Redfield Ratio Revisited – What are we doing wrong?

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Paulo Mallard Scaldaferri, João Carlos Basso, Marcos Augusto Bizeto, Miguel Mies, Roberto Denadai, Junio Melo

Introduction

Possible applications of the Redfield ratio in the management of the marine aquarium have been discussed over the year. Many have considered that knowledge very helpful while others consider it completely dismissible and useless to the reef keeping hobby. Our objective with this systematic literature review was to obtain the best information that could help clear this concept in terms of aquarium reef keeping.

It has been stated in the past that the well-known 106: 16: 1 carbon : nitrogen : phosphorus ratio should be aimed in order to keep a healthy aquarium. It has been suggested that it would prevent nuisance algae and cyanobacteria by many respected authors like Julian Sprung (1) and João Carlos Basso (2) among others (3). However, many hobbyists have questioned the actual usefulness of these predictions and possible corrections.

Origins

In 1934 Alfred Redfield published a widely known study (4) comparing the rates of oceanic organic compounds collected on the path of the "Dana" ship, between 1928 and 1929. Samples were obtained on the surface, and at depths of 700 and 1500 meters. The original publication identified the stoichiometric N : P ratio of 20:1, offering a light on a possible balance between these components in the ocean. In 1958, the same author (5) based on new published data recognized that a new C: N: P ratio of 106:16:1 was obtained and these are the numbers often referenced in our hobby.

A more recent and larger study (6) was made with extensive data collection (5336 distinct oceanic points). It showed a significant variation in ratios among different sites and revealed a more accurate global average for this relationship: 163:22:1. There was found a significant variation of rations between different sites.

The marine aquarium community has been using information from the original study to keep a healthier environment within the tanks. However, applicability of the original study values is very questionable since it was carried out in ocean waters.

The new interpretations of the N : P ratio

Every day more and more nitrogen-fixing organisms are studied and different N : P gradients have been identified in the nature and tested in laboratory (7). Different species benefit differently from various N : P ratios. Mathematical models have been developed to predict the prevalence of microorganisms where limiting factors are nitrogen, phosphorus or both (8).

A large number of studies include analysis of the carbon component of this equation. In order to simplify our focus, I will report more frequently the data specifically on the N : P ratio. These are the most available and practical parameters to the marine hobbyist (we usually use nitrates and phosphate tests, which are quite representative of the N: P ratio in the water column).

Could a ratio of nutrients really promote or limit the prevalence of different microorganisms in marine aquariums? Apparently yes, but it is not as simple as we previously thought.

Let's look at the biochemical basis for the different demand and composition of N : P (8). The largest pool of N is present in proteins and nucleic acids. It is also present in chlorophylls a, b, c and amino acids, and appears in diatoms chitin. In contrast, nucleic acids and phospholipids are the largest pool of P and it is less present in proteins. The main macromolecule that makes up for the cellular content in phosphate is the RNA.

But it is even a little more complex than that. Cyanobacteria have a high N: P cell ratio (> 25) comparing to most other eukaryotic microorganisms due to their significantly bulky light uptake apparatus (9). In contrast, other genera of eukaryotic phytoplankton such as green algae, diatoms and dinoflagellates have a higher content of phosphate. Evaluating just the cellular content of cyanobacteria, a high demand for nitrogen would be expected, but studies have shown exactly the opposite. Through repeated observation, it was hypothesized that cyanobacteria could supplement N deficiency with diazotrophic N2 fixation capacity (10). This N2 fixation capability also offers a better explanation for the competitive advantage of cyanobacteria over other microorganisms in nitrogen-deprived environments.

Relationships between N: P ratios and cyanobacteria

The unbalanced proliferation of cyanobacteria in estuarine and marine waters have been related to environments rich in nutrients, but lacking the N fraction of this equation. Smith has already verified in his studies that the N : P ratio below 29 favored the proliferation of cyanobacteria, which were more efficient in fixing N (11, 12).

