The oceans are warming due to climate change and this will impact all oceanic life, from the smallest plankton to the largest tuna. The web of life in the ocean is complex, and so exactly which ecosystems will be affected and how much they will change is currently being intensely researched. It is especially important to understand how phytoplankton, the foundation of the food web and producers of oxygen, will be affected by this changing environment. If phytoplankton decreases or changes distribution, then fisheries may also see reduced catches or fish moving out of the area entirely.

Cyanobacteria are a numerous, ancient, and widespread type of phytoplankton. They were believed to have transformed the ancient atmosphere of Earth from being high in carbon dioxide (CO₂ ) to high in oxygen, paving the way for the evolution of life as we know it. One cyanobacterium is microscopic, but when conditions are right, they can form extremely large colonies or blooms that can be visible from space.

In this study, researchers wanted to determine how the abundance of iron, an important nutrient for cyanobacteria, affects the growth rate of the cyanobacteria species Trichodesmium erythraeum at different temperatures. Trichodesmium is an ecologically important cyanobacteria because it transforms nitrogen gas into organic nitrogen, which can then be used by other animals, using a process called nitrogen fixation. Only a few species can change nitrogen from an inorganic form to a form that other living organisms can use, making these nitrogen fixers very important. It is frequently assumed in studies of phytoplankton that warming and nutrient abundance affect growth in an additive manner. This means that their growth changes by the effect of temperature plus the effect of the nutrient abundance. However, the researchers demonstrated in this study that it’s a little more complicated than that. Temperature increases can affect phytoplankton metabolism and chemical reactions by providing more energy, thereby causing additional changes to growth rate.

For this experiment, scientists from the University of Southern California and several Chinese universities grew Trichodesmium from 22℃ to 35℃ (the maximum survivable temperature) with either an excess amount of iron or a low level of iron. They observed the growth rate of the plankton and tracked the cells’ iron usage using radioactive iron compounds. 

When iron was plentiful, the cyanobacteria grew the fastest at 27℃, but when iron was limited, the maximum growth rate occurred at 32℃. A similar pattern was found with phosphorus, which is the limiting nutrient for Trichodesmium growth in areas where iron is abundant. This means that as oceans get warmer, up to the maximum of 35℃, the low levels of iron become less of an issue, and the cyanobacteria can grow as quickly as they would with unlimited iron at the same temperature.  As temperature increased, their rate of nitrogen fixation also increased at a steady rate. The researchers theorize that the cyanobacteria grow faster in warmer conditions when iron is low because when the surrounding environment is warm, reactions inside the cells proceed faster and the enzymes (which contain iron) get released and re-attached at a much faster rate. This means a smaller amount of iron (and phosphorous) can get the same amount of work done within the cell because everything goes faster at the warmer temperatures.

When scientists develop models to predict ocean productivity in a warmer future ocean, many do not take effects like this into account and therefore underestimate amounts of biomass and nitrogen fixation. Other studies have shown that high CO₂ also increases the rate of nitrogen fixation, but this process is also limited by iron. Since this study shows that less iron is required at high temperatures, it’s more likely that nitrogen fixation will increase even more with increasing CO₂ levels. According to the researchers from this study, if other species of cyanobacteria react similarly, then there will be an increase in nitrogen fixation in 2100 by an average of 21.5%. The percentage varies greatly by geographic area, however, because some areas are predicted to be greater than 35℃, meaning most phytoplankton could not survive at all.

In the areas that do not get too warm, this nitrogen fixation may help prevent some negative effects of ocean warming. Warming of the Earth’s oceans is predicted to reduce currents and therefore stratify the oceans into layers. This would prevent the normal exchange of nutrients like organic nitrogen from the depths to the surface. So, any increased fixation of inorganic nitrogen by cyanobacteria could provide the necessary organic nitrogen to surface waters if severe stratification occurs. 

Any change to the base of a food web will change everything in difficult to predict ways. Therefore, understanding the effects of climate change on plants and phytoplankton is of the highest importance for knowing what the future will bring.