What is the source of energy for heterotrophs in cellular respiration?

Food quality and quantity

Heterotrophs (consumers, including bacteria) live by consumption of biomass or nonliving organic matter. Due to the chemical composition of biomass (disregarding skeletal material or support structures) across all heterotrophs falls within a relatively narrow range, carnivores that feed on other heterotrophs are assimilating approximately the same mixture of elements that they will need in order to synthesize their own biomass (skeletal material and support structure typically pass through the gut unassimilated). Hence, their food quality is high. Detritivores also benefit from this carryover of elemental mixtures from one kind of organism to another, although detritus is more likely to show some selective loss of elements such as nutrients that would alter the balance typical of living biomass.

Unlike heterotrophs, photoautotrophs assimilate elements separately from water or, if they are rooted vascular plants, from sediments. For example, carbon is derived from H2CO3 and related inorganic carbon forms dissolved in water, and phosphorus is taken up separately as phosphoric acid that is dissolved in water. Large imbalances may develop when some essential components are much more abundant than others because the inorganic substances required to synthesize biomass are taken up separately. For example, phytoplankton has a high carbon:nutrient ratio under nutrient-depleted conditions. Thus, autotrophs face greater challenges than carnivores in assembling the necessary ratios of elements to synthesize biomass, but also herbivores (and bacteria) can experience imbalances of elements.

The approximate ratios of elements that are characteristic of autotrophic biomass have been extensively studied. Characteristic ratios of carbon to nitrogen and phosphorus are often the greatest focus of analysis. Because carbon is the feedstock for photosynthesis and phosphorus and nitrogen are the two additional elements that are often in short supply for conversion of photosynthetic products (carbohydrates) to other molecule types that are needed for the synthesis of protoplasm (e.g., amino acids which are rich in N, or RNA which is rich in P). The importance of C:N:P ratios in aquatic organisms was first brought out by Alfred Redfield (1890–1983), who discovered that healthy oceanic phytoplankton show a characteristic molar C:N:P ratio of about 106:16:1. Thus, the nutrient status of a phytoplankton community can be judged to some degree from the elemental ratios. For example, a phytoplankton community suffering phosphorus deficiency may show a C:P ratio of 500:1 rather than 106:1, as predicted by the Redfield Ratio for well-nourished phytoplankton. The analysis of elemental ratios for diagnosis of elemental imbalances is termed “ecological stoichiometry” (Sterner and Elser, 2002).

Imbalances in elemental ratios in one trophic level can create imbalances in the diet and thus an inefficient transfer of energy to the next trophic level. This is particularly true between primary producers and herbivores. For example, plants suffering phosphorus scarcity may pass biomass with a high C:P ratio to their grazers. The grazers must then consume extra food in order to obtain the correct balance for the synthesis of their own biomass because of an imbalance of elements in the food. Similarly, an especially low C:P ratio (e.g., 50:1) will provide an oversupply of phosphorus (e.g., when bacteria are consumed), a large part of which is released to the environment without generating any biomass.

Another strategy that herbivores may employ in improving the elemental balance of food intake is to consume heterotrophs in addition to autotrophs (omnivory, which is feeding at multiple trophic levels) as animals and bacteria are generally more nutrient rich than autotrophs. Thus, combining the consumption of a phosphorus-rich food (high quality) with a carbon-rich food (often available in high quantity, e.g., grass), enables a more efficient use of ingested mass than a single food type.

3.4 Physiological and Metabolic Aspects

Mushrooms are heterotrophs (i.e., they cannot perform photosynthesis). Consequently, they feed on organic matter. Chemical energy and useful materials are obtained from the digestion of substrates. Fungi are versatile in producing lytic enzymes active on many types of chemical bonds. Without their decomposing power, the earth would be possibly covered with dead organic matter, especially materials rich in lignin and cellulose [62].

The cell walls allow the passage of molecules having molecular weights up to nearly 4700. The hyphae membranes perform active and passive selective absorption and excretion of substances. Amino acids, for example, are transported through cell membranes by an active process (membrane associated proteins require energy to catalyze the process). In contrast, simple sugars are passively absorbed [48].

