Life began in the oceans. The richness of their biodiversity provides mankind with incredible and essential resources. This richness is under threat from human pressures.
A rich and microscopic world
Life first appeared in the ocean. In fact, the ocean has a history much longer than the land that emerged from it. Due to billions of years of evolution, almost all the branches of the tree of life are represented there. Even now, however, little is known about the biodiversity of the oceans, not least because of the number of species that it includes – which is a matter of some controversy: not including micro-organisms, it is estimated that 200,000 to 250,000 animal and plant species populate the oceans – figures that account for around 15% of all known species on the planet. However, some studies estimate the total biodiversity of the planet in the region of 2 to 100 million species, which shows the lack of data available to us.
Although the larger creatures are the most symbolic, such as whales, seals, sharks and turtles, almost all life forms are invisible to the naked eye. There are, in fact, several million species of micro-organisms – algae, bacteria and viruses; for example, there are up to 160 species of prokaryotes (bacteria) in a single drop of seawater, and between 229 and 381 species of eukaryotes (organisms with a cell nucleus) in one litre.
Phytoplankton – all of the unicellular organisms capable of achieving photosynthesis – themselves represent 98% of the oceans’ biomass: these micro-organisms, none of which measures more than one millimetre, produce resources to sustain the entire ecosystem, as well as recycling waste. They also produce most of the oxygen we breathe: these are the planet’s lungs.
As we can see, from the phytoplankton to the blue whale that feeds on it (and which can grow to 30 metres long and weigh up to 170 tonnes), all marine organisms are connected.
The coral reefs, oases of life
Other tiny organisms, the polyps, small gelatinous animals, are able to form a calcareous skeleton called coral, as immense as it is essential. The colours of coral reefs are caused by the microscopic algae in the tissues of the polyps. During the day, the coral collects nutritional substances in the algae it shelters using the sun’s energy. At night, each of the coral’s polyps becomes a formidable predator which captures its prey using its sticky and poisonous tentacles.
The reefs attract many organisms, crustaceans, molluscs, thousands of fish and a number of species of shark, which take advantage of the abundance of potential prey. Along with the equatorial forests, they comprise the richest and most complex ecosystems on the planet. Although they occupy less than 0.1% of the aquatic environment, they might be home to between one and nine million species, only 10% of which are known. Almost 5,000 species of fish – more than a quarter of all species of marine fish – have been identified there. Over one square metre of coral reef, the number of animal and plant species is more than one hundred times greater than those found in open water. The reefs therefore constitute genuine oases of life at the heart of the oceans.
As with the tropical corals, there are places in the depths of the ocean where life flourishes, particularly where there are hydrothermal vents which form along the oceanic ridges, at a depth of between 500 and 4,000 metres. Intense volcanic activity releases the thick black smoke, which gives its name to the “black smokers”. The liquid escaping from these chimneys can reach a temperature of 350°C; it contains a very strong concentration of chemical compounds such as iron, zinc, manganese, hydrogen sulphide and even CO2. In spite of the total lack of light and the extreme pressure (between 100 and 500 bars), significant local temperature variations (from 350°C to 2°C in a few metres) and water loaded with metallic compounds that are poisonous to most species, life develops and even flourishes there: animal concentrations around the chimneys can reach 50 kilos per square metre.
The life forms adapt and develop original strategies. For example, the Pompeii worm (Alvinella pompejana) is renowned for being the microscopic organism that can withstand the highest temperatures: it lives in a tube, in symbiosis with filamentous bacteria, and exposes parts of its body to temperatures up to 80°C!
The marine ecosystems and the diversity to which they are home provide mankind with a large number of resources with considerable economic potential – we call these “services rendered”. First and foremost, life in the oceans provides a significant food resource for nearly 3.5 billion people, for whom seafood is their main source of proteins. The oceans also supply a large number of ‘molecules-to-medicine’, such as anti-tumour drugs or AZT, as well as supplying the cosmetics industry. Finally, many areas, such as the Red Sea and Southeast Asia, have developed a strong tourist industry based on the diversity of the oceans and diving.
Nearly 30% of the services rendered by the ecosystems come from the oceans, and their accumulated value could reach 3,000 billion dollars per year.
A biodiversity under threat
However, when a single species disappears – even a rare one – this makes the entire ecosystem vulnerable. This phenomenon is particularly true for large predators: in the absence of the latter, jellyfish or small herbivorous fish, for example, begin to proliferate. In the same way, the introduction of a new species can have serious ecological and economic consequences. Along the Israeli coast, 50% of catches now involve species of fish which have entered the Red Sea by way of the Suez Canal. As a result, these species have growth and reproduction capacities much lower than those previously there.
The rise in temperatures, related to climate change, is changing the blueprint for the distribution of plant species and causing some animal species to migrate to higher latitudes. The shrinking ice caps result in the disappearance of the fauna and flora associated with the ice: the days of the polar bear, emblematic of the Arctic, are probably numbered. At the same time, the excess of CO2 in the atmosphere is causing water to acidify, with potentially grave repercussions for calcareous organisms like coral and some species of phytoplankton.