Scientists are obsessed with this lake - Nicola Storelli and Daniele Zanzi
Roughly 3 billion years ago,
a single-celled photosynthetic bacterium
began burping a new chemical
that was poisonous
to nearly every species on Earth.
Over the following hundreds
of millions of years,
more microorganisms began producing
this toxic gas,
first saturating Earth's oceans
and eventually its atmosphere.
Up here, this chemical changed
the composition of pre-existing gases
so drastically that it caused
a global ice age.
And the name of this powerful, poisonous,
world-changing gas? Oxygen.
In the millions of years since
the Great Oxygen Catastrophe,
most life, including all
multicellular organisms,
have evolved to rely on this gas.
However, there are some places
where oxygen-averse microorganisms,
like those from Earth's earliest days,
have re-emerged.
Many of these places are
in the ocean depths,
beyond the reach of researchers.
But there are other bodies of water
completely devoid of oxygen,
yet close enough to the surface
to explore.
And one such lake is hidden high
in the Swiss Alps’ Piora Valley.
Formed over 10,000 years ago,
Lake Cadagno is one of roughly 200
known meromictic lakes,
meaning it's actually two distinct bodies
of water stacked on top of each other.
The top layer functions like
a standard body of freshwater.
It’s safe for swimming and known mostly
for a plump and plentiful fish population
that’s been the subject
of local fishing legends for centuries.
But just 13 meters beneath that bounty
is a dense, sulfurous, oxygen-free pool
lethal to any multicellular life forms,
fish included.
In a typical lake, the entire
body of water would gradually mix,
diffusing oxygen
from the surface throughout.
But these two layers never mix,
as is the case with any meromictic lake.
And the reason for this divide in Cadagno
is the waters’ unique
chemical compositions.
Both layers are fed by rainwater
flowing down the mountains,
however, this water can take two paths.
The first is to trickle down the granitic
mountain directly into the top layer.
The second is to seep into the
Piora Valley’s vein of dolomite—
a porous rock full of salts
such as sulfate.
Rainwater that sinks into the dolomite
will slowly inch towards the lake,
all the while shedding its oxygen
and picking up salts.
Finally, this heavier water will cascade
from sublacustrine springs
below the lake’s surface,
forming the dense,
salt-rich bottom layer.
This lower layer is anoxic,
meaning oxygen free,
and will suffocate any
oxygen-dependent life.
But it’s ideal for the kind
of anaerobic bacteria that died off
in the Great Oxygen Catastrophe.
The flow from the sublacustrine springs
creates microenvironments
which feed large aggregates
of microorganisms
that emerge from the lakebed
in strange and otherworldly shapes.
Various anaerobic microorganisms
take in the water's sulfate
and emit toxic sulfide.
And at the border of these layers,
there’s a thin blanket mainly composed
of pink-bodied Chromatium okenii:
a photosynthesizing bacterium
that relies on this sulfur
the way most plants rely on oxygen.
However, while neither water nor organisms
move between the layers,
these ecosystems aren’t
completely out of touch.
Chromatium okenii live
at the top of the bottom layer
because they need to be as close
to the sun as possible.
But while they never cross
into the deadly oxygenated waters,
they’re close enough that organisms
like zooplankton can dive down,
eat them, and get back up.
In fact, this relationship forms
the bottom of the upper layer’s
robust food chain
supporting the lake’s
legendary fish population.
This unique ecology is more than
just a boon for Cadagno’s fishermen.
Having access to an isolated
anaerobic ecosystem
allows scientists to model the world
before the Great Oxygen Catastrophe.
For example, when Chromatium okenii
form their blanket,
they increase the density
of that thin layer of water.
As the water sinks, these microorganisms
are forced to swim back up,
creating a minute mixing of water
called bioconvection.
This billions of years old phenomenon
might be a clue
to how early life evolved
the ability to swim.
And it’s just one of the many insights
that await researchers
studying Cadagno’s mysterious depths.