Mushrooms are aerobic, using oxygen as a final electron acceptor, but are also able to operate anaerobic metabolic pathways by using organic molecules as final electron acceptors, generating products such as alcohols and acids [75,98]. They can grow very slowly in almost anaerobic conditions, but have no chance to compete with anaerobic bacteria if this condition is maintained for too long a time.

Mushrooms are able to reproduce both by sexual and asexual processes. When the mycelial net is divided, each part can continue to grow and form a new complete organism. When the carpophores are divided, most pieces have the potential to originate a new mycelial net. Some spores, such as those produced by the common button mushroom (A. bisporus), can directly generate secondary mycelium. Each new colony formed by these means are clones of the original organism, thus these are asexual processes.

The sexual cycle, within recombination events and the generation of variation, occurs by the combination of primary mycelia, generated by uninucleated spores, and the formation of heteronucleated secondary mycelium, which can produce mushrooms. The whole mushroom tissues are heteronuclear. Recombination events occur in basidia, where cariogamy gives rise to new spores. Thus, cultivations initiated with spores can be used as a source of variation in genetic improvement programs [99].

Depending on the ecological role of the mushroom species, metabolic pathways are complexly linked to the metabolic routes of the respective host or symbiotic partner. For example, species of the genus Cordyceps produce toxins that affect the nervous systems of infected insects, inducing behaviors (migration to open and high places) that enhance spore dispersion after the fruiting bodies emerge from the carcass of the insects [100]. Fig. 14.6 shows the fruiting body of a mushroom emerging from an insect carcass.

Also, mycorrhizal nets are known to exchange water and nutrients with the radicular system of vegetables. Mycelium can penetrate places and digest materials that the roots cannot, and supply the vegetable with further resources of water and mineral salts. In exchange, the vegetable uses its photosynthetic capacity to feed the mycelium with nutritive substances, including carbohydrates. Mycelium nets are not only nutrient channels, but also transmit information signals between vegetables. Symbiont physiology and metabolism complement in a synergistic manner [40].

Mushrooms' secondary metabolism is notably rich. Besides constitutive and vital metabolites, many substances are produced by these organisms as collateral by-products of the biotransformation of the substrates' molecules. Some of these are synthesized and accumulated just for being more stable and less toxic for the fungi than the respective precursor molecules. But some of these pathways are privileged by evolutive pressure, because certain secondary metabolites directly contribute for the strain’s survival (e.g., antibiotics and cryoprotectants) [101,102]. Some of these secondary metabolites are also useful for human applications.

Secondary metabolites are usually produced at a very slow rate and are difficult to accumulate to reasonable amounts under artificial cultivation. Moreover, the abilities to synthesize secondary metabolites that are less directly decisive for the survival of the species are less stable and more easily lost through evolution. These metabolic pathways, which direct the production of useful secondary fungi metabolites, should be better understood in order to be better controlled and preserved [103].

3.3 Diel cycling of microbial communities generated by indirect interactions

Direct interactions between phototrophs and heterotrophs are limited only to those species that can directly utilize photosynthetic products usually through the colonization of primary producers (Aylward et al., 2015; Frischkorn et al., 2018; McCarren et al., 2010; Seymour et al., 2017; Straub et al., 2011). In many cases, the diel cycling originating from a phototrophic microorganism is propagated relatively indirectly toward heterotrophic microorganisms as a result of complex biotic interactions within microbial loops (Deng, Cheung, & Liu, 2020; Fang et al., 2019; Hu, Connell, Mesrop, & Caron, 2018; Zhao et al., 2019). In this process, phototroph-derived organic matter plays an important role in transmitting and generating diel cycling in heterotrophs. For example, in Prochlorococcus—which is a dominant cyanobacterium in the marine environment—cell production and mortality rates are known to be tightly synchronized to the day/night cycle (Ribalet et al., 2015). Such diel cycling of organic matter contributes to shape community diel rhythms in aquatic ecosystems.

