By Charlie Whittaker
For sure, the abundance of terrestrial plants we share our planet with are weird and wonderful in equal measure, but why should they get all the glory when there’s an equally as important component to the biomass on Earth? I’m of course talking about the much maligned, often overlooked, and most definitely misunderstood microscopic creatures that inhabit the murky depths of our oceans!
The marine environment is by far the planet’s largest habitat. Covering over 70% of the land area, it contains a huge diversity of organisms, co-existing in a harmonious, yet fragile, balance. Underpinning all the life that the oceans sustain are photosynthetic organisms. Tiny, often microscopic and unicellular, these organisms are responsible for roughly half of all the primary productivity of the planet. Their ability to capture sunlight and use it to synthesise new organic compounds provides the energy for the diversity of marine life found in the ocean. They are, for want of a better analogy, the oceans’ invisible forest.
These primary producers are exceptionally diverse, ranging from tiny photosynthetic bacteria that hitch a ride on the tiny particulate matter found in seawater, to the phytoplankton. These represent a hugely diverse group of unicellular organisms. Contained within this group are the diatoms, which enclose their cell in a glass box made out of silica, as well as the dinoflagellates, that tend to employ semi-opaque plates of cellulose to separate themselves from the external environment. And then who could forget the coccolithophores? Unicellular like their other phytoplankton counterparts, these microorganisms cover themselves with ornamented plates called coccoliths made out of calcium carbonate.
So why does any of this matter?
80 million tonnes of marine seafood are caught globally each year. Seafood forms a common constituent of diets worldwide and provides more than 1.5 billion people with at least 15% of their protein requirements. The entirety of this marine life, whether directly (animals that feed on the producers themselves) or indirectly (in the case of organisms several trophic levels above the primary producers), relies upon the productivity and photosynthesis these organisms are carrying out.
They also represent an important carbon sink. The ocean plays a huge role in mopping up and buffering CO2 released into the atmosphere: and a significant proportion of the ability to do this stems from the simply huge amount of photosynthetically capable biomass present.
Okay, that’s fine and dandy then?
Not quite. Unfortunately things are getting progressively less peachy. Climate change poses a serious issue to the future productivity of the oceans and marine life. Changes to the oceanic average temperature has implications ranging from alterations to the vertical stratification of the water column (important in mixing, thereby ensuring all the phytoplankton receive all the nutrients they need) to impacting the chemical reactions responsible for the productivity of the primary producers. Whilst of course, the response to rising sea temperatures will not be the same globally (a paper recently published in nature showed that “Some phytoplankton like it hot” and that warming oceans may increase productivity in some areas) there are important marine areas of human concern that are set to suffer substantially: the Atlantic Cod population has plummeted in number in recent years. Partly this has been driven by overfishing, but it was also shown this was due to rising temperatures. The alteration modified and impacted the plankton ecosystem in such a way that it reduced the survival rates of young cod, and thereby facilitated the population’s rapid decline.
They may be invisible, but the effects of these tiny photosynthetic powerhouses are quite the opposite. And unless something is done soon, they may be at the forefront of drastic alterations to our current marine system.
On the effect of increased temperatures on cod and phytoplankton populations.
On the propensity some phytoplankton show for warmer temperatures.