Chapter 30: Microbes in Oceans, Lakes & Freshwater Ecosystems

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Welcome to The Deep Dive, our mission today.

A fast, thorough, and hopefully engaging into the incredible world of marine and freshwater microbial ecology.

We're really going deep into the source material here, trying to understand these, well, invisible engines that control so much of our planet's chemistry and stability.

Okay, let's unpack this.

We are going to start with something pretty stark, actually.

The most dramatic evidence that maybe these microscopy engines are stalling the rise of ocean dead zones.

It's a genuinely terrifying consequence, isn't it?

Global ecology knocked off balance like this.

Absolutely.

These dead zones, or hypoxic zones, technically, are regions where the dissolved oxygen drops so low that most complex life, fish, crabs, that sort of thing, they literally suffocate.

They die off.

What's alarming is that since the 1960s, these areas have been dramatically increasing, both in size and how often they appear around the world.

When we look at the mechanism behind it, it's entirely a microbial process.

It's not like the water just magically runs out of air.

Exactly right.

It's a chain reaction.

It kicks off when surface temperatures rise or you get a big influx of nutrients like nitrogen and phosphorus maybe pouring out from rivers.

This overload feeds a massive bloom of phytoplankton.

These are the tiny photosynthetic microbes at the surface,

primary producers.

Okay, so they bloom like crazy.

Right, but then when that bloom dies off, all that organic material sinks down and that's when the heterotrophic microbes, the decomposers in the lower water column in the sediment, they just go into overdrive.

Consuming all that dead stuff.

And respiration needs oxygen.

Precisely.

So that decomposition process strips the water of its dissolved oxygen very quickly, actually.

Once that oxygen is gone, the environment becomes anoxic and the only things left thriving are these really specialized anaerobic organisms, microbes that can get energy without oxygen, sometimes even fixing CO2 using weird chemistries.

It basically becomes a microbial wasteland for anything that needs to breathe oxygen.

That's a good way to put it, yeah.

The scale of this is just, yeah, well, it's staggering.

Our source material really highlights the dead zone at the mouth of the Mississippi River.

It's this massive area, something like the size of Connecticut and Rhode Island put together.

Wow.

And it's fueled largely by fertilizer runoff from huge swaths of agricultural land.

Think about the economic hit, the ecological hit to the Gulf Coast.

It's immense.

It really is.

But, you know, there is some hope here.

These dead zones aren't necessarily permanent fixtures.

The research shows they can be reversed.

The key strategies involve prevention,

mostly, things like more careful, prudent fertilizer use,

limiting surface runoff, and crucially, investing in restoring natural wetlands.

Wetlands are incredible natural filters.

They pull out a lot of that nitrogen and phosphorus before it even reaches the coast.

So understanding this whole picture means bringing together a couple of key fields, which is what we're doing today.

Yeah.

We need microbial oceanography.

That's the study of the big marine systems, their global impact on biogeochemical cycles.

And we also need limnology that focuses more on the continental aquatic systems, lakes, rivers,

wetlands, which are absolutely vital for terrestrial life, including us.

Okay.

Let's talk about water itself.

As a microbial habitat, I was genuinely amazed to read that the speed oxygen moves through water.

The flux rate is something like 10 ,000 times slower than it moves through air.

Yeah.

That seems like a massive physical constraint on life underwater.

It absolutely is.

But nature, well, it finds ways.

There are fascinating trade -offs.

While oxygen moves slowly, its solubility actually goes up in colder water and under higher pressure.

So counterintuitively, in the very deep ocean, dissolved oxygen can sometimes increase with depth, even though that water hasn't seen the atmosphere in maybe thousands of years.

Huh?

Interesting.

And microbes have adapted their metabolism to really exploit these subtle chemical and physical gradients.

They're masters of chemistry.

And sticking with the ocean, when we talk about the base of the food web primary production, the open ocean is almost totally reliant on the microscopic world, isn't it?

That's a really key takeaway.

Yeah.

I mean, near the coasts or in lakes, you'll see larger plants playing a role.

But out in the vast open ocean, roughly half of all the primary carbon fixation

is done solely by microbial autotrophs.

Half the planet's primary production by microbes.

They are truly the invisible engine driving the entire planetary carbon budget.

It's incredible.

We also need to think about temperature, how it separates water masses.

You get that warm, less dense water floating on top of cooler, denser water.

And that boundary layer is called the thermocline.

Right.

The thermocline.

It acts almost like a physical barrier.

