color shift was
due to a selective disappearance of chlorophyll and phycocyanin, thereby unmasking the carotenoids; that a yellowed culture could be made to return to green, sometimes within 24 hours, upon the addition of minute amounts of nitrogen..."
paper link
Other research in the 60 years since, has fleshed out how in nutrient deprivation, the cells become photobleached, and reduce their light-harvesting pigments while keeping more photoprotective carotenoids.
So I ran some spectrum test on samples from the cyano during increased carbon dosing, and I found the exact pigment shifts described above that are associated with N depletion.
On the left is the colony of cyano, on the right are the spectral measurements from the sample. Black is the actual measured spectra, grey dotted line is a fit made from the pigments listed in the graph header. Different colored lines represent different pigments that add up to the gray dotted line.
Top: emerald green cyano - 10/21 - note the strong phycocyanin (blue line).
Middle: light-green - 10/25 - Phycocyanin almost collapsed and Chlorophyll A is slightly reduced compared to carotenoids.
Bottom: yellow - 10/28 - total collapse of phycocyanin, and Chlorophyll A reduced compared to carotenoids which become dominant.
Additionally, the pigments not only shifted, the overall pigment density of these samples decreased during this process. Below compares absorption from light-harvesting pigments, to the scattering by the material after bleached with a couple of drops of sodium hypochlorite.
Solid lines are raw absorbance (normalized to 1) and the dotted lines represent what scattering remained when the sample was bleached.
The 10/21 - emerald green sample - blue line had over half 58% of its absorbance from actual pigments.
10/25 - light-green sample - working pigments had dropped to 40%
10/28 - yellow sample - pigment absorbance was only 13% of sample.
This indicates that by the end there was severe light-harvesting pigment loss, major photobleaching of the sample. It suggests that a lot of the sample was dead at this point (the strands detached from the pipe 4 days later) , All this data points to nutrient (Nitrogen) deprivation.
So what NO3 was actually measured?
10/8 - During heavy carbon dosing
NO3 - 0
PO4 - 0.09
10/21 - During no carbon, at height of cyano
NO3 - 0.40ppm
PO4 - 0.11
I then vacuumed and gently shook a sample out of the sand/debris
NO3 - 0.70
PO4 - 0.17
11/1 - Heavy Carbon Dosing - water
NO3 - 0.0
PO4 - 0.08
11/1- Sand/debris
NO3 - 0.0
My interpretation here is that the detectable NO3 values from the no-carbon dosing are not themselves near enough to grow happy cyano (h/t
@Dan_P), but they are small indicators in the water of much larger nutrient transfers happening in the debris. That sand/debris reads higher (near double) is one indicator of that. Another is that I sucked up sand and debris, and filtered down to 150 microns leaving only dust in 200mL water. After bubbling for 2 days in the dark it produced 1.6ppm NO3. The water volume was totally arbitrary, I could have bubbled the debris dust in much smaller water volume and read much higher NO3 concentration.
The point is, it's a constant ongoing production of N from the debris, but in the presence of large carbon dose, even that source of N seems to be snuffed out by other bacterial actors before cyano can get it. Bacterial strings and films are visible in all surfaces during this carbon dosing process, and my skimmate tripled, compared to no-carbon.
Congrats to those who read this far. Hope you found something interesting.
