pH and photosysthesis

[madmac]

I jus got a ph meter. These are my rough readings, observed manually :

time: pH

0600 : 8.04

0730 : 8.05 (2 t5s 20k comes on) *

0950 : 8.07 (2 t5s actinic comes on)

1100 : 8.11 (single 250W 10k MH comes on)

1400 : 8.25

1500 : 8.30

1800 : 8.47 (MH goes off)

2100 : 8.30 (t5s 20k goes off at 8pm; actinic offs at 8.30pm)

2300 : 8.25

* I dose baking soda in the mornings at 7.30am, to bring the dkH up by 2. There is no immediate effect on pH. I have primarily SPS for corals.

Assuming that only factor altering pH is the tank CO2 levels during photosynthesis...

then it can be seen that as the pH goes higher, so too is the collective 'rate of photosysthesis' of all corals in the tank.

This article presents a good read on the workings of corals : Photosynthesis - by Sanjay Joshi

How does irradiance affect photosynthesis? The rate at which photosynthesis occurs is dependent on the quality and quantity of light, density of zooxanthellae, composition of pigments, temperature and CO2 availability. The rate of photosynthesis is measured either by the O2 released or the CO2 fixed during photosynthesis. Without light, there is no photosynthesis and O2 is consumed and CO2 is released. As light is increased, some CO2 is consumed and O2 released. The point at which the CO2 consumption is the same as the O2 released is called the compensation point.

At the compensation point, the rate of photosynthesis equals the rate of respiration. As the light levels are increased beyond the compensation point, initially the rate of photosynthesis increases almost linearly with the increase in irradiance, but eventually the rate of photosynthesis begins to level off. The range of irradiance values where photosynthesis does not vary with irradiance is called light saturation. The light intensity required for saturation varies from species to species and also depends on other parameters, such as temperature and CO2 concentration.

* emphasis is mine

What I'm trying to get at is, can the pH readings actually tell your how much lighting to give.

ie. as long as your pH increases during lights on, :

... you're providing the primary means to meet the corals energy requirements, via lighting duration

... that if your pH remains the same during mornings to lights off, you are jus at compensation point.

.... if your pH rises for some time and then levels off, you have reach light saturation. This allows us to know when to reduce some lighting. Of course, this does not apply for all corals, e.g deep water species which will reach light saturation sooner, but collective speaking it should hold true, too?

I welcome any contribution to this and to if it'll holds water or not.?

I've made some assumptions above, e.g. room CO2 levels, use of a CR and thing that may alter CO2 leves and skew pH readings there. Thx all. -

**I've made a the post in RC to reach a wider audience

[strat]

Isn't ypur PH on the high side of 8.47?

[loster]

i notice my ph drop to 7.9 - 7.8 when i close my door and window and switch on my a/c at night. i guess my room have so much C02 (from me ) and that why my ph dropped so low. normal night ph is around 8.02 with my CR still running.

now i have moved my tank to my living room and there is much airy at night... my ph stay at 8.11 even my CR still running for past 3 night. but my tank ph will not go beyond 8.3 when all my lights is switch on.

[blueheaven]

So I guess that's why people run reverse photoperiod for refugiums with macroalgae

[weileong]

Isn't ypur PH on the high side of 8.47?

Might be caused by calibration of the pH meter.

Anyway for this purpose madmac is looking at relative pH value for a 24hr period so absolute pH point is not important.

What madmac trying to do is to see that during photosynthesis the pH value in the tank will slowly climb. When the light period gets too long the rate of photosynthesis will reduce and you start to see the pH value starts drop. This could be used to determine the duration of the photo-period.

I spoke to him last night and he suggested to usea high intensity light to kick start the photosynthesis and then after that reduce the light intensity to keep the photosynthesis going on. Remember this was mentioned by Anthony Carlfo during his talk too.

[iantoh]

hmm.. i think below 8.5 is still alright, as long as abiotic precipitation doesnt occur but i guess 8.47 is pretty high.

bro madmac,

cool hypothesis. i think it should hold true, though wouldnt you need to be able to test for either oxygen or CO2 levels to ascertain the direct correlation between PH levels and photosynthetic activity?

[weileong]

bro Ian,

concentration of DO (dissolved Oxygen) has no effects on the pH levels in the tank.

Both CO2 and DO can be at saturation at the same time.

[madmac]

What madmac trying to do is to see that during photosynthesis the pH value in the tank will slowly climb. When the light period gets too long the rate of photosynthesis will reduce and you start to see the pH value starts drop. This could be used to determine the duration of the photo-period.