Cyanobacteria are ubiquitous in marine aquariums, but their excessive proliferation is considered very problematic. Usually, some species are more prevalent in aquariums such as Oscillatoria , Lyngbya and Phormidium (3, 13, 14). Like other marine planktonic microorganisms, after an accelerated growth, cyanobacteria also develop a competitive advantage through secretion of allelopathic substances. These substances can inhibit the growth of other algae and cyanobacteria (15), even evidencing antimicrobial properties. This observation may partially explain the success of cyano treatment with macrolides, such as azithromycin or erythromycin. Other drugs from this group (macrolides) have already been isolated from different cyanobacteria (16). Could their mechanism of action have any similarity to allelopathy? Although it is interesting, we have not identified specific studies to verify the veracity of this hypothesis.

The initial imbalance that leads to accelerated cyano growth seems to be influenced by the N: P ratio. Ahlgren tested this hypothesis in closed environments, which we could call small aquariums (0.5L glass tubes). The researcher evaluated the proliferation of Oscillatoria agardhii in environments with limited nitrogen or phosphorus (17). He observed that in the studied species phosphorus depletion was a greater growth limit factor than nitrogen, evidencing a competitive advantage in nitrogen deprived environments. Levich (18) also studied the proliferation of cyanobacteria in detriment of other microorganisms with various N : P ratios and found very interesting balances: above 20 : 1 green algae was favored; on the other hand, cyano grew more rapidly in ratios below 5 : 1.

Implications of N : P ratios in carbon dosing

Carbon dosing is a widely used technique by marine aquarists for nutrient export and nitrate and phosphate reduction. Several carbon sources have already been tested such as sugar, vodka and vinegar. Principles of carbon dosing also seem to be explained with C : N : P ratio knowledge. Addition of carbon provides the limiting growth factor for the aquarium's bacterial population. Through this incorporation, bacteria also consume nitrate (in greater proportion) and phosphate, being later exported by the skimmer.

We also identified some studies (19) that investigated the C : N : P in these bacteria and found a ratio of 50 : 10 : 1. Exponential growth of marine bacteria in vitro was achieved when the ratio reached 32 : 6.4 : 1 in nitrogen rich and 45 : 7.4 : 1 in nitrogen poor environments.

In other words, we could expect in aquariums that the maximum export efficiency would theoretically occur in a ratio of 7 : 1 (N : P) and the expected theoretical result of consumption would occur in a ratio of 10 : 1 (N : P). This proportion was not the same in every single study, but it is closer to reality than the original Redfield (20).

Once nitrogen or phosphorus is eliminated, normally the increase in carbon supply will not remove the other residual element. Sometimes, over-dosing may stimulate the development of cyanobacteria, which can be well understood by the mechanism already mentioned: after removing the nitrogen source, cyanobacteria and others capable of fixing gaseous nitrogen (N2) could be stimulated.

Final considerations

We know that marine aquariums are closed environments with very diverse micro and macro fauna, and interpretation of this closed environment through studies carried out in nature is extremely complex. Alfred Redfield's initial studies were conducted in open waters, far away from coral reefs, so we discourage that the original Redfield ratio be taken as a rule for marine hobbysts. However, we identified the importance of his initial studies of C: N: P relationships, which were followed by the identification of new ratios in different environments.

However, many studies have demonstrated that different species usually preponderate under specific conditions. So we can check that the predominance of certain species with accelerated growth in marine aquariums seems to be influenced by the C : N : P ratio.

Higher N: P ratios (above 20 : 1) seem to favor green algae, while lower values (below 5 : 1) favor the growth of cyanobacteria. In situations where there is a critical limit in nutrients supply (a large reduction of both N and P), this interpretation seems to have less predictive value.

We must emphasize the limitations of these information and implications. The well-known imprecision of the tests that we usually use in the hobby demands that critical judgment should be used above all. Redfield did not predict with his studies events that occur in aquariums, neither it was his intention, but other studies already published seem to increasingly offer data that can help us manage marine aquariums. We believe that dissemination of this data may provide new horizons on nitrogen, phosphorus and carbon dynamics in aquariums.
 
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Bibliography

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