Interactions, feeding relationships and nutrient cycles

Symbiosis: the three ways of living together

Symbiosis means living together: two organisms of different species live in a close, long-lasting association. Cambridge divides symbiosis into three types, and the only thing that separates them is who gains and who loses. In parasitism the symbiont gains and the host is harmed. In commensalism the symbiont gains and the host is neither helped nor harmed. In mutualism both partners gain.

A parasite lives on or in its host and takes nutrients from it - blood, tissue fluid, mucus or already-digested food. The relationship is one-sided, so the host pays: it loses energy and material, grows more slowly, reproduces less successfully and is often more vulnerable to disease. A parasite that killed its host quickly would destroy its own food supply and habitat, so most parasites are adapted to take a steady supply without killing the host outright, and many are so specialised that they can live on only one species.

In commensalism one organism benefits and the other is unaffected. The benefit is normally transport, shelter or a hard surface to attach to, and the host is large enough, or tough enough, that the symbiont makes no measurable difference to it. Proving that a host is truly unaffected is difficult, which is why the examples you are given are ones where the symbiont is small and the host is enormous.

In mutualism both organisms benefit, and the benefits are usually different: one gains food, the other gains protection, transport or somewhere to live. Some mutualisms are so close that neither partner can survive without the other; others are looser, and each can live alone if it has to. In an answer, name the benefit to each partner separately, because 'they help each other' earns no marks.

Parasitism: copepods and marine fish

Copepods are small crustaceans, and most of them are free-living members of the zooplankton. Many species, however, are parasites of marine fish, attaching to the skin, fins, gills, lips or eyes. A parasitic copepod grips with hooked, anchor-like antennae and often a suction-cup base, and its mouthparts are modified into a siphon that pierces the host and sucks up fluid.

The copepod feeds on mucus, blood and tissue fluid, so its gut is a simple tube suited to a liquid diet, and it has lost the swimming legs and filtering mouthparts of its free-living relatives. The female carries her eggs in paired sacs, and the larvae are free-swimming for a short time, which is how the parasite finds a new host. Producing very large numbers of eggs compensates for the fact that most larvae never find one.

The host fish is damaged in several ways. Feeding on the gills destroys the delicate lamellae and reduces gas exchange; feeding on the skin leaves open wounds that admit bacteria and fungi; blood loss causes anaemia; and the constant irritation is a stress that reduces growth and reproduction. In the open sea a fish usually carries a few copepods and tolerates them, but in a crowded fish farm the parasites pass easily from one fish to the next and heavy infestations can kill, which is why salmon farmers monitor sea lice so closely.

Commensalism and mutualism: whales with barnacles, crabs with anemones

Barnacles on whales are the standard commensal example. A barnacle is a sessile crustacean that must settle on a hard surface, and the open ocean offers none - but the skin of a whale does. The barnacle gains a place to attach, transport across thousands of kilometres of ocean, and a steady flow of water over its feeding legs as the whale swims, which brings plankton within reach. Whale skin is thick and the barnacles are small, so the whale is not harmed in any way that matters, which is what makes the relationship commensal rather than parasitic. Whale barnacles such as Coronula diadema are found only on whales, and their free-swimming larvae settle directly onto the whale's skin.

Boxer crabs (Lybia tessellata) carry a small sea anemone in each claw and are the standard mutualism. The anemone's tentacles are armed with stinging cells, so a predator that comes for the crab is stung; the crab also waves the anemones at an attacker and uses them to wipe food from the substrate. The anemone gains transport to new feeding areas, which a sessile animal cannot manage alone, and it shares the food particles the crab disturbs. Both partners benefit, so the relationship is mutualism, and the crab can even split one anemone into two so that each claw is armed.

The key skill is telling the three types apart with the same kind of animal. A crab carrying an anemone that it also eats would be a parasite; a crab carrying an anemone it never harms and never helps would be a commensal; a crab whose anemone stings its predators while being carried to new feeding grounds is a mutualist. Always state the effect on each partner, separately and explicitly.

Feeding relationships: the vocabulary, chains and webs

A producer converts inorganic substances into organic substances using an energy source, and is the entry point of energy into a food chain. A consumer obtains its energy by eating other organisms, and is named by its position in the chain: a primary consumer eats producers, a secondary consumer eats primary consumers, and so on to tertiary and quaternary consumers. Herbivore, carnivore and omnivore describe the diet, not the position, so an omnivore can feed at more than one trophic level.

A food chain is a single linear sequence of organisms, each eating the one before it. The arrow shows the direction of energy flow, so it points from the organism being eaten to the organism that eats it: phytoplankton -> copepod -> herring -> cod. A food web is all the interlinked food chains in a community drawn together, and it is more realistic than a chain because most consumers eat more than one kind of food and are eaten by more than one predator.