Viruses are major factors affecting such diel cycling of organic matter released from phototrophic microorganisms (Figs. 1D and 2). In Section 2.3, for example, we describes how cyanoviruses reproduce viral progeny linked with photosynthetic activity during the daytime, and then lyse host cyanobacterial cells at night, resulting in the generation of diel cycling of organic matter (Aylward et al., 2017; Kimura et al., 2012; Liu, Liu, et al., 2019; Morimoto et al., 2019; Welkie et al., 2019; Yoshida et al., 2018). Furthermore, cyanoviruses redirect host metabolism for efficient viral reproduction during the infection (Hurwitz & U'Ren, 2016; Puxty, Millard, Evans, & Scanlan, 2015; Zimmerman et al., 2019), suggesting that organic matter content can be affected by viral infection. Such DOM, released via viral infection, indeed increases the diversity of DOM and induces the succession of heterotrophic microbial composition (Zhao et al., 2019) as well as altering gene expression in non-infected cells (Fang et al., 2019). Thus, viral-induced diel cycling of DOM and the resultant specific responses shape taxon-specific diel cycling in heterotrophic microorganisms in the environment (Figs. 1D and 2).

Another important factor affecting diel cycling in heterotrophic microorganisms is protistan grazing (Deng et al., 2020; Hu et al., 2018) (Fig. 1E). This process may reflect temporal adaptations to optimize resource availability. For example, it has been suggested that, during the daytime, the stoichiometric composition of the unicellular cyanobacterium Crocosphaera does not meet the nutrient demand of the protistan grazer; it is not until the cyanobacterium starts to fix atmospheric nitrogen during the dark period that it becomes preferentially grazed (Deng et al., 2020). Another adaptation to nighttime grazing activity may be associated with the reduced photosynthetic oxidative stress that predators may experience when ingesting phototrophic prey during the day (Uzuka et al., 2019). Such feeding during the night period provides additional DOM resources for the remaining photo- and heterotrophic community, leading to the induction of their response and generation of diel cycling (Fig. 1E). Indeed, Kelly et al. (2019) found increased diversity of genes associated with degradation of various carbohydrates at night. Altogether, diel cycling of phototrophic microorganisms is propagated toward heterotrophic microorganisms as a form of organic matter via viral lysis, or protistan grazing within the microbial community: a major mechanism generating diel rhythms for microorganisms that cannot utilize photosynthetic products as they are.

This section has shown that the primary diel cycle generated in phytoplankton according to the day–night cycle is transmitted to heterotrophic microbial organisms by direct or indirect microbial interactions. This microbial “interactions-driven” diel cycle generation not only seems to be specific to phototroph and heterotroph, but may also occur in heterotroph–heterotroph interactions (Fig. 1F). For example, different timings in expression maxima of genes associated with cellular activity were observed between co-occurring members of marine heterotrophic prokaryotes such as SAR11, SAR116 and SAR324 clades (Vislova, Sosa, Eppley, Romano, & DeLong, 2019). Although this may indicate simple temporal partitioning of their niche space due to competition avoidance, it may also be possible that this time lag was generated from the difference in the diel cycle transmitting pathway through microbial interactions in each taxon (e.g., differences in the number or species involved in the interactions). Consequently, this also suggests that bacteria exhibit higher diel variation in timing of the expression of different functional gene groups compared to phototrophic organisms. Recent studies revealed that transcriptional activity of individual populations of viruses is synchronized with their putative hosts (Aylward et al., 2017; Kolody et al., 2019; Martinez-Hernandez et al., 2020; Yoshida et al., 2018). Therefore, the viral lysis-mediated supply of organic matter multiply occurs in a day with different rhythmic pattern, and it may play a role in modulating the time lags of time partitioning in different heterotrophic taxa (Figs. 1G and 2). Because of the taxon-specific differences in nutrient requirements, the diel cycle of each heterotrophic taxon can depend on its specific supplier's diel lysis cycle with delay (Fig. 1D–E). Although dominant species of primary producer were shown to be as central determinants of overall community diel transcriptome dynamics (Aylward et al., 2015), considering the fundamental importance of microbial interaction for community structuring (Liu, Debeljak, Rembauville, Blain, & Obernosterer, 2019; Needham & Fuhrman, 2016), these heterotroph–heterotroph interaction also seems to be a modulating factor of diel community dynamics.

Introduction

Physiologically and morphologically, as obligately osmotrophic heterotrophs, the Peronosporomycetes are ‘fungi.’ They are phylogenetically separate from the Mycota (an alternative taxonomic name for the kingdom Fungi) and sometimes are described as Oomycota. The biflagellate, anisokont but nonstraminipilous Plasmodiophorales and the uniflagellate Chytridiomycetes likewise are unrelated. The Chytridiomycetes may be an early offshoot from the phylogenetic line leading to the nonflagellate Mycota.

