12.2: Production, Consumption, and Decomposition

Life is based on the production of an enormous variety of organic compounds, each of which serves a different function in the cells of living organisms. Organic compounds are created when carbon atoms are combined in chains or rings. Chemical properties of a compound are determined by the number of carbon atoms and how they are arranged, and by the number, position, and elements in other groups of atoms (containing, for example, oxygen, nitrogen, sulfur, or phosphorus) attached to the carbon chains or rings. Organic compounds made by living organisms can contain hundreds of carbon atoms and attached groups arranged in an enormous number of ways. Even the simplest living organisms consist of a bewildering array of organic compounds.

The most important use of organic molecules by living organisms is to provide energy for the biochemical processes that control their growth, movement, feeding, and reproduction. Energy is needed to combine carbon atoms into organic compounds. This energy can be released from organic compounds by decomposition (breakdown) of their molecules through catabolic pathways like respiration and fermentation. All living organisms use catabolic pathways to provide their needed energy.

There are two fundamental types of organisms. Those of the first type, autotrophs, create their own food from inorganic compounds by using an external source of energy. The process of converting inorganic compounds to organic matter is called primary production. Autotrophs use a portion of the food they synthesize to fuel their own life processes through respiration. Plants, algae, and some bacteria and archaea are autotrophs. Organisms of the second fundamental type, heterotrophs, cannot make organic compounds from inorganic compounds and must obtain organic matter as food. Animals and most bacteria and archaea are heterotrophs. Recent research has shown that the distinction between autotrophs and heterotrophs is not as simple as once thought. There is now strong evidence that certain microbial species in the oceans, collectively called mixotrophs, possess the genetic machinery for both autotrophy and heterotrophy and it is likely that these species can behave either as an autotroph or a heterotroph depending on the availability of light, nutrients and dissolved food (carbon in organic compounds that can be “burned” to produce fuel for metabolic processes).

The distribution of life depends on the distribution and growth rate of autotrophs. Heterotrophs can live only where and when autotrophs supply enough food and nutrients. Autotrophs synthesize organic matter by one of two fundamentally different mechanisms: phototrophy (predominately photosynthesis) and chemosynthesis.

Phototrophy

The dominant process of primary production in the surface mixed layer of the oceans is photosynthesis (CC14). The basic raw materials needed for photosynthesis are carbon dioxide, water, and nutrients. An ample supply of carbon dioxide is dissolved in seawater in the form of carbonate and bicarbonate ions (Chap. 5). Of course, water is readily available. The source of energy used to combine carbon atoms is solar radiation. Therefore, light availability is an important determinant of where photosynthesis can occur and at what rate. Photosynthesis in the oceans is dominated by phytoplankton that include species of algae and cyanobacteria. The general term for organisms using light energy to produce organic matter for growth of living organisms is phototroph (CC14). Organisms that perform traditional photosynthesis are photoautotrophs. 

Non-Photosynthetic Phototrophy

Certain archaea, and bacteria are phototrophs that do not use chlorophyll or photosynthesis and do not release oxygen. Instead they use a simpler metabolic pathway to capture light energy using compounds called rhodopsins. Rhodospins do not rely on the electron transport chain to generate energy, but instead use light-activated proton pumps. When these pumps are activated by light, they pump hydrogen ions across the membrane generating ATP. 

Microbes utilizing rhodopsin-dependent phototrophy are heterotrophs, unable to fix carbon dioxide on their own (i.e., photoheterotrophs). Organisms capable of photoheterotrophy tend to thrive in waters where nutrient concentrations are very low. This is especially true in iron-depleted environments where chlorophyll synthesis, which requires iron, is limited. Additionally, rhodopsins are not dependent on temperature and can absorb a wide range of light waves, allowing these microbes to thrive in marine environments where other organisms may not be able to. Recent research also shows that certain marine bacteria and phytoplankton use the rhodopsin based pathway to supplement photosynthesis when conditions are unfavorable for photosynthesis (e.g., cold, low iron concentrations, and/or high light intensity). This versatility speaks to why the use of rhodopsins is more widespread than originally thought. In fact, it is estimated that over 60% of microbial cells found in ocean surface environments carry rhodopsin genes. While it is not exactly clear what impact rhodopsins have on biogeochemical cycling in marine environments, it is clear that the use of rhodopsins is advantageous, increasing energy efficiency and survival in microbes living in an ever changing ocean. 

Nutrient elements, such as nitrogen, phosphorus, magnesium, and sulfur, are also necessary for phototrophy. For example, nitrogen atoms are part of all protein and chlorophyll molecules, which are essential to photosynthesis (CC14). The following sections of this chapter explain how the distribution of light and nutrients and the processes that affect their distribution control the distribution of life in most of the oceans.

