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RE: CO2 in Low Light tanks

> Date: Wed, 13 Feb 2002 03:17:11 -0800 (PST)
> From: Scott Hieber

> Naomi Mizumoto said, among other things:

>> ...I suppose if extra CO2 is not going to hurt
>> anything, why not? I was worried that it might
>> have a similar effect as over-fertilizing.

> Tom, didn't you point out recently that added CO2
> can be good in a (relatively) low light tank?
> Question: if the added CO2 helps to bring the tank
> to a NO3 or PO4 limited (depleted?) state, would
> that be inviting algae or worse, the dreaded BGA?
> I don't mean that one (CO2 in low light) will
> always cause the other (algae or BGA)...

Although this question is earmarked for Tom, I thought to add a couple of
comments since I'm the one that keeps touting the carbon:nitrogen ratio.

Adding CO2 to a light-limited tank doesn't necessarily (even generally)
indicate a _need_ for extra fertilization - light still acts as the primary
"throttle" for metabolism. However, ensuring a sufficient amount of carbon
definitely affects the _efficiency_ of nutrient utilization. This, in turn,
affects the growth _pattern_ of the plant itself.

The glucose produced through the action of chlorophyll serves two basic
functions - the conversion of light energy to the potential of stored
chemical energy for future use, and as the basic building block for tissue
generation. The plant uses primarily nitrogen, sulfur and glucose to
synthesize its proteins and tissues, and the balance of glucose to the other
essentials eventually determines the plant's overall "energy level".

When glucose production starts to lag behind nutrient absorption, the result
is that the available sugars are almost entirely dedicated to tissue
generation in support of vegetative growth. Very little, if any (situation
dependent, of course) is left over for some of the other "niceties" of
growth. One of these "niceties" is the structure of the generated cell,
particularly the cell walls.

Outside of obvious exceptions - "tuber"-type foods such as potatoes and
beets, "bulb"-type plants such as Aponogeton or Barclaya - there isn't a
wide range of specialization when it comes to energy storage in a plant.
Most of it occurs as more complex sugars such as starch, which is then used
to line the cell walls and serves a secondary function of strengthening
those structures.

All this changes when the plant shifts from vegetative growth to
reproductive. Phosphorus then becomes a primary nutrient over nitrogen as
the plant ceases nearly all functional vegetative growth and starts
concentrating on storing energy for its future progeny - fruit and seed
production. The increased need for phosphorus is normally accounted for in
the _type_ of energy being stored for use by the offspring - think of the
mechanics of ATP energy to understand why and you'll realize it's an easier
route than simpler sugar storage.

Agricultural research points toward all of this as being an evolved,
adaptive response to nutrient availability during a "typical" temperate
growing season. The decay of Winter and the release of nutrients is
finalized through the storms of Spring to distribute those nutrients
throughout the surrounding environment. Summer becomes the primary growth
season, which also means an accompanying depletion in available nitrogen.
Autumn's shift in climate, as well as the frequent heavy rains of late
summer and early fall that wash away most of the remaining nutrition, means
having to prepare for the next "cycle" of Life as quickly as possible. The
return of Winter then ensures a redistribution of the nutritional wealth and
the continuation of the cycle.

The overall effect is that nitrogen availability becomes a *secondary*
"throttle" to vegetative growth while carbon affects the ability to
_support_ that growth. As with anything else, if both are available in
balance, then growth is healthy and "normal". The effects of shifting this
balance are dramatic enough to be readily discernable through even casual

Farmers are keenly aware of the benefits of controlling nitrogen
availability, and for the most part no less aware of the importance of
timing in its application. Even "lawn junkies" who strive for that _perfect_
suburban "look" can tell you what happens when you lay down a nitrogen- rich
fertilizer *too late* into the Autumn.

Now that you have a little background info, ponder on this:

Most of the plants we keep in an aquarium have adapted to life where the
water column's nitrogen content in ppm can be counted on the fingers of a
single hand. How many do we try to grow in an environment where you'd need
_both_ hands *and* _both feet_ to begin to count those same ppm? And don't
forget that the type of tank most likely to encounter those levels aren't
those with stadium lights hanging over them.

It wasn't until we moved indoors to greenhouses that we were able to affect
the C:N ratio to the carbon side with any predictable efficiency. Just as
with our tanks, the "optimal" method turned out to be simple manipulation of
the carbon dioxide concentrations in relation to the other growth factors.
Now, some 25 years or so after I'm reading all of this out of some 1941
edition of a USDA "handbook" and gaining some practical experience with a
mix of hydroponics thrown in, research along the same lines is beginning to
duplicate the results in a more natural "lab" as the ambient atmospheric
level is on the rise  ;-) ...

In a nutshell, the best way to describe the differences in growth as the C:N
ratio becomes more skewed towards N is to picture a plant in phototropic
response patterns (but only as far as _growth_ is concerned - not health or
coloration in general). Without light, the cell walls elongated to produce
quick, "spindly" growth in an effort to "find" the light. Growth energy
comes from stored reserves in the absence of light and sugar production, and
if the plant doesn't reach light quickly enough it will essentially starve
itself to death.

Plants on a nitrogen "rush" will exhibit a lot of the same symptoms. Growth
is accelerated through elongation of the cell walls as well as an increased
pace in tissue production, resulting in a thinner, taller, more "spindly"
appearance. If glucose production can't keep pace with nitrogen consumption,
then the plant will dip into and strip itself of any reserve supplies. This
is most usually the supporting starches (IIRC, primarily pectin) lining the
cell walls, which serves basically to reduce structural integrity and
"sturdiness" of the plant as a whole.

Having an excess of glucose to nitrogen consumption is like giving the plant
a "collagen treatment". The end result is a plant that's shorter, stockier,
sturdier, more rich in color saturation with heavier, "fleshier" leaves.

So, to finally answer the opening question, the likelihood of your scenario
is pretty slim. The two primary "throttles" are already set - light and
nitrogen. Carbon will have a lesser effect on the speed of the growth than
it will its overall health and appearance...


David A. Youngker
nestor10 at mindspring_com