Marine ecosystems

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Oceans cover two-thirds of the earth’s surface. Their productivity depends on the latitude which influences the amount of light received and the temperature of the water. The latter modulates the amount of carbon dioxide and dissolved oxygen and therefore the efficiency of photosynthesis and respiration because cold water dissolves these two gases better. The availability of mineral salts such as iron, phosphate and that of nitrogen in a form that can be assimilated by eukaryotic algae such as nitrate are also parameters that will strongly affect primary productivity. They will depend on the proximity of the continents and ascending or descending sea currents. In general, the productivity of waters at extreme latitudes is greater than that of tropical and equatorial waters, although there is less light there. This is easily seen by their turbidity: warm waters are generally crystal clear, indicative of low biomass. In contrast, temperate and cold waters are very opaque due to the micro-plankton present. Carbon fixation in marine ecosystems is also strongly modulated by the many species that manufacture calcium carbonate tests, such as Haptophyte “coccoliths” or Foraminifera. Calcium carbonate dissolves at depth, and in present-day seas disappears below 4,500 meters deep. Nevertheless, the mineralization of the tests participates in the precipitation of carbon in the sediments and therefore in the global carbon cycle. The return of carbon but also of other elements necessary for metabolism (phosphate, nitrate, mineral salts, etc.) in the pool available for primary producers is done by consumers, parasites and viruses.

Traditionally, marine ecosystems have been dissociated into five major types: the shallow pelagic ocean, the deep ocean, the ocean floor with warm hydrothermal areas and the rest of the cold benthic and coastal areas (Figure 354).

The shallow pelagic ocean is the area of the oceans that receives light, called the photic zone, and in which most of the marine photosynthetic activity occurs. Its depth depends on the turbidity of the water and does not exceed 200 meters deep in the high seas. It is therefore a very shallow layer, but which will concentrate a large fraction of the oceanic biomass. High throughput sequencing analysis of planktonic organisms shows that in superficial pelagic ecosystems the biodiversity of eukaryotic protists is much greater than that of animals. However, the Eumycota are not very diverse there compared to terrestrial ecosystems. Instead, protozoa and microalgae, but also many parasites, are responsible for the great diversity. In this environment, primary production is ensured mainly by microscopic organisms, eukaryotic microalgae and cyanobacteria. The relative proportions of the contributions of the two groups are still not well known and it is possible to find widely diverging estimates in the literature. Nevertheless, the most recent data suggests that eukaryotic algae are the most important. Indeed, the analysis of satellite images makes it possible to estimate the quantities of photosynthetic pigments at the global level. Such pigments are relatively specific to each group of photoautotrophs (Table 5). The estimated quantities for the year 2000 measured in kg of chlorophyll a would be 2.5×109 for Haptophyta algae, 1.3×109 for Bacillariophyta and 1.1×109 for cyanobacteria. Note that this estimate ignores Dinophyceae, the third important group of eukaryotic microalgae in the oceans, possibly because their photosynthetic pigments are too diverse (Table 5) to give a correct estimate. A simple calculation therefore indicates that the eukaryotic biomass represents at least 77% of the total biomass measured in mass equivalent of photosynthetic pigments. The vast majority of carbon fixation in the oceans therefore effectively occurs through eukaryotic microalgae. They occupy the base of food chains in the shallow pelagic ocean. Another source occupying the base of the food chains in this environment are the recyclers, mainly osmotrophic bacteria, which will consume the organic matter in solution, part of which comes from the death of the various organisms present in the environment.

The image that emerges from this superficial pelagic ecosystem holds a complex dynamic, where microalgae continuously reproduce by their photosynthetic activity. These microalgae are consumed first by a host of phagotrophs, which includes mixotrophic algae such as Dinophyceae, but also exclusive phagotrophs such as Rhizaria, heterotrophic Stramenopila, and especially Alveolata, some known Ciliophora and others unknown. Eukaryotic protists are also the major consumers of bacteria and have a major impact on the dynamics of pelagic bacterial populations, an impact probably greater than that of bacteriophages. Recent data indicate that a majority of phagotrophic activity in the North Atlantic is in fact due to pico-planktonic algae, that is, having a size of the order of a micron, and partly feeding on bacteria with a mixotrophic type diet. They are responsible for around 70% of the consumption of bacteria which would provide them with around 25% of their final biomass. Small phagotrophic protozoa are in turn eaten by larger predators and so on. As predators increase in size, animals become more common among them, and animals occupy the last stages of these trophic chains. Eukaryotic and viral parasites seem to play an important role at all levels, because metagenomic analyzes show that they are diverse and abundant in these superficial pelagic ecosystems.