It prevents nutrients from the deep dark water from easily mixing up into the sunlit surface layer where the phytoplankton need them.

This separation holds pretty firm until usually in the autumn, in temperate regions, the surface water cools down, becomes denser and sinks.

This causes the whole water column to mix an event we call an overturn.

And that overturn is critical, isn't it?

Oh, absolutely.

That sudden pulse of nutrients mixing up from the deep water triggers these sudden rapid microbial blooms.

You see the whole system kick into high gear.

So the ocean's clearly dynamic, but it also performs this huge buffering service for the whole planet.

Let's get into that crucial carbonate equilibrium system.

Okay.

This system is basically why seawater is naturally buffered.

It keeps its pH quite stable, usually between about 7 .6 and 8 .2.

What happens is atmospheric CO2 dissolves in the ocean, forming carbonic acid.

That acid then quickly dissociates.

It splits into bicarbonate ions and carbonate ions.

And those ions are the buffer.

Exactly.

They work together, reacting back and forth to effectively soak up changes and maintain the pH stability of the ocean.

It's a really elegant chemical system.

Okay.

Here's where it gets really interesting, or maybe worrying.

The massive amount of CO2 we humans have pumped into the atmosphere is starting to overwhelm this natural buffer.

That's the problem.

The ocean absorbs about a quarter of all the anthropogenic carbon we release.

And that leads directly to ocean acidification.

We've already seen the average ocean pH drop by about 0 .1 units since the industrial revolution started.

We're down to around 8 .1 now.

And that 0 .1 unit might sound tiny, but remember, pH is a logarithmic scale.

So that represents a pretty significant increase in acidity, about 30 % more hydrogen ions.

It's a huge structural change chemically.

And what are the real world consequences?

What are we seeing right now because of that shift?

The main issue is that this acidification reduces the availability of those carbonate ions, the ones needed to build calcium carbonate, CaCO3.

This directly hits any organism that needs to make a shell or a skeleton out of calcium carbonate.

Like corals?

Corals are the poster child, definitely.

But also really important planktonic organisms like coccolithophores.

These tiny algae produce maybe a third of all the marine calcium carbonate.

Their ability to form their shells is impacted.

And for corals, yeah, their very structure depends on precipitating these carbonate skeletons.

Models are suggesting that continued acidification could lead to many coral reefs eroding faster than they could actually grow.

It's a serious threat.

Okay, so while the global ocean faces these huge, slow chemical shifts, let's zoom into the coastal zones, like estuaries.

These are places where fresh river water meets salty ocean water.

Life here must be adapted to some pretty wild swings, right?

Oh, estuaries are incredibly dynamic.

Yeah, very changeable environments.

The microbes living there have to be halo tolerant.

They need ways to cope with big, sometimes daily shifts in salt concentration and the resulting osmotic stress.

How do they do that?

They manage their internal water balance mostly by accumulating specific molecules inside their cells, things like potassium ions or amino acids like proline or a compound called

These act as compatible cell elutes.

They balance the internal osmolarity without interfering with cell functions.

And estuaries are often ground zero for pollution impacts, right?

If a system gets overloaded with too much organic matter, it becomes eutrophic.

Eutrophic, exactly.

That's a major threat in coastal areas.

When you dump too much organic material in sewage, run off the microbial respiration, the decomposition, starts consuming oxygen faster than photosynthesis can possibly replace it.

And that leads directly to localized anoxia, little dead zones.

Even worse, if those nutrients come in sudden pulses, you can trigger massive blooms of just one or two species.

That gives rise to harmful algal blooms or HABs.

Which we often hear called red tides.

What's the impact of those?

They can be devastating.

Some HABs are toxic.

We see certain diatoms like pseudonychia producing domoic acid.

It's a neurotoxin that builds up in shellfish and fish.

If humans or We also have dinoflagellates like species of alexandrium that cause paralytic shellfish poisoning in humans.

This is a direct threat to public health and it can shut down fisheries completely.

Very serious stuff.

You know, we can actually visualize these layered microbial communities, especially in coastal sediments, using a classic lab model, the Winogratsky Column.

Can you sort of paint a picture of that for us?

Imagine a glass cylinder filled with mud, water, some nutrients.

Right.

The Winogratsky Column is brilliant.

It's like a miniature ecosystem built entirely on chemical gradients, usually incubated under light.

At the very bottom, where there's no oxygen, it's dark and anoxic.

You have microbes like Clostridium fermenting organic carbon from the mud.

As they do that, other microbes like ditulfovibrio use sulfate, which is usually plentiful and marine mud as an electronic scepter.

