Imagine that you make delicious apple pies, but you know nothing about growing apple trees. You make a lot of pies-more than you need or eat. When you need more apples, you go to a farmer friend who has a whole orchard. He doesn’t know how to cook, so you are happy to bake apple pies in exchange. This type of win-win interaction is called mutualism, and it happens all the time in the microbial world.
We know how valuable plants are for us. They are the ones that convert light and carbon dioxide into the food we eat. In the nutrient-poor open ocean, microscopic phytoplankton are the primary producers that transform light into food. Picocyanobacteria, tiny photosynthetic bacteria, are the most numerous population of primary producers on Earth.
Scientists have long observed that phytoplankton produce more food than they use, and this extra food is delivered to other marine life. This food clearly benefits non-photosynthetic organisms, but scientists wonder how producing extra food benefits the producers themselves. Do picocyanobacteria produce more than they need by accident, or do they do so in a controlled manner? One explanation is when eating this extra food, non-photosynthetic organism break down smaller sized particles and nutrients trapped within this food, which are in turn consumed back by picocyanobacteria. Different species of phytoplankton show the same interaction pattern with organic carbon-eating organisms, which suggests this is a general trait, and not something specific of a certain species.
To understand why phytoplankton make excess food, scientists grew the phytoplankton species, called Synechococcus, with different food-eating Roseobacteria and other strains. We call these bacteria “heterotrophs”, as they cannot produce their own food. In contrast, organisms that make their own food, like phytoplankton, are called autotrophs. The mixed culture was grown only with sunlight and carbon dioxide. No other food or nutrients were added, so the entire bacterial community would be fully dependent on what the autotrophs produced. Then they took a sample from these different cultures, and looked at the food they produced, as well as the type and quantity of proteins they produced. Proteins inside the cell form systems with very specific objectives. There is a system to acquire nutrients, one to store compounds, one to respond to a stressful situation, etc. Looking at proteins can thus help scientists understand which system is working, and which system changes when Synechococcus is alone or together with microbial heterotrophs.
In the presence of various heterotrophs, Synechococcus remained alive for over 10 months, whereas alone they died after 4–6 weeks. That was expected, and confirms previous anecdotal observations. The big question is why. Part of the explanation is that Synechochoccus produces as much organic matter when it is alone than when in cultures with others. This trait is already part of their nature, it is programmed into their DNA. So when they are alone, excess food builds up and becomes toxic to them. When others are present, however, they eat this excess food and Synechochoccus lives longer.
Two of the systems that changed were those of nitrogen and phosphorous. Instead of reusing phosphorous and nitrogen inside the cell, scientist saw that in the mixed cultures, Synechochoccus prefered to uptake these compounds from outside. This meant that heterotophs were degrading all these complicated molecules that Synecochoccus produced into simpler compounds and releasing nitrogen and phosphorus. In this way, heterotrophs help phototrophs tofocus on what they do best: make food from the atmosphere.
This study is the first time scientists took a detailed look at how microbial phototrophs and autotrophs share food and nutrients, and they found that evolution sometimes favors cooperation and specialization of functions. Knowing how microbes interact with others in the oceans is useful to us. Knowing how microbial communities behave in response to the conditions around them can help us intervene if the ecosystem is under threat, in the lab or in nature itself.