All living things are composed of cells. These require certain chemicals like
glucose and oxygen to function but also produce byproduct chemicals that need to be removed.
In simple cells like bacteria, they rely on passive diffusion to get the chemicals in and
out. This relies on there being a difference in the concentration outside the cell compared
to inside to drive this process but requires no additional work to be done by the cell itself.
This works extremely well for small cells but if we were to make the cell larger and larger
the chemicals will take too long to get into the centre of the cell and will end up being
far too diluted when it does. This puts a physical limit on how large a cell can be.
Generally large and more complex cells, like human cells, overcome this by using elaborate transport
systems that shuttle nutrients to where they are needed but these require energy to operate.
These types of systems don't exist in bacteria and this was thought to put a size limit on bacteria
but a new finding seems to break all these rules. Let's dive in and find out more...
Bacteria are very small cells and you cannot see them with the naked eye.
It normally requires a microscope to see them but this new bacteria
is so large that it is easily seen with the naked eye. Until recently only a
handful of scientists knew it existed and they had no idea it was a bacterium.
A marine biologist at the University of the French Antilles noticed centimetre-long white
strands attached to submerged mangrove leaves. At the time it was suspected to be fungi of an
unrecognised species as only about 7% of fungi have been characterised. So they took samples and
returned to their lab for further analysis. Early experiments suggest the organism was not fungi
but instead a type of bacteria some 50 times larger than the biggest bacterium ever discovered.
This is too large for it to rely on passive diffusion and yet bacteria should not have
active diffusion. The massive microbe has evolved a rather unique solution to the problem
which fall between both of these approaches.
It contains a fluid-filled sac, which squashes the intracellular content
up against its outer cell wall. This means that chemicals have a much smaller distance to diffuse
as they are limited to the edges of the bacterium.
This is not the only unusual feature it possesses and breaks another big rule. They seem to have
something resembling a nucleus. This was thought to be THE defining structure of eukaryotic cells
like animal cells. The nucleus is what separates simpler prokaryotic cells from the more complex
ones. Somehow these big bacteria seem to have evolved into a very strange direction
and calls into question whether the current tree of life is wrong.
The tree of life depicts our understanding of evolutionary relationships right from the
start and has undergone radical changes in the past. In 1977 the Monera Kingdom fell.
It had only been added 50 years prior to this and consisted of bacteria and similar microbes.
At the time it was the most populated kingdom that had ever existed. The downfall
is often attributed to Carl Woese, who was a microbiologist and biophysicist. Using genetic
analysis he discovered that the borders of the Monera Kingdom were drawn incorrectly. Instead,
he determined that the former kingdom needed to be separated into two domains, Bacteria and Archaea.
This new finding is hinting that maybe we are on the verge of another crumbling empire.
So this raises the question about how well we know and understand the tree of life?
A recent discovery of ancient insects in amber dated to the late Cretaceous
highlights this problem. A piece of amber contained fossilised larvae of ancient lacewing.
Lacewing are part of a superorder of insects in which the insect undergoes holometabolism. This
is a complete metamorphosis, as its body develops and transitions between each life stage.
The success of these types of insects has been attributed to the niche differentiation between
the adult and the often highly aberrant-appearing larva. This has caused there to be a persistent
idea in biological circles that the larvae, the stage before they reach the adult stage,
of these types of insects are necessarily underdeveloped, worm-like things, having not
yet had a chance to grow the pronounced features such as eyes, antennae, mouthparts, and legs.
Some even suggest that they are something like early hatched embryos
and that this is why they possess a type of 'net-yet-ready' morphology.
And yet in these amber deposits, they identified a number of lacewing larvae
that seem to challenge this line of thinking. These showed numerous lacewing specimens
with a variety of elongated antennae, jawbones, limbs, trunks, and necks.
In particular, they showed straight venom-injecting, piercing-sucking
mouthparts that protrude from the jaw, forming what is known as a 'stylet'. These work as a
means of keeping their wounded victims at a distance until the toxins begin to take effect.
Nothing is known about their prey.
At this stage, they can't say for sure how all these body parts might have functioned
but there is enough evidence to challenge the old argument that
creatures like lacewing can only grow such prominent features in adulthood.
The idea that larvae being defined as stubby things with a lack of diverse body parts
reflects scientific ignorance of larval forms, and of how complex they can sometimes become.
So the question then becomes what might have caused lacewing larvae
to have lost these pointy bits? This loss to some extent would help to explain our
lack of understanding of immature lacewing and similar creatures.
Perhaps it is we who are constrained in our thinking
not the lacewing in their protruding. Absence of evidence is not evidence of absence.