And the waste product of that is hydrogen sulfide, H2S, that rotten egg smell.

Okay, so you get this toxic gas building up.

Exactly.

But as that H2S fuses upwards towards the light and maybe a tiny bit of oxygen near the top, you see these distinct beautifully colored zones appear.

Those colors are the key, aren't they?

They are.

You'll see a green layer, often chlorobium, and maybe a purple layer, like chromatium.

These are amazing

photolithoautotrophs.

They use sunlight for energy, but instead of using water like plants, they use that toxic hydrogen sulfide as their electron donor.

They're literally feeding off the waste product of the microbes below them and detoxifying the environment in the process.

Wow.

It's a completely self -sustaining chemical loop, all driven by light and microbial metabolism in a jar.

Precisely.

A perfect illustration of biogeochemical cycling.

Okay, let's leave the bustling coast now and head way out into the open ocean, the oligotrophic photic zone, sometimes called the invisible rainforest.

Here, the challenge is the exact opposite of the coast.

There are hardly any nutrients.

It's like a desert.

It really is.

Survival out there depends on incredibly efficient nutrient recycling.

This is described by the concept of the microbial loop.

You see, even healthy phytoplankton leaks some organic matter.

Plus, marine viruses are constantly bursting microbial cells.

All this released organic material called dissolved organic matter, DOM, doesn't just sink and get lost.

Instead, heterotrophic bacteria and archaea rapidly snap it up.

They consume this DOM, keeping those essential nutrients, carbon, nitrogen, phosphorus cycling, near the sunlit surface where the primary producers need them.

It's super efficient.

But some carbon does manage to escape this loop and sink down, right?

Which is important to the climate.

That's the biological carbon pump.

Yes, the biological carbon pump is the process by which photosynthetically fixed CO2 gets exported to the deep ocean.

It often travels down as marine snow, these sort of fluffy aggregates of dead cells, fecal pellets from zooplankton, other bits of organic detritus.

So it's like a slow drizzle of carbon heading downwards.

Kind of.

But what really highlights the incredible efficiency of that microbial loop in the water column is this fact.

Less than one percent, often much less, of the carbon originally fixed by photosynthesis at the surface actually reaches the deep sea floor unaltered.

Less than all of it during its slow descent.

They are incredibly effective recyclers.

Since that recycling is so tight, growth in the open ocean is often limited by nitrogen.

So bringing new nitrogen into the system through N2 fixation must be absolutely crucial.

Who are the key players doing that?

Oh, there are some fascinating microbial specialists.

One major player is trichodesmium.

It's a type of filamentous cyanobacterium, forms these visible rafts or puffs on the surface sometimes.

It solves the oxygen problem because nitrogenase, the enzyme for N2 fixation, is destroyed by oxygen by having specialized cells called diazocytes.

These protect the enzyme, allowing it to fix nitrogen even while the other cells are photosynthesizing and producing oxygen.

Very.

And then there's another really unique one called UCYNA.

It's a cyanobacterium, but it's a photoheterotroph.

And intriguingly, it seems to have completely lost Photosystem II, the part of photosynthesis that actually produces oxygen.

No oxygen production.

Right.

Which means it doesn't have the same oxygen conflict.

It can fix nitrogen during the day using light energy captured by Photosystem I without poisoning its own nitrogenase.

A really neat evolutionary solution.

Okay.

If we're talking about microbial success stories in this nutrient poor world, we absolutely have to mention the most abundant organisms on the planet.

These are tiny alpha proteobacteria.

In typical surface ocean waters, they make up something like 25 to 50 percent of all microbial cells.

25 to 50 percent of cells in the surface ocean.

Yeah.

And when you factor in estimates for the deep ocean, sediments, even soil, some researchers estimate SR11 might account for roughly 25 percent of all microbial cells on Earth.

Period.

Okay, wait.

Let me process that.

A quarter of all microbial life on the planet belongs to this one group.

That is truly mind -boggling.

Their success must hinge on some incredibly clever adaptations for living lean, for oligotrophy.

It absolutely does.

They have some brilliant metabolic tricks.

Let's look at two key ones.

First, many of them have

proteordotoxin, PR.

Rhodopsin, like in our eyes.

Related, yeah.

It's a pigment bound to a protein, but instead of vision, it acts as a light -driven proton pump.

When light hits it, it pumps a proton out of the cell, creating a gradient that the cell can use to make ATP.

So they can get energy from sunlight even though they're heterotrophs eating organic matter.

