As part of this blog, we have talked about some pretty small
life. Wolffia globosa is the smallest flowering plant, only 0.6 mm long. We also talked about archaea, a different
kingdom than bacteria, but still on the smallish side of life. The tardigrade is the toughest animal, but is also one of the smallest, at 100 µm (0.00394 inch).
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The
organism on the top is T. dieteri,
and arthropod, just
as
is any crab or spider. The size is deceiving. The pictures
on
the bottom are to scale and are copepods, also
arthropods.
The organism on the top is a parasite of the
organisms
on the bottom. The small blue line? That would
be
the scaled size of T. dieteri. So…. it’s
SMALL.
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Here’s one thing we should be able to agree on, viruses
don’t get to play in our game. Viruses are very small, but they're not life!
We’ve talked about this before - the seven characteristics of life (see this post). Viruses need a host in order to replicate, they don’t manage
homeostasis, and they aren’t cells, so they aren’t life.
So how small has actual life become? Let’s assume that since
tardigrades and T. dieteri are over
50 µm, huge when compared to some bacteria, our current minimum for life is
probably a bacterium or archaea.
Let’s go straight to the genus of smallest bacteria we know
about – the mycoplasma (from mykes = fungus, and plasma = formed). They were first described in 1898, but the
observer didn’t have a clue what he was looking at; hence the fungal part of
the name.
Mycoplasma don’t have the traditional cell wall of many
bacteria, so they look different and this might be why they were mistaken for
fungi. Whatever the scientists thought of them, they were confusing enough to
be roundly ignored for 50 years. Rediscovered in the 1950’s-1960’s, this time
they were thought to be L-forms of bacteria. L-forms are organisms that for
some reason have lost their cell wall.
There are stable forms of L-bacteria; they can live divide
and live on without their cell wall. There are also unstable L-forms as well; those
that may revert to walled bacteria at any moment. Are mycoplasma simply
bacteria that have lost a cell wall? Nope. They didn’t have a cell wall to
lose. They have no cell wall genes, so if they had a cell wall, it was millions
of years ago, before they became their own genus.
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The
difference between some free living cells. You can
probably
see the E. coli in bright green, but
you may have
to
squint to see the mycoplasma above it. It’s pink. Really,
it’s
there. Compare these sizes to those of the arthropods
above.
1 mm is equal to 1000 µm.
Mycoplasma
is really, really, SMALLLLLLL.
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So mycoplasma are small, but they still have to play by the
rules. They contain DNA and salts and proteins and ribosomes and other things
that take up room. A single ribosome is about 50 nm in diameter (0.05 µm or
0.00000005 m), so there must be a certain volume required for the cell to
function – a minimum size for life.
Which of the mycoplasma species is the smallest? Mycoplasma genitalium is considered to
be the smallest mycoplasma known, and the smallest form of free-living organism -
my gosh – you can fit about 400 M.
pneumoniae inside one E. coli! As
such, it is the current minimum size for life that we have. M. genitalium is 200 nm (0.2 µm) x 600
nm (0.6 µm), so they’re pretty dawg on small. Let’s put it this way, there are
25,400,000 nm in one inch – mucho small.
It is important to note that M. genitalium is free living, but does need some help. It uses
cholesterol in its membrane but doesn’t make it itself. It picks it up from
the cells that it lives near……wait for it….. your genital epithelium.
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One
of the human diseases that is becoming more
convincingly
associated with M. genitalium is
pelvic
inflammatory
disease (PID). Resulting when many
different
sexually transmitted diseases go untreated,
PID
can cause permanent damage to the reproductive
organs
of women. It is important to get treatment early.
The
inflammation of PID may be associated with the
fallopian
tubes or ovary, and will cause a chronic pain
in
the lower abdomen, bleeding and pain on urination.
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The idea of free-living is important because M. genitalium also has a very small genome (amount of DNA in one cell,
including the list of all its genes). M.
genitalium has about 580 kbp of DNA where kbp = kilobase pairs. Remember
that DNA is doubled stranded (usually) so each base is paired with another.
Knowing this, we count them as a unit. In all, M. genitalium has just 520 or so genes; it can make about that
many proteins.
Genome size could be another way of determining the minimum
size of life - what's the minimum number of genes or number of base pairs of
DNA for an organism to still meet all seven characteristics of life? As of
summer 2014, no organism smaller in size than M.
genitalium has been described, but there have been some other organisms
discovered with smaller genomes.
Nanoarchaeum equitans
was thought to have the smallest gene for a while, with only 491 kbp of DNA. It
is an archaea that lives on the edge
of hydrothermal vents at the bottom of the ocean. But it is an obligate symbiont with another archaea; it can’t
survive without its partner, so can you say it has the minimal genome? It
relies on another organism’s DNA.
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On
the left is the leafhopper in which N.
deltocepahlinicola makes
his
home. Well inside its cells that is. The leafhopper survives on
phloem
and xylem; high in carbs but little protein. The bacterium
makes
the amino acids the leafhopper can’t in exchange for energy
in
the form of ATP. On the right is a colored photomicrograph of
the
abdomen. The red is one type of endosymbiont bacteria,
the
green is N. deltocephalinicola.
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So why is it important to find the minimal size and minimal
genome for life? So we can use the information. J. Craig Venter (of the
human genome project) wanted to develop a synthetic form of life (synthetic biology); a bacterium that
could be developed to provide hydrogen for energy or eat waste to reduce
pollution. Others say we need to know so that we can better recognize life on
other planets, or life that may have come here from other planets (astrobiology).
Being J. Craig Venter, develop a synthetic form of life is
exactly what he and his research institute did. It’s interesting that Venter
was one of the scientists that first sequenced the entire M. genitalium genome in 1995. Some 15 years later, Venter’s JCVI-syn1.0 (2010) was the first synthetic life, housing 1000
kbp and 500 or so genes. The genome was based on that of another mycoplasma, M. mycoides. They modified the genome,
and introduced it into a cell membrane that had been evacuated of all its
constituents. The resulting cell was capable of growing, dividing, you know….
living.
If M. genitalium represents
our current estimate for the minimum size of life, it’s only because we’re
thinking of life as we know it. Perhaps
we have already found life that is smaller, and the minimum size is actually much
smaller than M. genitalium.
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This
is a photomicrograph of a meteorite from Mars. The
small
spheres (like the ones the arrows point to, are
supposedly
nanobacteria. Proof of life on Mars,
contamination
from Earth nanobacteria, or just mineral
spheres
that look a little like incredibly tiny bacteria?
The
answer is C.
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But nanobes/nanobacteria have their proponents. Some scientists say that
since no DNA has been exhibited, they are a completely different form of life,
so size restriction (big enough to hold ribosomes) doesn’t apply. Nanobacteria
are also claimed to be important in human disease, as these structures are
found in many calcifications of diseased tissues.
On the other hand, nanobacteria are probably just mineral
formations. A 2013 study showed that they form spontaneously from many
different biological fluid samples, and their appearance in diseased tissues is
more a sign of disease than a cause of it. We’ll just have to keep looking for
something smaller.
Next week, another question tackled and dissected - think pink.
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