Chemotrophy 

Some types of bacteria and archaea can convert carbon dioxide and water to organic matter by using energy from chemical reactions rather than from light. This process, called “chemosynthesis,” uses energy obtained from chemical reactions including the conversion of methane gas to carbon dioxide and water; and the oxidation of inorganic compounds a process known as lithotrophy) including:

• hydrogen sulfide to sulfate
• metals from a reduced to an oxidized form, including iron, arsenic, manganese and uranium
• hydrogen to water
• ammonia to nitrite
• nitrite to nitrate

Note that “oxidation” does not necessarily require oxygen atoms to be present. Oxidation is the general term for a chemical reaction in which an electron is removed from a molecule. This means that molecules without oxygen atoms can act as the electron acceptor for example in the conversion (oxidation) of compounds, for example ferrous iron (Fe²⁺) compounds to compounds of ferric iron (Fe³⁺). These reactions release energy that can be used by organisms. Some organisms can perform chemosynthesis in the complete absence of oxygen so chemosynthesis is possible almost anywhere on Earth and other planets even if they have no free oxygen. Indeed, there was no (or extremely low concentrations of) oxygen in Earth’s atmosphere or free dissolved oxygen in Earth’s oceans when life first evolved. The first life forms were almost certainly chemosynthetic. 

Hydrogen sulfide, metal sulfides, hydrogen, and methane are all oxidized readily by chemical processes in environments where oxygen is present. Therefore, most of the reduced compounds that fuel chemosynthesis are not currently present in the atmosphere or in most ocean waters, because these environments contain appreciable concentrations of oxygen. In reducing environments where chemosynthetic fuels are abundant, free oxygen is absent because it has been consumed by respiration or chemical reactions. In the oceans chemosynthesis currently occurs in geographically limited environments where both reducing and oxygenated waters meet and mix, including hydrothermal vents and some seeps. Chemosynthesis also occurs in seafloor or near seafloor anaerobic environments such as sediment pore waters and oxygen free dead zones (Chap. 15). It also occurs in transitional environments, such as oxygen minimum zones discussed later in this chapter, where dissolved oxygen is not totally depleted but where complex microbial communities include chemosynthetic species. 

Chemosynthesis is common near hydrothermal vents on oceanic ridges and volcanoes, but it also occurs in water released to the oceans by vents or in seeps from ocean sediment. Some vents or seeps are in subduction zones, where water with dissolved methane and hydrogen sulfide is squeezed out of subducted ocean sediment. Others are in certain areas of the continental shelf where groundwater from the continents migrates through the sediment and accumulates methane or hydrogen sulfide. Chemosynthesis also occurs in the surface sediments of salt marshes and swamps, at the interface between oxygenated surface waters and oxygen-deficient bottom waters in fjords, and in other ocean areas where oxygen is depleted. Living microorganisms have been found in many extreme environments, including deep within ocean sediments and even within oceanic crust that is millions of years old. Many of these organisms are probably chemosynthetic, obtaining their energy needs by chemical reactions such as the combination of hydrogen and carbon dioxide to form methane.

Secondary Production and Decomposers 

Nonautotrophic marine organisms are heterotrophic and must obtain food to supply all or part of their needs for organic matter and energy. Heterotrophs convert organic compounds to carbon dioxide and water during respiration, thereby releasing energy that was originally derived from the sun (phototrophy) or chemical sources (chemotrophy). The released energy fuels the metabolic processes of the heterotrophs.

Heterotrophs include all animals and many species of archaea, bacteria and fungi. Animals eat plants, algae, bacteria, archaea, other animals, and organic detritus formed by the partial decomposition of dead organisms. Animals that are herbivores eat photosynthetic organisms, carnivores eat only animals, omnivores eat herbivores, carnivores or other omnivores, and detritivores eat detritus. The production of animal biomass by animals that consume primary producers or other heterotrophs is called “secondary production.”

Heterotrophs use food inefficiently (CC15). They excrete part of their food as solid waste organic particles, called fecal pellets, or as dissolved organic matter in liquid excretions. Humans are no exception to this rule.

Many species of microorganisms, including some bacteria, archaea, and fungi, are heterotrophs but are also called decomposers. Aerobic decomposers obtain energy from organic particles or dissolved organic compounds, which they convert to carbon dioxide and water. Decomposition can also occur in oxygen-poor environments through anaerobic processes that result in the production of methane gas. Some species of microorganisms are capable of growing either autotrophically or heterotrophically, depending on their environment.

Phototrophic and chemosynthetic organisms, as well as food-eating and decomposing organisms, function together as a community within an ecosystem (Fig. 12-3). In an ecosystem, carbon dioxide, water, and nutrients are synthesized to organic matter, which is then processed through herbivores, carnivores, omnivores, detritivores, and decomposers. Each of the organisms in this food chain breaks down some organic matter and releases carbon dioxide, water, and nutrient elements back to solution.

Figure 12-3. Simplified model of the energy and mass (organic matter and nutrient element) cycles in a marine ecosystem. Sunlight (or chemical energy) and dissolved nutrients are essential for autotrophs to perform the phototrophic (or chemosynthetic) primary production of organic matter from carbon dioxide. Energy collected and mass produced by autotrophs is cycled through various heterotrophs, followed by decomposers. The mass is transferred back to solution by decomposers, and the energy is eventually released as heat. Primary production of organic matter takes place only in the photic zone (except for limited areas where chemosynthesis occurs). All other steps in the ecosystem take place partly in the photic zone and partly in the aphotic zone.

Nutrients and other elements are recycled through ecosystems (Fig. 12-3). Recycling is an important factor in the distribution and abundance of ocean life. In parts of the oceans, life is limited because nutrients are not recycled fast enough to support photosynthesis.