Hi, and thanks for your patience.
I’d start by saying that my word is hardly the last word on the subject of nutrient sequestration in recirculating aquarium systems. But I hope some of the following is helpful!
The first thing I’d say is that I personally wouldn’t get too wrapped up in any effort to “balance” nitrogen and phosphorus per Redfield’s ratio. This ratio is NOT necessarily a “normal” or “ideal” ratio of nitrogen and phosphorus in all natural ecosystems; it rather is the recipe for phytoplankton. In other words, this ratio is ideal for phyto and other algae, but not necessarily for corals and other reef animals. Wait, but what about zooxanthellae, you might ask...
In reality, natural seawater very often has different ratios than this (giving us the concept of limiting nutrients). Actually, I’ve scoured many scientific papers looking for “normal” nitrate and phosphate concentrations for reef ecosystems; all I learned is that there are no “standard” values for these parameters. They vary from site to site, and may even vary from within a particular site according to season.
One thing, however, seems to be the norm in zooxanthellate coral reef habitats:
Oligotrophy. While reefs do tend to be nitrogen-limited, both nitrogen and phosphate are typically in very low supply. This I think is important to remember, as it is the reason that coral reefs can be coral reefs in the first place; if nitrate and phosphate levels are high, benthic algae grows unchecked and (as I’m sure you already know) outcompetes corals and most other sessile inverts. The take-home, I think, is that nutrient concentrations (low ones, to be most specific) are way more critical than nutrient ratios when it comes to promoting a coral-friendly environment. Far better to have oligotrophic (especially low-N) conditions with a slightly imbalanced Redfield ratio than to have eutrophic (especially high-N) conditions with a “perfectly balanced” ratio.
That’s not to say that nitrogen deficiency and phosphorus deficiency are equivalent. To the contrary, it appears that phosphorus deficiency is considerably more stressful to the coral holobiont (most specifically to the zooxanthellae). Again, “deficiency” is relative in the Redfield sense, as a deficiency of phosphorus may be brought on by an excess of nitrogen.
Again, I don’t wish to present confusing or contradictory ideas here; absolutely Redfield ratios matter to corals because they matter to the algae that live within them. The findings in
this paper, for example, strongly suggest that phosphorus deficiency (or nitrogen surplus, if you want to look at it that way) can adversely affect coral (i.e. zooxanthellae) health. What I’m trying to point out is that ratio AND concentration matter. And low nitrogen in particular seems to be key. Corals seem to be pretty well adapted to N starvation (especially compared to P starvation). And, nitrogen limitation appears to be the very thing that keeps clear water/coral reefs from becoming green water/algal turfs.
It’s interesting that you had a green algae bloom just as your NO3 tests began to read zero. This is a common paradox in our hobby. Why would a bloom occur (especially green algae, which in general are most nitrogen-hungry) just as your nitrates drop to undetectable levels? The test kit offers a snapshot that measures concentration in a given moment, but cannot reveal anything about rates of production/assimilation. It’s certain that an abundance of dissolved inorganic nitrogen was being produced in your tank; the algal bloom couldn’t occur otherwise. Indeed, the reason that your nitrate concentrations were so low were very possibly BECAUSE of aggressive uptake by the algae. Sure, the algae were simultaneously sequestering phosphate as well, but there apparently was an excess in input (perhaps from foods). Without more info, though, I find it a lot harder to explain exactly how your phosphate concentration more than tripled soon after ADDING nitrate.
Honestly, you did what I would’ve done (considering the growth of green algae) when you attempted to reduce PO4 without adding NO3. To condense the above long story into something shorter, I suggest aiming for very low nutrient concentrations overall. Nitrogen in particular should be very low (considerably less than 2 ppm, in my humble opinion). I realize that a lot of hobbyists, including experienced coral farmers, might question that. And it’s true that some enjoy success (in terms of coral growth rates) with elevated nutrient levels. But this comes with the price of potentially horrendous nuisance algae blooms. These systems generally require intensive mitigation in the form of UV and/or ozone sterilization, massive algivorous CUCs, etc.
