Nutrient uptake rates of algae vs plants

[Thomas Barr <tcbiii at earthlink_net>]

This is not for newbie, but there's no test afterwards either:-)
It's heavy on the physiology but is a much better explanation of the
observations than this nutrient limited/competition business everyone seems
to claim happens all the time. Seldom are things that simple and it's also
wrong. If I add PO4 I would get algae under that theory's assumption but we
simply do not see this happen.

http://www.picturetrail.com/gallery/view?p=999&gid=776024&uid=536313&members
=1

Click on the graph.
V= nutrient uptake rate(i.e. pmol/cell/h)
Vmax= the maximum possible uptake rate
Ks is the half saturation constant(value of S where V= 1/2 Vmax)
S is the substrate concentration.

Ks is basically how hungry the enzyme that grabs a nutrient is. Oligotrophic
algae are very very hungry. Plants are not so hungry but will eat more at
higher nutrient concentrations. If the nutrient concentration is below or at
where the two lines meet, algae will bug you. Higher and the plants will win
out. This graph assumes both are in the water column, none of this substrate
stuff for extra plant nutrients via roots etc.

If you continue adding more and more nutrients, at some point this whole
system becomes unstable. Vmax is only so high for the uptake of the plants.

This explains _some_ of what we see in our tanks but not everything.
Photo respiration is another area that finds some interesting differences
between algae and plants.

Algae normally do not photorespire when growing(they use -HCO3 commonly). If
you add CO2 to the system in the 20ppm range they do photorespire.
As they photorespire, it cuts down on their ability to fix carbon ( O2 also
effects Rubisco => can lead to a 20-40% decrease in production). This is a
very large factor. Plants also do this but have much more regulative
abilities to control free radical build up. C4 plants(corn, sugar cane etc)
don't do photorespiration (they/some do but it's extremely small). At low
CO2 levels, they win out over a C3 plant.
As the CO2 levels rise on the earth I guess we'll be eating sugar from beets
instead of sugar cane at some point:-)

Bicarbonate plants use a similar C4 plant process where they concentrate the
CO2 by producing acid on the lower sides of the leaf, and this works
extremely well when there is high light and NO3. This is accompanied by a
"polar" leaf since the upper side has all the high extra cellular pH and you
get that marl formation on leaves such as Potamgetons.
The anatomy  of these leaves is also different than non HCO3 leaves.

Algae use the same HCO3 process to fix carbon and generally have very high
Rubsico efficiency since there's no O2 to interfere with Rubisco. But when
CO2 is added this pushes algae too far and while they will grow, the high O2
environment is very hostile to them and their large surface to volume area
also takes in all that external O2. Plants have a cuticle and are somewhat
better able to fend off this problem. Algae also are more prone to leaking
of photosynthate (up to 50% in diatoms) at high production rates than most
plants. Some comparative studies between these two groups would be
interesting.  

I'd had the idea to add O2 to tank to see if this works or not but still
have not gotten around to it. Adding H2O2 is not the same and has not
produced quite what I think I would see.

It certainly fits very well with the observations of great plant growth =
bad place for algae even if the nutrients are available to both the plant
and the algae.

The other theory simply fails. Leibig's law is being misapplied in this
case. Algae are not being limited, the plants are. You can try partitioning
the nutrients in the gravel but this only works at lower lighting levels and
is also limited by plant transport via the roots. Many aquatic plants don't
have roots(i.e. Riccia) or use them much for substrate uptake.
But this same theory I am suggesting also works the same at low lighting
also and with better results than the "limited" approach and with better
reduced algae results also. I've done this and was surprised at just how
well it worked. 

Uptake rates were slower, but you could maintain the high levels that occur
in high light tanks, you just don't need to dose as often (Weekly instead of
every 2-3 days etc).
    
These are the four good difference models to consider:

Nutrient uptake rates
Photo respiration
Leakage rates
Surface to volume ratio

There's likely more but I'm not going to go into it yet.

Oh well, enough rambling for now. I'll develop this further when I get a
chance. I'll put this in a book at some point
Regards, 
Tom Barr