Rust Never Sleeps: The Red Tide That Made The World
If you've spent any time on the old blue briny, particularly the Pacific side of the island continent, but almost anywhere in the world's oceans, you'll be familiar with the red slicks that form across stretches of sea surface, sometimes so vast they're visible from space.
Yet even if you've never seen them, you've heard the term: red tide.
The micro-organism responsible for red tide gets a bad rap, mostly as a portent of a toxic future, but it's very far from being a bad guy and worth a moment of our time to investigate.
First of all, what is this red tide? I've heard it called many things: whale sperm, coral spawn, red algae, and more, yet it's none of those things. The vast majority of so-called red tides are a type of single-celled Cyanobacteria called Trichodesmium.
This is a deeply unsexy organism. There are no Surfers for Cyanobacteria, nor likely to be. We don't give it attention as we do whales, dolphins, or coral reefs, but this colony-forming, filamentous organism is a Very Big Deal in the marine ecosystem, particularly in the nutrient poor tropics and sub-tropics. They also have a cool little backstory, which involves creating the conditions for all life on Earth.
Cyanobacteria are some of the most ancient organisms on Earth. They're considered 'simple' uni-celled Prokaryotes, as distinct from more complex multi-celled Eukaryotes, which is almost everything else: plants, animals, algae, fungi etc.
Prokaryotes are found in every habitat on Earth, from the deepest ocean, to the driest soil, buried inside our guts, or atop the highest mountains, airborne in the atmosphere, even adrift on Arctic ice sheets. A stream of airborne micro-organisms, including Prokaryotes, circles the planet above our weather systems but below commercial air lanes. Some micro-organisms are swept up from terrestrial dust storms, but most originate from marine micro-organisms in sea spray.
In 2018, scientists reported that tens of millions of bacteria are deposited daily on every square metre around the planet. Life at the microbial level is mind-bogglingly profligate.
The first Prokaryotes show up in the fossil record 3.5 billion years ago, only a billion years after the Earth was formed. These prokaryotes ruled the Earth, particularly the marine environment for a couple billion years, before the Eukaryotes showed up. Importantly, Cyanobacteria were the first, or pretty fucken close to the first, organisms to figure out oxygenic photosynthesis. In short, Cyanobacteria were the pioneers of incorporating energy from the sun via photosynthesis. This puts them at the head of a long line of evolution. No Cyanobacteria, no plants. No plants, no you, no me. No life on Earth as we know it.
Aside from starting a new evolutionary path, the newfound ability of Cyanobacteria to photosynthesise had another impact on life on Earth. Some 2.5 to 2 billion years ago, before Cyanobacteria developed oxygenic photosynthesis, the world was a fucked up place. The Earth had a “weakly reducing atmosphere”, with practically no free oxygen. It was a toxic soup of nitrogen, carbon dioxide, and methane suitable only for the hardiest bacteria.
A side-effect of photosynthesis by our ancestral Cyanobacteria was free oxygen. O2. Evidence suggests that biologically-produced oxygen started to accumulate in the atmosphere and change it from a weakly reducing atmosphere into an oxidizing atmosphere containing abundant oxygen. Those ancient red rocks in WA? Cyanobacteria produced enough free oxygen to make everything containing iron, rust.
Then, once the world went rusty, the free oxygen produced more convulsions. A mass extinction of the old Prokaryotes and a new explosion in multi-cellular life - the Cambrian Explosion. It wouldn't have been possible without the liberating effect of chemical energy released by the abundance of oxygen.
No life on Earth, as we know it now, would be possible without the Great Oxygenation Event, and it was kickstarted by marine Cyanobacteria.
A small round of applause is appropriate then, I think you'll agree, for the stinky, slimy mess that washes up on the beach every summer and to which we owe our existence.
But there's more. Cutting back to the initial subject of our investigation, the Trichodesmium, it also lives up to the Cyanobacterium level of high achievement.
If you've ever scuba dived you'll be familiar with the process of controlling depth by releasing and adding air (gas) to a buoyancy control device. It's pretty cool. Via the same mechanism, small gas filled vesicles, Trichodesmium can rise and sink in the water column on a diurnal (daily) basis. They do this to hoover up molecules needed for their other main ecological achievement: fixing nitrogen.
Nitrogen, if you've forgotten high school biology, is essential to life. While nitrogen gas is abundant in air, it is not available to most plants and animals in that particular form. Trichodesmium deserve our respect because they are 'nitrogen fixers' - this means that they can take nitrogen gas from air and 'fix' it in a form that can then be transferred into the food chain. Diazotroph is another deeply unsexy term, referring to the ability of certain micro-organisms to fix nitrogen out of the atmosphere, in this case in the marine environment.
OK, so Trichodesmium is the main nitrogen fixer in the ocean, which means all the other plankton and diatoms rely on it. And thus the copepods, the crustaceans, and upwards to the fish, dolphins, and whales.
Sentient creatures get all the attention, and that is fair enough I suppose. They are more like us. It's harder to relate to some of the most ancient organisms on Earth, especially when we only experience them as stinking slime on the sand, or vaguely irritating slicks in the line-up. Still, it seems worthwhile, once in a while, to spare a thought for our oceanic microbial ancestors and the role they played in making the Earth habitable, as well as creating the continuing foundation of the marine ecosystems we all love to play in.
Viva la Trichodesmium!
// STEVE SHEARER