I think something closer to nature is preferable: A situation where nutrients (especially NO3) are maintained at very low concentrations OUTSIDE the coral, but are nevertheless maintained at optimal concentrations WITHIN the coral. Part of this of course is possible due to the efficient N recycling between corals and zooxanthellae. But relatively recent research has shown that the high productivity of coral reefs owes primarily to symbiotic
relationships between corals and nitrogen-fixing bacteria (zooxanthellae cannot fix nitrogen!). We’ve already established why corals cannot thrive in eutrophic environments (overgrowth by algae); nitrogen fixation (especially by rhizobial bacteria (Family Bradyrhizobiaceae))
explains why corals can grow so prolifically in oligotrophic environments.
The bacteria in PNS ProBio (
Rhodopseudomonas palustris) are rhizobia and have indeed been shown to benefit corals in this manner. It’s too simple to say merely that they fix nitrogen and thereby fertilize the coral’s zoothanthellae. In reality, they do more than that: They actively
regulate nitrogen concentrations. This is because they preferentially take up excess nitrogen (whether ammonia, nitrite, nitrate or nitrogenous organic compounds) from the environment. They
only fix nitrogen when nitrogen is in extremely short supply. Only makes sense right? Makes more sense energetically to grab a slice from the pizza that’s already on the table than to go make another slice from scratch.
All this makes these microbes AT LEAST as beneficial in captive systems (where NO3 is much more often in a state of excess) than in nature.
So, why are they mainly doing this inside the coral, but not everywhere in the tank? There are at least three reasons for that, actually. First, they prefer anaerobic environments (and they can carry out nitrogen fixation
only in anaerobic conditions). Sure, this can be deep in the substrate, within passive flow biomedia, etc., but also deep within the coral’s gut and mucus. Second, they prefer an organic source of carbon (which is plentiful with the coral’s gut and mucus). Third, while they can survive in the dark, they are photosynthetic and prefer strongly photic conditions (and zooxanthellate corals are always exposed to intense sunlight). For these reasons, these microbes may be hundreds of times
more abundant in the coral’s tissues than in the surrounding water column.
This brings us to the dinos… maybe. There are many, very different, species of dinoflagellates that can grow in aquaria. Some can be particularly frustrating to eliminate because they’re either heterotrophic or mixotrophic. In other words, you can control NO3 all you want, but they’ll still grow so long as there is some form of nitrogenous organic matter present (I’ve seen this with hobbyists that feed heavily, especially those that don’t strain out the juice from frozen foods, or overdosing aminos).
I can think of at least a couple cases where people have reported a reduction of dino growth after using PNS ProBio. HOWEVER, it’s difficult to ascribe causality in these cases. In at least one of these cases, the individual simultaneously tried multiple methods of control. I’d say that IF this bacterium helps to control dinos, it most plausibly is through competition for organic carbon as well as for nutrients. One more thing--dinos do often live in close association with nitrogen fixers; however, dinos are obligately aerobic, and therefore are far more likely to live closely to aerobic nitrogen fixers (i.e. cyanobacteria) than to anaerobes such as
R. palustris.
Though I didn’t necessarily give conclusive answers to all of your questions, I hope that I’ve given you a little more insight into what MIGHT have happened! But I wrote a lot of stuff here in a short amount of time and certainly might have forgotten some points or not have explained some things as well as I could have. So please let me know if any of this requires elaboration or clarification!
In terms of dosage, I’d just use as directed (per directions on the bottle). But I’d also suggest
adding the doses directly to your fish foods and especially to any coral foods you might use.
Also, please stay in touch and update!
My tank has been running for about 6 years now, and I have always had trouble with nitrates running in the 20's even though I have a pretty good skimmer, dose vinegar, and have a reverse-lit refugium with Ulva (used to be Chaeto, but the Ulva outcompeted it). My phosphates have always run between 0.100 and 0.200 unless I'm super consistent about dosing lanthanum chloride.