Little is known about the deep pelagic environment, in particular with regard to the populations of eukaryotic protists. It is fed mainly by organic matter that falls from the photic zone, mainly in dissolved form. The scarcity of food resources make it a very diluted environment regarding life. For example, the osmotrophic bacteria that appear to be at the base of the food chain in this environment have a concentration of around 104/105 cells per milliliter, while they are ten to a hundred times more concentrated in the photic zone. However, the large volume of this biotope, measuring more than 1.3 billion cubic kilometers, means that even if the organisms are not very concentrated there, their contribution to the total biomass is not negligible! Analyzes of eukaryotic organisms that consume bacteria from deep pelagic environments suggest that “nanoflagellates”, “nano-eukaryotes” ranging in size from two to ten microns, are the most important. They are responsible for the disappearance of 30% of daily produced bacteria through their phagotrophic activity. The dilution of the medium suggests a partially osmotrophic mixotrophic diet of these protists to make up for the scarcity of their prey. Little is known about the next links in the chain, but very large animals, like giant squids, inhabit the abyss…

Ocean floors are as poorly understood as the deep pelagic environment for the same reasons, namely the difficulty of collecting samples and keeping them intact on the surface. Deep hydrothermal vents are relatively well explored, where geochemical energy is used by prokaryotes to make biomass (Box 24). The rest of the nutrients in this environment derive from the surface and are the remainder that was not consumed in the water column. In these abyssal deeps, the average temperature is -1°C to 4°C, which does not allow much active metabolism. Protists and animals inhabit the hydrothermal vents and feed either by living in mutual symbiosis with chemo-autotrophic bacteria, by filtering water or by eating bacterial biofilms. Metagenomic data indicate that the majority of phagotrophic eukaryotic groups appear to be present in this ecosystem. The Eumycota are also represented and diverse, mainly Dikarya and Chytridiomycota. Cultures of such isolates show that these organisms are adapted to life at great depths and therefore are not temporary inhabitants coming from the surface. In cold deeps, the amount of organic matter is high and bacterial populations are dense with cell densities 10 to 10,000 times greater than on the surface, although the sedimentation provides few nutrients: of the order of 1 gram per square meter per year. The reason for the richness in bacteria probably stems from the bacterial metabolism being slowed down by cold temperatures, in addition to low consumption by bacterivorous protists. The organisms involved are not well known, but the explorations of the depths indicate that animals are not at the top of the food chains. Giant Foraminifera seem to dominate these ecosystems, especially in arctic and antarctic regions. They live in association with many other protists and Eumycota. These also live below the surface and can be detected up to 1,500 meters deep in the oceanic crust. Their metabolic activity nevertheless seems to cease around -350 meters.

Coastal environments are better known and very diverse: rocky coasts, beaches, estuaries and deltas, mangroves, etc. In these areas, the multicellular algae of the Phaeophyta, Rhodophyta and Chlorophyta generally dominate primary production. However, in coral ecosystems symbiotic zooxanthellae and zoochlorellae as well as Rhodophyta coralline algae are important. The contribution of coastal ecosystems to the global carbon cycle is not negligible although the geographic area is very small. In fact, coastal algae could contribute up to 30% of the total productivity of the oceans. They serve as a source of food and shelter for many animals, thus creating biotopes very rich in species, especially animals. Phagotrophic protists are also abundant there, in particular Ciliophora. These ecosystems are often modified by the presence of humans, probably more than pelagic ecosystems. In particular, the introduction by man of fertilizers on coastal lands can lead to their leaching towards the sea. This causes significant blooms of algae which can no longer be consumed sufficiently quickly by the animals. These eutrophication processes - eutrophy means rich in nutrients as opposed to oligotrophy which means poor in nutrients and which is often the normal state of an ecosystem - are more and more frequent, especially in regions with intensive agriculture, but also near large coastal towns. Algae accumulate, die and end up being degraded by bacteria that consume large amounts of oxygen. The decrease in the oxygen concentration as well as the release of organic and/or mineral matter such as ammonia strongly disrupt ecosystems and allow the proliferation of anaerobic bacteria which further accelerate the process, often resulting in the death of wildlife. However, not all algal blooms are created by humans and can have natural causes such as the seasonal upwelling of nutrients by updrafts (Box 26). The effects are no less devastating for wildlife (Box 26).


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