12.1 Modes of Nutrition

Every organism needs a source of chemical energy to make ATP and a source of matter from which to build cells. Autotrophs synthesize organic carbon compounds from inorganic carbon, usually carbon dioxide. Photoautotrophs supply the required energy through photosynthesis; chemoautotrophs obtain it by oxidizing inorganic substances such as ammonia, nitrite, hydrogen sulfide, hydrogen or iron compounds. The bacterium Nitrosomonas is a nitrifying chemoautotroph: it oxidizes ammonia to nitrite and uses the released energy for carbon fixation. Calling it nitrogen-fixing would be incorrect because nitrogen fixation converts atmospheric nitrogen into ammonia.

Heterotrophs obtain organic carbon from other organisms or their products. They also obtain chemical energy by oxidizing organic molecules. The labels autotroph and heterotroph describe carbon acquisition, whereas phototroph and chemotroph describe energy source; these axes should not be collapsed. Metabolic diversity is especially clear among bacteria and archaea, including organisms that use light without oxygen-producing chlorophyll systems, organisms that oxidize inorganic chemicals, and organisms that consume organic compounds.

Oxygen use is another independent classification. Obligate aerobes require oxygen for aerobic respiration. Obligate anaerobes are harmed by oxygen or cannot use it and rely on anaerobic pathways. Facultative anaerobes use aerobic respiration when oxygen is available but can switch to fermentation or anaerobic respiration when it is absent. A nutritional category therefore does not determine oxygen requirement: heterotrophs and autotrophs can each include organisms with different respiratory strategies.

Some species change nutritional mode. Euglena can photosynthesize in sufficient light but can obtain organic material heterotrophically when light is inadequate. Such organisms are mixotrophs. Mixotrophy exposes the limits of forcing all organisms into exclusive plant-like and animal-like feeding categories and shows why classifications are revised when observations do not fit an existing scheme.

Consumers ingest living or recently killed organisms, digest food internally, absorb soluble products and assimilate them into their tissues. Herbivores consume autotrophs, carnivores consume animals, and omnivores feed from both plant and animal sources. Detritivores such as earthworms, millipedes and many crustaceans ingest non-living particulate organic matter and digest it internally. Their fragmentation of litter increases surface area for microbial action.

Saprotrophs carry out external digestion. Fungi and many bacteria secrete hydrolytic enzymes onto dead material or waste, then absorb soluble products. Decomposer activity converts organic compounds into forms that can eventually return inorganic mineral nutrients to soil or water. A saprotroph is therefore not simply a small detritivore: the location of digestion is the decisive distinction. Both groups connect dead biomass to nutrient cycling, while their respiration transfers chemical energy to the surroundings as heat.

A trophic level is a feeding position. Producers occupy the first trophic level because they introduce newly fixed organic carbon into the biological part of the ecosystem. Primary consumers feed on producers, secondary consumers feed on primary consumers, and tertiary consumers feed on secondary consumers. In a food-chain arrow, the arrow points from the food to the feeder because it shows the direction in which chemical energy and matter are transferred. It does not point toward the organism being eaten.

Real communities are food webs rather than isolated chains. One consumer usually has several foods and several predators. An omnivore may occupy different trophic levels in different feeding events: a fox eating fruit acts as a primary consumer, while the same fox eating an herbivorous rodent acts as a secondary consumer. Trophic level is therefore a property of a particular pathway, not an unchangeable badge attached to every species.

Food webs remain selective models. They rarely show changing diets through life, seasonal resource shifts, parasites, decomposer routes, competition, facilitation or abiotic constraints. Generalists use a broad range of resources and conditions, so they may persist when one resource disappears. Specialists occupy narrower niches and can exploit them efficiently, but habitat loss or loss of a particular food can affect them severely. Neither strategy is universally superior.

In parasitism, the parasite benefits while the host is harmed. Endoparasites live inside a host; ectoparasites live on its surface. Attachment organs, resistant reproductive stages, high reproductive output, reduced digestive structures and mechanisms that avoid host defenses can support a parasitic life. A successful parasite does not normally benefit by killing its host immediately, because the host supplies habitat and resources, although severe disease can still occur.

Mutualism benefits both partners. Reef-building corals house photosynthetic dinoflagellates within their tissues; the symbionts receive a protected, nutrient-rich environment, while photosynthate and oxygen support coral metabolism. Some ants protect aphids from predators and receive carbohydrate-rich honeydew. These outcomes are ecological costs and benefits, not acts of intention. Environmental change can alter the balance, and some associations range from optional to obligate.

Feeding adaptations reveal selection by resource properties. Broad molars with cusps crush or grind plant tissue and seeds; sharp crests shear flesh; enamel thickness resists wear and fracture. Predator adaptations include forward-facing sensory systems, rapid acceleration, grasping structures, venom, camouflage and cooperative hunting. Herbivores may have long digestive tracts, microbial fermentation chambers and continuous tooth growth. Inferences from teeth or skulls indicate capability and likely diet, but wear marks, isotopes and associated remains strengthen the conclusion.

Plants are active participants in feeding relationships. Spines, hairs, silica, thick cuticles and tough tissues deter ingestion mechanically. Alkaloids, tannins and other secondary metabolites reduce palatability, impair digestion or act as toxins. Some defenses are induced after damage and some recruit predators of herbivores using volatile signals. Defenses cost resources, so their distribution reflects trade-offs among growth, reproduction and probability of attack.