Exactly.

It's a form of photoheterotrophy.

It supplements the ATP they get from breaking down scarce organic molecules.

And the system is beautifully tuned.

The specific type of proteordotoxin they have is adapted to the light available, absorbing green light near the surface, shifting to absorb blue light deeper down where blue penetrates best.

Wow.

Okay, what's the second strategy?

The second is lithoheterotrophy.

This is using simple inorganic compounds as an extra energy source, even while you're primarily eating organic carbon.

Inorganic compounds?

Like what?

A great example comes from another marine alpha proteobacterium, silicobacter palmaroi.

It can actually oxidize carbon monoxide CO, turning it into CO2.

It breathes carbon monoxide.

Sort of, it gets energy from that oxidation.

And by getting energy this way, it means it can use the more complex, precious organic carbon molecules it finds purely for building new cell parts, for biosynthesis and growth, instead of having to burn them just for energy.

That makes sense.

Save the good stuff for building.

Exactly.

These organisms are the ultimate metabolic minimalists, masters of energy efficiency.

We can't finish talking about the surface ocean without hitting on the most numerous microbes of

the marine viruses.

Oh, yeah.

They are everywhere.

We're talking staggering numbers.

Densities can reach up to 10 ,177 ,000 virus -like particles in a single milliliter of surface seawater.

10 million per mil.

Yep.

And they are major, major agents of mortality.

Estimates suggest viruses kill about 20 % of the total microbial biomass in the ocean every single day.

Wow.

So viral lysis bursting open cells isn't just about death, then.

It's actually critical part of that nutrient cycling, feeding back into the microbial loop we talked about.

Precisely.

Viral lysis effectively short -circuits the microbial loop.

Instead of nutrients being locked up in cells and slowly passed up the food chain when something bigger eats them, viruses cause rapid cell bursting.

This accelerates the conversion of living microbial biomass back into that particulate and dissolved organic matter, PDOM.

It dramatically speeds up nutrient turnover in the photic zone, keeping things moving quickly.

Okay.

Let's take a deep breath and clench down.

Way down.

To the deep benthos, the sediments on the deep sea floor.

This is the pisosphere, right?

Where pressures are immense, over 100 atmospheres easily.

That's right.

And what's incredible is that the total microbial biomass living in these vast, cold, high -pressure deep sea sediments is estimated to be roughly equal to all the microbial life floating in the water column of all the world's oceans combined.

Seriously.

As much biomass in the mud as in all the water.

Roughly.

Yeah.

These microbes are specialized.

They have to be pressure -loving, pisophilic, and cold -loving, psychrophilic.

And they live life in the ultra -slow lane.

We think their doubling times aren't measured in hours or days, but potentially in hundreds, maybe even thousands of years, they are really pushing the known limits of life.

And things get energetically very strange down there too, don't they?

Very strange indeed.

In really deep sediments, say below 400 meters or so, researchers have observed what they call upside -down microbial energetics.

Upside -down?

How so?

Well, normally, based on thermodynamics, you expect microbes to use electron acceptors in a predictable order.

Oxygen first, if any, then nitrate, then manganese oxides, then iron oxides, then sulfate, and finally CO2 for methanogenesis.

That's the order of energy yield, high to low.

But deep down, this predicted thermodynamics gratification seems to be reversed.

You find evidence of sulfate reduction happening below methanogenesis, for example.

What does that imply?

It implies something profound.

There must be unknown deep sources providing these electron acceptors, maybe nitrate or sulfate, diffusing up from below.

We don't fully understand it yet.

It fundamentally challenges our classic models of how life gets energy in the deep earth.

And speaking of the deep earth and energy, this realm holds a potentially massive economic factor.

Methane hydrates.

Yes.

Methane hydrates, sometimes called fire ice.

These are literally cages of water ice that trap methane gas molecules inside, formed under the high pressures and low temperatures of the deep seabed.

The methane itself is produced by methanogenic archaea living deep within the sediments, and the scale is enormous.

Estimates suggest there could be something like 10 trillion metric tons of methane locked up in these hydrates worldwide.

10 trillion tons.

That's an unbelievable amount of potential energy.

It's a colossal energy reserve, yes.

A resource we might eventually tap into, but one that currently represents this vast hidden microbial activity happening miles below us.

Okay, for our final section, let's come back up to the surface, but leave the oceans behind.

Let's shift focus to freshwater systems.

Field of limnology.

It's easy to forget just how precious fresh surface water is only about 0 .2 % of all the water on earth.