The Peronosporomycetes are algae fungi or cellulose fungi, form a class within the Stramenopilen, and therefore are much closer to brown algae, golden algae, and diatoms used as the genuine fungi. The taxa include several plant pathogens, such as the causative agent of late blight of potato and downy mildews.

The Peronosporomycetes include the most numerous, most important, and earliest known (with mid-eighteenth century reports for Saprolegnia on fish) water molds (see Figure 1). Study of the Peronosporomycetes has received attention since the 1840s, because of the sociohistoric significance of late blight of potato (Phytophthora infestans) and downy mildew of vines (Plasmopara viticola). Some of the most damaging groups of pathogens of food crops are Peronosporomycetes.

Many of the parasitic species, other than the root pathogens, have restricted host ranges; most are obligate parasites not available in axenic culture (a culture of an organism that is entirely free of all other ‘contaminating’ organisms). The downy mildews (Peronosporales on advanced dicotyledons and Sclerosporales on panicoid grasses) are leaf and stem parasites; nematodes and rotifers are parasitized by the Myzocytiopsidales; arthropods by the Saprolegniales and Salilagenidiales; vertebrates by the Saprolegniales and Pythiales; and other Peronosporomycetes by related fungi.

Most species of Peronosporomycetes are freshwater or terrestrial; few are strictly aquatic, but many are characteristic of wet marginal sites or are from seasonally or intermittently waterlogged soil. Aqualinderella fermentans is the only obligate anaerobe. In terrestrial and freshwater ecosystems, the saprobic Peronosporomycetes have a major ecological role in degradation and recycling, as deduced from estimates of activity and biomass production from spore population sizes. Many of the saprobic and facultatively parasitic species are abundant, with worldwide distributions. A few taxa are confined to the pantropics or to a continental landmass, but strictly psychrophilic or thermophilic species have not been identified. Saprobic taxa survive in estuarine conditions, but such habitats may not be their primary niche: A few parasitic Peronosporomycetes are oligohaline or marine.

The Peronosporomycetes contains at least 900 and perhaps as many as 1500 species, depending on the species concepts used for the obligate parasites of angiosperms. The principal families in terms of numbers of species, frequency of isolation, and economic importance are the Peronosporaceae, Pythiaceae, Sclerosporaceae, and Saprolegniaceae.

Marine Microfossils

Marine microfossils, including protistan autotrophs and heterotrophs, have traditionally been thought to be one of the broadly construed ‘taxonomic’ groups most affected by the end-Cretaceous extinction. Review of the family-level fossil record largely bears this out. Among the major marine microfossil groups (Figure 5), only diatoms fail to exhibit a Maastrichtian extinction-intensity peak. For coccoliths, benthic foraminifera, and radiolaria this peak is more-or-less isolated from background patterns of family richness variation, suggesting they reflect operation of a causal process or processes that was/were confined to the Maastrichtian. For dinoflagellates and planktonic foraminifera though, the Maastrichtian peak appears to be part of a larger pattern that encompasses both the Maastrichtian and Paleocene intervals. Overall Maastrichtian extinction intensities across all six marine microfossil groups are less than 20%, suggesting an overall species loss similar to that of the total Maastrichtian estimate (see above).

Interpretation of this Maastrichtian–Danian microfossil record is complicated by several factors. A considerable controversy regarding the correct interpretation of Cretaceous planktonic foraminiferal species routinely found in lowermost Danian sediments continues. Some specialists regard Cretaceous isotopic ratio values obtained from the analysis of particular species' skeletons, along with the widespread chaotic disruption of bedding patterns in lowermost Danian sediments, as indicative of widespread shelf failure at the Maastrichtian–Danian boundary with consequent reworking of Cretaceous species into Danian sediments. This shelf failure was presumably caused by the physical shock of bolide impact (see below), along with subsequent earthquakes. Others regard the recovery of Danian isotopic results from other Cretaceous species, the pristine preservation of many millions of Cretaceous microfossil skeletons in Danian sediments (fully comparable with those of undoubted Danian species and distinct from obviously reworked Cretaceous species), and the fact that Danian occurrences of Cretaceous species exhibit both a clear and consistent biogeographic signal of greater penetration into the Danian in higher latitudes (where the effect of boundary disturbances is known to be reduced) as evidence for the survivorship of some species into Danian times.