A trophic level is the feeding position of an organism in a chain: the first trophic level is the producers, the second is the primary consumers, the third is the secondary consumers, and so on. A trophic level is a position, not a species, so in a web one species can occupy more than one level - a fish that eats both algae and small crustaceans feeds at two levels at once.

Decomposers - bacteria and fungi - break down dead organisms, shed parts and faeces, releasing the nutrients locked in them back into the environment. They are rarely drawn at the end of a marine food chain because they feed from every trophic level at once, but they are essential, and in the deep ocean they work on the marine snow that falls from above.

Producers: photosynthesis and chemosynthesis

Producers are the organisms that harness an energy source to convert inorganic substances into organic substances, and that organic matter carries energy into the rest of the food chain. There are two routes. Photosynthetic producers use light energy, and include phytoplankton such as diatoms and dinoflagellates, macroalgae and seagrasses. Chemosynthetic producers use the chemical energy released when inorganic substances such as hydrogen sulfide are oxidised, and they do not need light at all.

Photosynthesis can be summarised as a word equation: carbon dioxide + water -> glucose + oxygen, with light energy absorbed by chlorophyll supplying the energy. The point of the equation is not the formula but what it does: it captures the energy of sunlight and stores it in the chemical bonds of glucose, so energy that was unavailable to animals becomes available to the food chain. The oxygen released is a by-product.

Chemosynthesis is the reason life exists at hydrothermal vents, far below the depth where any light penetrates. Bacteria there oxidise hydrogen sulfide, ammonia or methane and use the energy released to build organic compounds from carbon dioxide. Those bacteria are the producers of the vent community, and some live as symbionts inside animals such as the giant tubeworm Riftia pachyptila, which has no mouth or gut and depends entirely on them.

Not all of the glucose made stays as glucose. Some is used to build biomass - cellulose, starch, proteins, lipids and new cells, which is what a plant or alga is made of and what a consumer eats. Some is broken down in respiration to provide usable energy for the producer's own life processes, summarised as glucose + oxygen -> carbon dioxide + water. Respiration happens in every living organism, producer and consumer alike, and it is the reason energy is lost at every step of a food chain.

Productivity, energy losses and ecological pyramids

Productivity is defined as the rate of production of biomass per unit area or volume per unit of time, so its units always have three parts - for example g m-2 year-1, or g m-3 year-1 for a volume of water. Productivity depends on light (which falls with depth, turbidity and in winter), on temperature and above all on the supply of dissolved nutrients. Where primary productivity is high, more energy enters the food chain, so the ecosystem can support more consumers, larger populations and, often, more trophic levels - which is why upwelling areas and estuaries support such huge fisheries.

Energy is lost at every transfer, and four processes account for almost all of it. Respiration burns much of the food an organism takes in and releases the energy as heat, which cannot be used by the next trophic level. Some of the organism is not eaten at all - shells, bones, scales and tough structural tissue. Some of what is eaten is not digested and is lost in faeces, and some is lost as excretion of nitrogenous waste. The result is that only a small fraction of the energy, often quoted as about 10 per cent, is passed on, so a pyramid of energy always narrows sharply at each level and chains cannot be long.

The arithmetic is straightforward. If phytoplankton in a square metre fix 24 000 kJ in a year and the copepods that eat them contain 2 400 kJ, the transfer is 2 400 / 24 000 x 100 = 10 per cent, and the energy lost between those two levels is 24 000 - 2 400 = 21 600 kJ m-2 year-1. If the fish that eat the copepods contain 240 kJ, only 1 per cent of the original energy has reached the third trophic level.

Pyramids of energy show the energy at each trophic level per unit area per unit time, in kJ m-2 year-1. They are always upright, because energy is lost at each transfer - an inverted pyramid of energy would mean the consumers held more energy than their food contained. Pyramids of numbers show how many individuals are at each level and can be inverted: one large host fish can carry hundreds of parasitic copepods, so the parasite level is wider than the level below it. Pyramids of biomass show the mass of living material at each level and are usually upright, but they can be inverted in the sea, because phytoplankton have a tiny standing crop at any one moment yet reproduce and are eaten so fast that their total production is large, while the zooplankton that feed on them live longer and accumulate more mass. A plankton or algal bloom temporarily reverses that picture, since the phytoplankton biomass becomes very large for a few weeks before the zooplankton catch up.

Nutrients: the molecules organisms build and the elements they need

A nutrient is a general term for any substance an organism needs for growth, repair, energy or normal metabolism. Nutrients are not one kind of chemical: they include gases such as carbon dioxide, ions such as Mg2+, CO3 2-, PO4 3- and NO3-, and organic compounds such as carbohydrates, lipids and proteins. That is why 'nutrient' in a marine question can mean a dissolved nitrate ion taken up by a diatom or a piece of food swallowed by a fish.

