Let’s say you and I are on a small boat in the middle of the ocean, armed with nothing but a simple spoon. If you bend over and scoop up some ocean water, what do you expect to find? Surely you wouldn’t catch a whale, and you’d be incredibly lucky to catch even a small fish. With the naked eye, it appears that you have caught nothing special. Nonetheless, you would be wrong to think that what you see is only a mixture of salt and water.

Indeed, when we think about the ocean, images of sharks, fishes and coral reefs often come to mind, and we tend to forget about major actors in the marine life: microbes. The micro-organisms we find in the ocean are as diverse as their bigger counterparts (if not more!), and have as large an impact as them on the ecology, biogeochemistry and response of the ocean to climate change.

And let me tell you, you have caught some interesting stuff in that spoon of yours. So let’s dive in and take a closer look!

## Phytoplankton#

The first group we’ll focus on may also be the most important one: phytoplankton. Basically, phytoplankton are tiny unicellular plants that live in the surface layer of the ocean, where they are able to perform photosynthesis. They come in various shapes and forms, and each type (or taxonomy) has a different abundance. We’ll focus on two particular types here: diatoms and cyanobacteria.

Diatoms are amongst the biggest phytoplankton there is, with their size ranging from 5 to 200 $\mu$m, making them as big as the width of one of your hair. About 5 of them end up in your spoon, but don’t let this small number fool you, as they are key actors in capturing CO2. We estimate that as much as 50% of the oxygen produced each year comes from diatoms. Imagine that: every second breath you take on average comes from the ocean!

Cyanobacteria, on the other hand, are important because they are ubiquitous. Where there is water, you can be sure to find cyanobacteria (even on land!), and your spoon has about 500,000 of them right now. Contrary to diatoms, they are prokaryotic, which means that they do not have a nucleus holding their DNA. Prokaryotes are way older than their nucleated counterparts (the eukaryotes), and we believe that photosynthesis first appeared with cyanobacteria.

Phytoplankton are the foundation of the marine food web, as they provide the organic matter necessary to sustain all life in the ocean. We’ll delve into more details about it later, but suffice it to say that without phytoplankton, we wouldn’t have the diversity of life we find today in our seas!

## Heterotrophic bacteria#

As their name suggests, cyanobacteria are a type of bacteria, but they are not alone in this category. As a matter of fact, they aren’t even the most abundant type of bacteria you caught! Indeed, in your spoon you will find no less than 5,000,000 heterotrophic bacteria. This very diverse group of prokaryotes are unable to perform photosynthesis, and thus rely on ingesting organic matter and respiration to produce their energy.

They are at the heart of the microbial loop, which is a main point of focus of my Ph.D. Once again, we will have time to discuss its role in marine ecology in depth later, but in a word the microbial loop acts as a recycling unit in the ocean: part of the organic matter that would have been lost by sinking to the depth of the oceans are re-injected in the food web due to the action of heterotrophic bacteria.

## Viruses#

This one may surprise and even disgust you, but the most abundant entity (it is still debated whether we can consider viruses ‘alive’) you have caught with your little spoon are viruses. Indeed, you should find about 50,000,000 individual viruses floating around in that tiny pool you scooped up!

Frightening as they may seem, viruses in general are pretty specialized, so you shouldn’t be too worried about getting infected next time you go to the beach for a swim. The viruses you caught are, for the most part, bacteriophages, meaning that they are specialized in infecting bacteria. As their sheer number suggests, they are an important (but yet not fully understood) force behind the functioning and evolution of marine ecosystems. Part of my Ph.D. focuses on their ecological role on the microbial loop and the consequences for carbon cycling, so be prepared to hear about them quite often!

Let’s recap what we found in our spoon so far: we found about 5 diatoms, 500,000 cyanobacteria, 5,000,000 heterotrophic bacteria and 50,000,000 viruses. To put these numbers in perspective, let’s ask ourselves the following question: what are the chances that we catch something bigger?

Let’s start with zooplankton. We can say that zooplanktons are to animals what phytoplanktons are to plants: their microscopic counterparts. They typically are bigger than phytoplanktons (with some being visible to the naked eye), and feed on pretty much anything that is smaller than them. Small as they may be, you would only have a 1 in 20 chance to catch one with your spoon.

From then on, the chances only get slimmer: the probability to catch any type of fish in the surface ocean is close to 1 in 10 million. For blue whales, optimistic estimates would put you at 1 in 3$\cdot 10^{15}$ chances, which means a 3 with 15 zeros behind. Good luck with that!

## Taking another point of view#

As we can see, when we think about number of individuals, microbes dominate the oceanic scene. But then again, each individual is very small and weighs practically nothing. In fact, we can shift the perspective from total number of individuals to how much biomass each community represents. Biomass is simply the technical term used to talk about the mass of all living things in a particular context, and it can be a more useful metric than individual counts in some cases – for instance, to assess how much carbon flows through a given ecosystem, it is easier to think in terms of phytoplankton biomass than count.

When looking at the different groups we talked about by focusing on mass, we find that:

• Phytoplankton represent around 1 billion tons of biomass in the oceans.
• Heterotrophic bacteria represent roughly the same amount, with their total biomass estimated at 1.2 billion tons.
• Viruses, despite their huge number of particles in the oceans only account for 200 million tons.
• Zooplankton, on the other hand, clock at 7.4 billion tons of biomass.
• Fishes represent about 700 million tons in the oceans
• Finally, blue whales represent a mere 1 million tons.

It is interesting to classify more broadly these groups in one of two categories: producers and consumers. Producers are comprised from individuals that can synthesize biomass from inorganic nutrients, such as the autotrophs who perform photosynthesis. Consumers, on the other hands, depend on producers to eat, as they need organic biomass to sustain growth. Here, the producers are the phytoplankton and consumers are everyone else.

We can see that for every ton of producer, we have about 10 tons of consumers. Compare that with terrestrial ecosystems, where producers (mostly plants) have a total biomass of 450 billion tons, whereas consumers are only 20 billion tons.

We can note 2 interesting things here. First, we can see that phytoplankton are a whopping 450 times less abundant in terms of biomass than their terrestrial counterparts, while still accounting for roughly half the photosynthesis on Earth. Go team phytoplankton! Secondly, the ratio between producers and consumers is completly reversed when compared to terrestrial ecosystems. This stems from the fact that phytoplankton have much shorter lifespans than marine consumers, allowing for rapid turn-over and high productivity at the cost of low biomass concentrations.

This short lifespan can be attributed to the small size of phytoplankton and their unicellular quality. We can intuitively explain why the major functional group of the oceans is unicellular: when compared to a stable life on land, where you know the environmental conditions of the soil are pretty fixed and you can’t move easily, life in an ever-changing environment due to mixing favors small floating individuals. Indeed, since there is no soil to plant ones roots in, it is advantageous to be small and floating.

## Conclusion#

You can now put your spoon down, as we have finished our little headcount. The ocean is a very diverse and lively place, so keep in mind that we have only scratched the surface here! There is a world to discover in how all these organisms interact with each other and their environment. As we’ll continue our journey through my Ph.D., we’ll learn about the special role that heterotrophic bacteria play in the marine ecosystem, how viruses shape the evolutionary destiny of many important actors and what we can expect for them as oceans become warmer and more acid. All of these questions will be looked at through my own lenses, that of a theoretical ecologists whose job is to design and analyse mathematical models. Don’t worry though, it is true that you’ll see some equations in the future, but I plan on explaining every step of the way as simply as possible!