do seaweed, algae need high alkalinity?
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vlangel
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I use Instant Ocean which is low in calcium and alkalinity and I can grow lots of macro algae. If you are trying to grow the calcareous algae then you might need a salt higher in calcium and alkalinity.
Randy Holmes-Farley
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This is an interesting question, but the simplistic answer is that 7 dKH is perfectly fine.
A different question is whether 10 dKH might or might not be better.
It boils down to whether the macroalgae is getting CO2 from CO2, or from bicarbonate. It seems there are some of each type:. That question also has a bearing on what pH is optimal for any given species.
Photosynthesis and the Reef Aquarium, Part I: Carbon Sources by Randy Holmes-Farley - Reefkeeping.com
Photosynthesis of Macroalgae as a Function of pH
One of the side effects of the necessity of taking up carbon dioxide to photosynthesize is that pH may affect the rate of photosynthesis, because the amount of carbon dioxide (as CO2 or H2CO3) in the water varies with pH. Assuming constant carbonate alkalinity, the effect is quite strong. A drop of 0.3 pH units implies a doubling of the carbon dioxide concentration. A reef aquarium at pH 8.5, for example, has one fourth the carbon dioxide of a reef aquarium at pH 7.9, assuming the carbonate alkalinity is the same.
Aquarists may rightly wonder whether organisms are able to photosynthesize efficiently as the pH is raised. The answer is mixed. Some can and some cannot. Those organisms that rely solely on carbon dioxide may not. Those that rely on both carbon dioxide and bicarbonate have a better chance of retaining efficiency at higher pH because a much larger amount of bicarbonate is present, and it does not change as rapidly with pH over the range of interest to aquarists.
Table 1 shows the response of a variety of macroalgae in terms of their ability to photosynthesize at pH 8.1 and 8.7. In seawater with constant carbonate alkalinity, there is 20% as much carbon dioxide at pH 8.7 as at pH 8.1, so an organism relying on carbon dioxide alone might experience a large drop in photosynthetic rate over this range. Clearly, the response varies with species. Chaetomorpha aerea, in particular, may be of substantial interest to aquarists. It is not necessarily the exact species that many grow in refugia (which is unidentified as far as I can tell), but this species of Chaetomorpha shows a 25% drop in photosynthesis when exposed to the higher pH. That drop is not as large as some other species, but may still be important, and it is more than many other species of macroalgae.
Of course, the photosynthesis rate does not necessarily translate to growth rates. If other nutrients are limiting growth (nitrogen, phosphorus, iron, etc.), then it may not matter if the rate of photosynthesis is reduced at higher pH. But because these nutrients are often present in surplus in reef aquaria, it may well be that carbon uptake is limiting in some cases, and in those cases aquarists might benefit from ensuring that the pH is not too high.
Table 1. Relative rates of photosynthesis19 in seawater (measured by oxygen evolution) at pH 8.7 relative to pH 8.0. A value of 100 means that the rates were the same, and values below 100 indicate less photosynthesis at pH 8.7.
A different question is whether 10 dKH might or might not be better.
It boils down to whether the macroalgae is getting CO2 from CO2, or from bicarbonate. It seems there are some of each type:. That question also has a bearing on what pH is optimal for any given species.
Photosynthesis and the Reef Aquarium, Part I: Carbon Sources by Randy Holmes-Farley - Reefkeeping.com
Photosynthesis of Macroalgae as a Function of pH
One of the side effects of the necessity of taking up carbon dioxide to photosynthesize is that pH may affect the rate of photosynthesis, because the amount of carbon dioxide (as CO2 or H2CO3) in the water varies with pH. Assuming constant carbonate alkalinity, the effect is quite strong. A drop of 0.3 pH units implies a doubling of the carbon dioxide concentration. A reef aquarium at pH 8.5, for example, has one fourth the carbon dioxide of a reef aquarium at pH 7.9, assuming the carbonate alkalinity is the same.
Aquarists may rightly wonder whether organisms are able to photosynthesize efficiently as the pH is raised. The answer is mixed. Some can and some cannot. Those organisms that rely solely on carbon dioxide may not. Those that rely on both carbon dioxide and bicarbonate have a better chance of retaining efficiency at higher pH because a much larger amount of bicarbonate is present, and it does not change as rapidly with pH over the range of interest to aquarists.
Table 1 shows the response of a variety of macroalgae in terms of their ability to photosynthesize at pH 8.1 and 8.7. In seawater with constant carbonate alkalinity, there is 20% as much carbon dioxide at pH 8.7 as at pH 8.1, so an organism relying on carbon dioxide alone might experience a large drop in photosynthetic rate over this range. Clearly, the response varies with species. Chaetomorpha aerea, in particular, may be of substantial interest to aquarists. It is not necessarily the exact species that many grow in refugia (which is unidentified as far as I can tell), but this species of Chaetomorpha shows a 25% drop in photosynthesis when exposed to the higher pH. That drop is not as large as some other species, but may still be important, and it is more than many other species of macroalgae.
Of course, the photosynthesis rate does not necessarily translate to growth rates. If other nutrients are limiting growth (nitrogen, phosphorus, iron, etc.), then it may not matter if the rate of photosynthesis is reduced at higher pH. But because these nutrients are often present in surplus in reef aquaria, it may well be that carbon uptake is limiting in some cases, and in those cases aquarists might benefit from ensuring that the pH is not too high.
Table 1. Relative rates of photosynthesis19 in seawater (measured by oxygen evolution) at pH 8.7 relative to pH 8.0. A value of 100 means that the rates were the same, and values below 100 indicate less photosynthesis at pH 8.7.
| Species of macroalgae: | Relative photosynthesis at pH 8.7 compared to pH 8.1 (as a %): |
| Chaetomorpha aerea | 75 |
| Cladophora rupestris | 100 |
| Enteromorpha compressa | 67 |
| Ulva rigida | 100 |
| Codium fragile | 76 |
| Asparagopsis armata | 45 |
| Gelidium pusillum | 33 |
| Gelidium sesquipedale | 18 |
| Gymnogongrus sp. | 39 |
| Osmunda pinnatifida | 46 |
| Porphyra leucosticta | 110 |
| Fucus spiralis | 86 |
| Colpomenia sinuosa | 100 |
| Dictyota dichotoma | 53 |
| Cystoseira tamariscifolia | 57 |
| Padina pavonia | 53 |
We typically call them macroalgae (algae and seaweed both work, but calling them macro/macros/macroalgae seems to be a bit more specific when you're meaning ornamental algae for reef tanks).
I'd suggest searching "macros" or "macroalgae" if you're just looking for any kind; if you're looking for a specific kind - like "Sea Lettuce" (Ulva spp.) for example - then I'd suggest searching for those specific names (i.e. for "sea lettuce" or "ulva").
Edit: Forgot to add, the forum linked below is a good place to start looking for these:
Macroalgae (all types of desirable algae)
This is the forum for discussing all types of macros (chaeto, caulerpa, gracilaria, halimeda, or any other kind of macro you can think of)! Need tips for growing or troubleshooting desirable algae? Wanna show off your macroalgae display or refugium? Post here!
www.reef2reef.com