Large molecules are built from smaller ones. Starch and cellulose are both polymers of glucose; proteins are polymers of amino acids; lipids are built from fatty acids and glycerol. The elements follow from the monomers: carbohydrates contain carbon, hydrogen and oxygen; lipids contain the same three; proteins contain carbon, hydrogen, oxygen and nitrogen, and some also contain sulfur.

Some nutrients supply essential elements with specific biological roles, and these are worth learning as a list of five. Nitrogen is needed to make proteins, chlorophyll and DNA. Carbon is the backbone of all organic compounds. Magnesium is needed to make chlorophyll. Calcium is needed to make bones, shells and coral skeletons. Phosphorus is needed to make DNA and bones.

The reservoir of dissolved nutrients: what empties it and what refills it

Most of the nutrients that producers need are soluble, so the ocean holds an enormous reservoir of them dissolved in the water, available to producers and consumers alike. Phytoplankton take up nitrate, phosphate and other ions straight from seawater, and those ions are then passed along the food chain in the bodies of the organisms that eat them. The reservoir is depleted by uptake into organisms, which is why the sunlit surface waters of the open ocean are often nearly empty of nitrate and phosphate - the producers have used them up.

Six processes replenish the reservoir, and each one brings the nutrient from a different place. Upwelling lifts cold, nutrient-rich water from the deep ocean to the surface. Run-off carries nitrates and phosphates from the land into coastal water. Tectonic activity releases nutrients from rock, both through weathering at the coast and through hydrothermal and volcanic input on the seabed. Dissolving of atmospheric gases adds carbon dioxide to the surface water. Excretion by animals releases soluble nitrogenous waste, and decomposition by bacteria and fungi releases the ions locked in dead tissue and faeces.

Marine snow is the link between the sunlit surface and the deep ocean. It is made of aggregates of dead plankton, faecal pellets, mucus and fine detritus that clump together and sink, carrying energy-containing organic material downwards out of the photic zone. In the deep water, bacteria decompose it and release the nutrients at depth - nutrients that are out of reach of the phytoplankton until upwelling brings them back to the surface. This is why the deep ocean is a nutrient store rather than a nutrient dump.

Two consequences follow. Harvesting removes nutrients: when fish are caught and taken ashore, the nutrients in their bodies leave the ecosystem, and a heavy fishery can therefore remove nutrients faster than they are recycled. And productivity is limited by the nutrient in shortest supply - adding more of a nutrient that is already plentiful makes no difference if another one is missing. In much of the open ocean that limiting nutrient is nitrate or phosphate, which is why the central gyres are clear blue deserts while the upwelling zones are green and support vast fisheries.

• Upwelling: deep water, rich in nitrate and phosphate, rises to the surface - the main route back for nutrients that sank as marine snow.
• Run-off: rivers, streams and coastal drainage carry dissolved nitrate and phosphate from the land into the sea.
• Tectonic activity: weathering of rocks and hydrothermal or volcanic input release minerals, including nutrients, into the water.
• Dissolving of atmospheric gases: carbon dioxide dissolves at the surface and forms carbonate and hydrogencarbonate ions.
• Excretion: animals release soluble nitrogenous waste, such as ammonia, directly into the water.
• Decomposition: bacteria and fungi break down dead organisms and faeces, releasing the ions they contain.
• Depletion: uptake by producers and consumers locks the nutrients into biomass, removing them from the dissolved reservoir.

The marine carbon cycle

Carbon is stored in four places that matter here: as carbon dioxide in the atmosphere and dissolved in seawater, in organic compounds in living and dead organisms, in fossil fuels, and in rocks containing carbonate ions such as limestone and chalk. The cycle is the set of processes that move carbon between them, and Cambridge limits it to six.

Photosynthesis removes carbon dioxide from the air and from seawater and fixes it into organic compounds in producers; respiration releases it again from living organisms; decomposition releases it from dead tissue and waste as bacteria and fungi break them down. Combustion - the burning of fossil fuels - returns carbon that has been locked away for millions of years to the atmosphere very quickly, which is why it matters so much.

Formation of fossil fuels is the slow opposite of combustion: dead organisms are buried before they can decompose, and over millions of years heat and pressure convert them into coal, oil and gas. Formation of rocks containing carbonate ions happens in the sea, where organisms use dissolved calcium and carbonate to build shells and coral skeletons; when they die, the carbonate settles as sediment and eventually becomes limestone or chalk. Weathering of those rocks reverses it, because rainwater with carbon dioxide dissolved in it is a weak acid that dissolves the carbonate and carries the carbon back towards the ocean and the atmosphere.

Describe the cycle as a set of processes rather than as a circle with a start, and be precise about direction: photosynthesis and carbonate formation remove carbon dioxide from the water or air, while respiration, decomposition, combustion and weathering return it.