Relatively low Maastrichtian extinction intensities for dinoflagellates and diatoms have been accounted for by noting that the biology of these groups includes resting cyst stages and that these may have enhanced their overall survivorship potential. Species-level data for both groups from the K–Pg section on Seymour Island, Antarctica do not support this interpretation. Seymour Island cyst-forming dinoflagellates and diatoms exhibit progressive turnover patterns across the K–Pg boundary, suggesting that extinction-inducing environmental changes were not confined to any single horizon. There is an increase in diatom resting spores in the Upper Maastrichtian interval of this high-latitude section, but this occurs throughout the succession and is not confined to any single stratigraphic horizon. Moreover, since no modern diatom resting spore has been revived successfully after more than two years' dormancy, this sets an inferred maximum duration of environmental disruption that could be tolerated before wholesale extinction of the indigenous diatom flora would occur. There is no evidence of sudden and very short duration environmental disruption in these high-latitude sections.

Introduction

Secondary production is the generation of tissues by heterotrophs, regardless of the fate of the generated materials; death, consumption by predators, molting, etc. do not matter, as the tissue was produced at some point before any of those occurred. Secondary production is ultimately the result of a series of ecological efficiencies, starting with the assimilation efficiency (AE) of consumed food, and then the efficiency at which assimilated materials are converted to production, or net production efficiency (NPE). The overall efficiency of conversion of ingested materials to production is the gross production efficiency, or GPE. These efficiencies, and thus secondary production, are influenced by a wide range of intrinsic and extrinsic factors spanning scales from individuals and populations, to communities and habitats, to regional climate differences. Secondary production has been referred to as the “ultimate dynamic variable” because it integrates all elements of fitness, including abundance, biomass, growth, survival, and other important population metrics in to one value (Benke, 1993). Secondary production studies draw upon elements of both population biology and ecosystem ecology (Benke and Whiles, 2011).

The two main components of secondary production are biomass and individual growth rate. As such, biomass can be considered a predictor of secondary production in some cases, but not in cases where there is great disparity between biomass and individual growth rate. For example, high biomass of adult unionid mussels in a mussel bed in a river would not necessarily translate into high production because of slow individual growth rates of the long-lived adult mussels. Conversely, moderate levels of biomass of dipteran (true flies) Chironomidae larvae in sediments of a stream pool can result in relatively high production because of rapid individual growth rates. Thus, the occasional use of biomass as a proxy for production can be inaccurate; however, empirical estimates of production, rather than application of statistical models based on biomass, temperature, and literature values of P/B, are better. Obviously, the highest production estimates are associated with animals with both high biomass and high individual growth rates.

The annual production to biomass ratio (mean annual production divided by mean annual biomass), a measurement of turnover, is a proxy for individual growth rate. Annual P/B values vary tremendously across stream-dwelling taxa as a function of individual growth rates and life histories. Small, rapidly growing taxa often have annual P/B values that exceed 10, and in some extreme cases exceed 100. In contrast, slower growing species may have values around 1–2, and even < 1. Many univoltine stream invertebrate taxa with moderate growth rates have annual P/B values ~ 5–6. Annual P/B values can be used to estimate actual tissue turnover times by dividing the number of days in a year by the P/B. For example, a stream invertebrate with an annual P/B of 5 will have a tissue turnover rate of 365/5 = 73 days for complete turnover of the tissues as the individual grows.

Introduction

Cellular life depends upon a constant supply of energy. Energy, in the form of ATP, depends upon the availability of glucose. Glycolysis is a series of 10 chemical reactions that convert the energy stored in a molecule of glucose into four molecules of ATP and two molecules of NADH. In the presence of oxygen, the two pyruvates produced as a result of glycolysis can be further catabolized into three molecules of CO2 and a molecule of GTP. The resulting NADH and FADH2 molecules then carry their electrons to the electron transport chain, driving the production of more ATP through oxidative phosphorylation.

What is the energy source for heterotrophs?

Do heterotrophs get energy from cellular respiration?

What cellular process do heterotrophs use to get energy?

What is heterotrophs in cellular respiration?