Nutrient Limitation
As previously mentioned, controlling the concentrations of nutrients, and their
precursors, in reef aquaria is extremely important; the long-term overall health of the
system is highly reliant upon it. This entails extracting dissolved and particulate
organic material, as well as phosphate and nitrogenous molecules such as ammonia,
nitrite, and nitrate from the system efficiently and effectively. In the past, aquarists
have utilized protein skimming, activated carbon, and ion-exchange resins in
conjunction with the natural biological and chemical reactions taking place within
sediment and porous substrates to accomplish this goal. The aspect of this method
that makes it “unique†is the utilization of a family of selective zeolites for the dual
purpose of extracting ammonia/ammonium and providing a substrate for the
colonization of beneficial microbes. It follows that an aquarium with efficient
extraction of ammonia before it is utilized in the nitrification process will have a very
limited propensity for nitrate accumulation. Similarly, by extracting dissolved and
particulate organic material from the water before it has a chance to be broken down
via microbial processes and/or photodegradation, the constituents of that material are
not released into the system; this is accomplished through protein skimming and the
use of high-quality activated carbon. The rates of nitrogen and phosphorus input and
the presence of adequate organic carbon largely determine whether or not some
amount of measurable nitrate and/or phosphate persist in an aquarium; nitrate is
removed via denitrification and/or the use of a sulfur reactor, and phosphate is
addressed by utilizing some form of phosphate-adsorption media. The ultimate goal
is to maintain microbial biomass production at such a rate that the concentrations of
nitrate and phosphate never become measurable.
Biological Nutrient Limitation (Microbes and Organic Carbon)
The role that microbes play in a zeolite filtration method is extremely important: they
convert nutrients existing in excessive concentrations into biomass, which is then (in
the case of planktonic bacteria, or “bacterioplanktonâ€Â) consumed by corals and other
suspension-feeding organisms and utilized in biological processes, and/or removed
from the aquarium by protein skimming. The process may be thought of as nutrient
recycling and export, and in that regard it provides some of the same benefits that a
refugium housing macroalgae provides; the main difference is that the nutrients
assimilated into microbial biomass are at least partially available to corals, which is not
the case when it comes to nutrients assimilated into macroalgae tissue. A percentage
of the microbes form biofilms on inanimate objects (often appearing as a brownish
film on the panes of the aquarium), which can be brushed off and captured by
aquarium inhabitants and skimmer intakes, again exporting nutrients from the system;
we will return to the topic of biofilm shortly. Before we move on, it is likely that some
readers will ask the questions, “Why is this means of nutrient export any different than
the natural processes taking place within any cycled aquarium? Microbes are going to
colonize the NeoZeo media regardless of seeding it. What’s the big deal?†The
primary difference between a “traditional†system in which biological filtration media
is set into place and allowed to become passively populated with microbes and a
system in which a select group of microbes is supplemented on an ongoing basis is
that (again) the enthusiast maintains more control over the rate of nutrient export,
and simultaneously encourages the biological processing of various substances
known to negatively impact water clarity and/or the health of the aquarium
inhabitants.
It is here that the discussion turns to carbon limitation and the implications
this has with regards to the ability of microbes to assimilate phosphorus and nitrogen.
Covering this topic requires elementary thinking and relies on the ratios of nutrient
uptake in bacteria being somewhat analogous to that of marine phytoplankton.
Using the molar ratios of 106:16:1 C:N(carbon : nitrogen : phosphorus) in marine
phytoplankton as a model for nutrient uptake in bacteria, it follows that bacteria
require considerable carbon to assimilate nitrogen and (particularly) phosphorus. In a
system with low- to immeasurable-concentrations of nitrate and phosphate, there is
presumably sufficient organic C present to enable bacteria to utilize the N and P that is
available; resultantly, bacterial biomass is regulated by the relative abundance of N
and P. Conversely, in a system with relatively high concentrations of nitrate or
phosphate (e.g. one that tends to be heavily-fed), their uptake by bacteria will be
inhibited by inadequate organic C. The solution is to supplement the system with
organic C, which enables assimilation of the existing N and P into bacterial biomass.
As previously described, the bacteria, and hence the N and P, are then removed from
the system via filtration and/or converted into biomass of suspension-feeding
organisms capturing bacterioplankton and pieces of biofilm that become dislodged
from static surfaces.
Even than, it is actually highly variable between different species of algae. Using it as an assumption for bacteria is...well you know what they say about that word 