It's incredibly scarce, relatively speaking, and when we study freshwater, we generally divide systems into two main types.

You have the moving water's lotic systems, like streams and rivers, and you have the standing water's lentic systems, like ponds and lakes.

Lotic systems, the rivers and streams, are typically net heterotrophic.

What that means is most of their energy and carbon comes from outside the river, washed in from the surrounding land.

We call that allochthenous input.

Think leaves, soil, organic matter.

Lentic systems, especially larger lakes, are often the opposite.

They tend to be net autotrophic.

Most of their carbon is fixed within the lake itself by phytoplankton and other aquatic plants.

That's autoclossness production.

Rivers, being flow through systems, show a really predictable pattern when they encounter localized pollution, don't they?

They do.

It's the classic oxygen sag curve.

If you have a specific point source of pollution, like an untreated sewage pipe emptying into a river, you see a sharp drop in dissolved oxygen immediately downstream as microbes decompose the waste.

Then, as the river flows further, the waste gets diluted and degraded, and oxygen from the atmosphere dissolves back in, so the O2 level gradually recovers.

It creates this sag shape on a graph of oxygen versus distance.

But that's for point sources.

What about non -point sources, like agricultural runoff spread over a large area?

Yeah, that's often a different story.

Non -point pollution tends to cause more chronic, widespread nutrient loading.

This doesn't usually create a sharp sag, but it can lead to persistent algal and cyanobacterial blooms throughout large sections of a river or watershed.

And lakes, the lentic systems, have their own dynamics, particularly stratification and mixing with the seasons.

Absolutely.

Deeper lakes in temperate climates stratify seasonally, much like the ocean.

In summer, you get a warm, well -mixed oxygenated surface layer called the epilimion.

This sits on top of a cold, dark, often low -oxygen bottom layer called the hypolimion, where nutrients released from decomposition tend to accumulate.

Separated by that thermocline again.

Exactly.

And just like in the ocean, the seasonal overturn usually in spring and fall when the water temperature becomes uniform mixes these layers completely.

This brings those nutrients from the hypolimion up to the surface, fueling temporary blooms of phytoplankton.

We also classify lakes based on their nutrient status.

Oligotrophic versus eutrophic.

We do.

Oligotrophic lakes are nutrient -poor.

They tend to have very clear water, low algal growth, and often high oxygen down deep.

Think of pristine mountain lakes.

Eutrophic lakes, on the other hand, are nutrient -rich.

They are often turbid, with lots of algal growth, prone to low oxygen conditions in the bottom waters, and frequently dominated by cyanobacteria, especially in late summer.

And cyanobacteria seem to really thrive in those nutrient -rich eutrophic conditions.

They absolutely do.

They often have competitive advantages.

Many prefer warmer temperatures and the slightly higher pH often found in productive lakes.

Plus, they are masters at scavenging nutrients.

Some produce compounds called siderophores, which are incredibly effective at binding and acquiring essential trace metals like iron, effectively stealing them from other algae.

Some even produce toxins, deterring glazers.

Okay.

Wow.

We have covered a massive amount of ground from ocean dead zones to the deep biosphere and freshwater lakes.

So what does this all mean?

Let's try to recap.

We've clearly seen that microbes are just.

They are the fundamental engines driving the ocean's massive carbon and nitrogen budgets, which in turn influences global climate instability.

We've uncovered some of their incredible adaptations for survival in the vast, nutrient -starved open ocean.

These amazing metabolic tricks, like using proteo -dopsin for light energy or heterotrophy to use inorganic energy sources.

Real masters of efficiency.

And we found that the deep biosphere, the sediments beneath the ocean floor,

represents this enormous, incredibly slow -burning reservoir of life operating under energetic rules that we're still trying to figure out.

Yeah, that upside -down energetics is fascinating.

And to tie it back to the bigger picture, we established how viruses act as accelerators, speeding up nutrient cycling in the surface

and how human -produced CO2 is actively disrupting the ocean's natural carbonate buffering system, leading to acidification.

So maybe a final thought for you, the listener, to ponder.

We know viral lysis speeds up nutrient recycling near the surface.

How might changes in the rate of viral activity, maybe due to changing ocean conditions, directly influence the efficiency of that biological carbon pump?

How might that interaction affect the ocean's ability to soak up CO2 and ultimately impact climate change mitigation efforts?

That's a complex feedback loop to consider.

A lot to mull over, indeed.

Thank you for joining us on this deep dive into the microbial machinery that runs our planet's waters.

We'll catch you next time.