I spoke to him last night and he suggested to usea high intensity light to kick start the photosynthesis and then after that reduce the light intensity to keep the photosynthesis going on. Remember this was mentioned by Anthony Carlfo during his talk too.

Yup, and you have another good use for the pH meter.

I could, for example, shift the MH light period to start earlier. Because currently with the early 20k-T5s turning on, it doesn't bring up the pH until the MH comes on. So assuming you're trying to target for a level 8.3 pH reading throughout the day, the time to switch off the MH would be when that target is reached, you then let the T5s hold the pH value for you until lights off.

You'll be able to save electrical cost, knowing same time, you're also meeting the lighting needs for the overall tank.

Of course this is all theory and will work of one individual coral, but with more corals and their different needs and different placement, you may need to just over-cater the duration a little.

Do you guys think it makes sense? or it won't work practically at all.

[madmac]

cool hypothesis. i think it should hold true, though wouldnt you need to be able to test for either oxygen or CO2 levels to ascertain the direct correlation between PH levels and photosynthetic activity?

The origin of that idea, actually came from WeiLeong, in a post he made in his own thread. It does make sense, it was so simply obvious and yet not. Now the only thing to do is to test out the theory and see how it goes.

[weileong]

I've read about using a low wattage U-V lamp (UV-B and UV-C) to induce the corals to produce more pigments for photoprotection.

[madmac]

I don't follow you DK, I know I'm not talking abt O2 and its effect on the pH.

I'm talking about using the pH levels to determine sufficient photo-period... colours will be those tweaking thingg you do later, to get the best output.

Unless I understanding wrongly, compensation point is the minimum level where the coral can be sustained. Above that, the coral will continue to take in CO2, which increases the pH... no?

[weileong]

When the rate of photosynthesis slows down we should see the pH slowly taper down. However it is not reasonable to expect the pH to raise in proportion as it increases as pH is log scale meaning pH 7 and pH 8 has 10 times the difference of H+

[madmac]

DK, see, I'm easily confused. . you're absolutely right there.

[madmac]

When the rate of photosynthesis slows down we should see the pH slowly taper down.

When it slows down, as in, along with the decrease of light... then yes.

but at light saturation, with further increase of irradiance, the ROP, doesn't follow linearly, it starts to level off.. while still producing energy for corals, implying the pH should stay level too.

[weileong]

but at light saturation, with further increase of irradiance, the ROP, doesn't follow linearly, it starts to level off.. while still producing energy for corals, implying the pH should stay level too.

Sounds logical but if one is using CR and when the CR kicks in (CR not running 24/7) then the pH will starts to slowly reduce too making it a little difficult to judge if one had light saturation.

[tineng]

bro madmac..like what weileong said, oxygen has no effects on the pH of the tank..so at compensation point..the pH should increase as all CO2 produce for respiration is used for photosynthesis...so there is no CO2 to hold the pH...and i was also thinking...some degree of colour from sps are for photoprotection..if u off the MH once pH lvl maintains..then those which achieve light saturation will not colour up as nice..

in any case..i have to agree thats a good hypothesis..

acuatlly while the increase of O2 might not have impact on PH....increase CO2 does lower PH.....

in my early days of 2 footer.....after removing the fully enclose hood, the PH swing at night becomes less drasitc.....however, i notice for my current tank, 4.5ft....the impact is not that evident....

bro Madmac. you tank is quite small also, do you notice a significant increase of stablility of the PH when you switch from cannister to a sump set up as your effective surface area expose to the air has doubled?

[madmac]

TinEng, before this, I never had a pH measuring instrument, not even a pH kit... (I had it initially, and about all the tests done showed almost the same result, so decided not to buy another when the it finished. ) So can't really comment, except now with some pH data, I trying to make some sense from all of it... and yes the tank is small abt 150 liters only, but there is no hood... opened at the top, so I think the water molecules are normalized with the air and should be relative stable.... well until the lights come on.

It is such an eye-opener to have one of the meters constantly attached to the tank... oh well I suppose it'll grow old on me soon.

[madmac]

Sounds logical but if one is using CR and when the CR kicks in (CR not running 24/7) then the pH will starts to slowly reduce too making it a little difficult to judge if one had light saturation.

Yes.. thats a good point.. I'm sure there can be a way to still determine the level, with some tests...

I'm not sure if this will work, perhaps timing for the pH to level off(i.e. at saturation level), before turning on your CR. Once you've determined the time it takes to reach that level, adjust your light timers to match the duration and you're done. When the CR comes on, should not matter much as one is trying to determine the optimum time...?