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RE: salt v. fresh, substrate

> > > Charley Bay <Charleyb at Cytomation_com> wrote:
> > I wanted to add one more thing, though:  There's a TON of stuff
> > > suspended in marine systems (many kinds of salts, calcium, lots
> > > of other compounds).  This makes the water more viscous <snip>
> > and much easier to support life <snip>

> > Carmen Robinett <ccr67 at uclink4_berkeley.edu> responded:
> > I was a little confused by your comments on the experience of saltwater
> > organisms.  I'm not sure how viscous (or salty?) water is better able to
> > support life?  This doesn't seem to make sense in my mind.
I was mostly going on the thought that we have more of
life's building blocks suspended in marine systems, so
it's easier to maintain a diversity of species.  Further,
the compounds are closer to creating an isosmotic 
environment which favors animals over plants (see below).

Since we are talking substrate critters, they may not be 
advanced swimmers but "scurriers", and it may be easier 
to get around in a more viscous solution because they
can be suspended for miles and months for migration and
speciation.  (I don't know if I'm full of cr*p on that or not.)

> > Carmen continues:
> > Freshwater animals have more salts (of various flavors) inside them--not
> > quite impurities--than their water environment.  For bony fish, their
> gill
> > tissue is a site of osmotic water gain, and their body fluids are
> > constantly under the threat of overdilution.  <snip, physiological
> compensation to address this problem.>
> > But living in saltwater doesn't guarantee an easy time of it.  <snip>>
> Also "Marine bony fishes
> > constantly lose water [through their gills] by osmosis to their
> > hyperosmotic surroundings.  They compensate by drinking large amounts of
> > seawater and using the epithelium of their gills to pump the excess salt
> > out of the body."  They also put out very little urine.  <snip>
Yes, if environmental concentrations are not identical to
internal concentrations, then some sort of osmoregulation
is necessary to maintain an internal solute concentration 
at an acceptable level.  However, for critters, a marine 
system is a much closer starting point than a freshwater 

> > As for the notion that the marine tanks have more substrate critters,
> seems
> > to me like alot of the organisms (or eggs and larval forms) may be
> brought
> > in on live corals and anemones...?  <snip> 
Yes, but my basis is that there simply isn't the same
number of species to draw from.  If a typical freshwater
river bank has thousands of species of substrate
critters, a typical reef should have millions or tens of
millions of substrate critter species.

> > <snip, Charley thinks more animal diversity in marine
> > systems because of suspended salts and stuff>

> > > I've given this issue a lot of thought over the years, and I've
> concluded
> > > that we CAN have things called "freshwater reefs", but they can't be
> > > a flora/fauna match with marine reef systems.  In freshwater, we
> > > are greatly blessed with plant diversity and greatly penalized with
> > > substrate-bound animals.  (For physiological reasons we can move
> > > into next, plants typically can't handle salts and most animals tend
> > > to require salts.)
"Here's the 'next' that I referred to," Charley states, as he 
nervously looks around the room.  He is well aware that 
the audible groans he hears relate to these good people
recalling his long posts.  "I'll keep it short..." he quickly
adds in his nervous tone.

Quick terms:

When comparing two solutions we have:

  hyperosmotic - the solution has a greater solute concentration
  (Atlantic Ocean is hyperosmotic to Lake Michigan)

  hypoosmotic - the solution has a lesser solute concentration
  (Lake Michigan is hypoosmotic to the Atlantic Ocean)

  isosmotic - the solutions are equal in solute concentration
  (the left side of the tank is probably isosmotic to the right side.)

Recall:  Animal cells do not have cell walls, and plant cells do.

Animal cells are more viable in an isosmotic environment,
unless they employ some type of special adaptation to
regulate the uptake or loss of water across the cell 
membrane.   If you are fighting the gradient, you're going
to pay.  Let's talk "active transport".

  active transport - moving a molecule against its
gradient across a membrane in a process that 
requires expending of metabolic energy.

A common example in many biology texts is the
sodium-potassium pump, and there have been many
discussions on this list over how ATP is used as
metabolic energy in many of these reactions.

We simply won't have the species diversity in our
freshwater systems because we don't have all those
nice building blocks (suspended solutes) floating 
around, and the gooey guts of those insect-like 
substrate critters have an easier time in marine 
systems maintaining an isosmotic balance with 
their environment.  Rather than expending energy 
maintaining equilibrium, marine critters can spend 
more energy growing and reproducing (and evolving, 
and learning to read and write, and making democratic 
decisions, and entertaining us.)

To keep this post plant-related, I'd like to mention that
plant cells (because of their cell walls) are healthiest
in a hypotonic environment because when the cell
is full of water, the wall exerts pressure back into the
cell and water molecules are then able to diffuse in 
and out at a constant rate (the cell is turgid).  If it 
were in an isosmotic solution, the cell would simply 
be flaccid (limp), at equilibrium, at whatever dimension 
the cell wall suggested.

Most importantly, a cell wall is no advantage if
the cell is placed in a hyperosmotic environment.
Like an animal cell, the water molecule would be
exported and the cell would shrink.  However, as
it shrivels, the plant cell's plama membrane 
pulls away from the cell wall in a process called
"plasmolysis" which is usually lethal.  Bummer.
That's why there are very few (almost no) plants
that are truly marine.  (As an "FYI", fungi and bacteria 
also plasmolyze in hyperosmotic environments).

If you want to add salts to your very soft freshwater
system to enhance plant growth and fish health, 
you would ideally do this to the point where the 
plant cells remain turgid with a maximum number 
of suspended ions in the water.  Theoretically, you
could do volume experimentation with plant mass
to determine this amount.  When you start to
decrease plant volume, you've added too much
(this will be plant species-specific.)

charleyb at cytomation_com