Most of the oxygen in the atmosphere is produced not by the rainforests but by tiny marine organisms. Phytoplankton collectively form the main lungs of the planet, although their importance was only realised recently. Phytoplankton are commonly known as algae and are single celled, photosynthesising protoctists. They are found in every sea on the planet, sometimes in huge numbers, and play the same role in the ocean as plants do on the land.
The organisms classified as phytoplankton are extraordinarily diverse and currently are classified into at least thirteen classes. They don’t all use chlorophyll to photosynthesise, some use red or brown pigments instead. Since phytoplankton are not in fact plants they exhibit other features that are nothing like what we would associate with plants. Some have one or two flexible appendages (flagella) which means that they can swim.
The dinoflagellates (from the class Pyrrophycaeae) are one such group. Their two flagella make them motile, although they are hardly strong swimmers and their movement is quite localised. This is the second most abundant group of phytoplankton in the oceans and they are found everywhere the sunlight can reach. About half of them are photosynthesising autotrophs with the others being heterotrophs eating other organisms. This other half belong to the zooplankton, not the phytoplankton. A few are mixotrophs and are capable of both methods of sustaining themselves.
Dinoflagellates are one of the groups responsible for algal blooms. Under especially favourable conditions they can reproduce extremely rapidly, creating tides of phytoplankton visible to us. These are usually caused by an excess of nutrients. Algal blooms are not usually good news for anything but the phytoplankton. When they die there are too many for the occurring zooplankton to eat and so they decompose.
The decomposition removes oxygen from the water, resulting in anoxic conditions unsuitable for most marine life, such as fish.. In some cases the low oxygen conditions are so extensive they become effectively dead zones, like the enormous annual dead zone in the Gulf of Mexico. Sometimes the species that got lucky turns out to be toxic and numerous cases of shellfish food poisoning have been linked to algal blooms.
The most abundant and most studied group of phytoplankton though are the diatoms (from the class Baccariophyceae). These all have hard external skeletons made mainly of silica and coming in a variety of delicate, geometric structures. Many of these tiny shells also have spines and under the microscopic they are rather beautiful. They are also so abundant that the shells of long dead diatoms make up large quantities of the ocean sediments, which sometimes become rocks. Another group, the coccolithophorids, produced the shells that make up the famous white cliffs of Dover.
Diatoms are unable to swim and since they are dependent on light to survive they must have various means that ensures they float. Cell size and shape is one example, others form long chains to increase surface area to volume. Most reproduce asexually although sexual reproduction also exists. This applies to most other forms of phytoplankton as well. Some diatoms can turn themselves into tough spores during adverse environmental conditions.
Less is known about the other classes of phytoplankton but what we do know about phytoplankton as a group is fascinating. They form the basis of the ocean food chains, they form rocks and ocean sediments, they can cause dead zones in the oceans, and they produce much of the oxygen we breathe. In fact they are some of the most important lifeforms on the planet.
Given this, the recent news that global populations of phytoplankton have been steadily declining since at least 1950 is disturbing. Scientists don’t know what the result of this will be or whether anything should be done to reverse the trend. What is obvious though is that a lot more study needs to be done.