Essential Nutrient and Microbiota Transfers Through Trophic Levels

Table of Contents

Abstract

Like for any engineering system, it is senseless to try and understand a natural habitat without analyzing the sum of its parts. It with this insight that scientists develop trophic networks, ecological models that help in more accurately measuring the true impact that one species has on all others in its environment. This article will shed a light on some of the primary ways we can quantify these impacts ranging from vitamin to fatty acid to even microbiota flow. Whilst these various aspects all exhibit different dynamics, certain concepts such as that of trophic levels give a clearer understanding on how there nonetheless remains certain fundamental properties which form the basis of a deeper investigation into a given habitat.

Introduction to Trophic Levels

All organisms go through metabolism, the process in which they use energy and matter in order to grow, perform actions, and sustain their vitality (Brown et al., 2004). The metabolic rate is, as Brown et al. (2004) put it, “the fundamental biological rate […] it is the rate of energy uptake, transformation, and allocation.” In order to sustain its metabolism, organisms must be able to access and gather the materials they need from their environment; if not, it may result in the organism’s growth being stunted or the organism’s death (Brown et al., 2004). For example, many previous studies have shown that plant growth is limited and controlled through the abundance of certain compounds or elements in its environment, such as water and nitrogen (Figure 1) (Brown et al., 2004).

Fig. 1 A graph illustrating how nitrogen abundance in the thylakoids of various plant species affects oxygen evolution (generation). Note that as the amount of nitrogen in the thylakoid decreases, there is less oxygen being produced, indicating lowered metabolic rate. Adapted from (Evans, 1989).

Many organisms are able to obtain the materials and energy they need through mainly abiotic means; producer species are able to capture energy, either through photosynthesis or chemosynthesis, and use it, along with the simple inorganic materials that they have available to them such as water and carbon dioxide, to create complex organic molecules like glucose as a mechanism of energy storage (Corliss et al., 1979; Lindeman, 1942). These molecules can then be used in other metabolic processes, either as an energy unit in respiration to drive important reactions like protein synthesis (Petrie, 1943), or as a building block itself for the development of more complex organic molecules such as cellulose or starches (Nelson & Pan, 1995; Purushotham et al., 2020).

Not all organisms are able to produce or obtain all of the nutrients and energy they need to survive through their abiotic environment, however. Organisms that exhibit this inability, termed consumer organisms, must prey on and consume other organisms to sustain their metabolic processes (Lindeman, 1942). One of the main reasons why this occurs is because the organisms are not able to fix energy from abiotic sources into chemical bonds themselves – they must take it from structures such as glucose in other organisms (Corliss et al., 1979; Lindeman, 1942). This idea of consumers is not limited to organisms that are unable to perform energy fixation through – for example, the Venus flytrap must consume insects in order to suffice its nitrogen needs; since it grows in nitrogen-poor soil, it must find alternative methods of obtaining nitrogen as it cannot rely on obtaining it from abiotic environmental sources (Schulze et al., 2012).

This stepwise motion of energy and nutrient transfer from prey to predator to higher-predators is the foundation for trophic organization, the systemization of the transfer of nutrients and energy from one level to the next (Figure 2) (Brown et al., 2004; Lindeman, 1942). It places species in set levels depending on their place in the food chain, with producer species at the bottom, herbivores (primary consumers) next, and then further omnivores/carnivores (secondary consumers, tertiary consumers…) until you reach the top of the food chain (Lindeman, 1942; Polis & Strong, 1996).

Fig. 2 A diagram illustrating the stepwise motion of trophic levels and their creation from food webs. Note that the primary production required (PPR) to sustain each trophic level decreases by a factor of 10 per trophic level. Adapted from (Pauly & Christensen, 1995).

This transfer of nutrients and energy from organism to organism, and therefore trophic level to trophic level, is not efficient. Mean energy transfer efficiencies between trophic levels hover around 10% (Lindeman, 1942; Pauly & Christensen, 1995). This loss is generally accounted through several factors: firstly, there will be energy loss in energy transformations or transfer between species, as well as in an organisms’ own metabolic processes such as in respiration (Brown et al., 2004). This is governed by the Second Law of Thermodynamics, which states that “any real process can only proceed in a direction which results in an entropy increase”(Schneider & Kay, 1994) and is fundamentally shown through the equation:

∫ {dQ \over T}≤0

where Q is equal to the heat transfer in calories and T is in degrees Kelvin. This equation may be explained in that any energy transfer will result with the loss of useful work, as though the useful energy may be transferred into heat, the heat may not be converted entirely back into useful work (Schneider & Kay, 1994) .

Another source of loss of energy and matter is the inability of a predator to digest certain tissues and organic materials – energy and molecules in these tissues and materials are considered lost, as the predator is unable to break down and use these molecules and the energy stored in the chemical bonds of these molecules (Lindeman, 1942).

Both the proceeding phenomena generally explain the natural distribution of productivity from trophic level to trophic level, commonly represented through an ecological pyramid; since energy and mass transfer is inefficient from prey organisms to predator organisms, there is less access to nutrients and energy for organisms in higher trophic levels, therefore higher trophic levels will be less productive than lower levels (Trebilco et al., 2013). Although this generally holds true for pyramids of total biomass as well, occasionally these pyramids may look like stacks of equal sizes or even an inverse pyramid (Trebilco et al., 2013). It is hypothesized that these oddly shaped pyramids (Figure 3) are due to the fact that there are outside energy and materials may be obtained outside the predator-prey relations in the trophic levels that are investigated (Trebilco et al., 2013; Woodson et al., 2018). This may also be explained through the idea that the productivity and reproductive capacity of the producers are high, which leads to them being more available to predator species whereas their own populations are kept relatively low (Gasol et al., 1997).

Fig. 3 Example of an Atypical Ecological Pyramid. The size of the rectangles is representative of each level’s respective biomasses. Adapted from (Woodson et al., 2018).

Using the principles developed through the trophic level model, scientists may be able to gain a better sense of an organism’s niche in its ecosystem. For example, in Post’s (2002) research, he uses the principle of matter and energy flow from level to level and applies it to isotopic analysis of nitrogen-15 and carbon-13 in order to estimate an organism’s trophic level, allowing scientists to gain greater insight into the role that an organism may play in an ecological system.

Challenges of Trophic Level Modelling – The Cyclic and Nonlinear Movement of Nutrients

Before proceeding to further topics in this paper, it is important to recognize some of the fallacies that occur with the model of trophic levels.

Due to the law of conservation of mass, matter cannot spontaneously burst into and leave existence, which means that the finite amount of matter in an ecosystem must be recycled from the higher trophic levels back to the producers at the bottom. Modelling through trophic levels are generally unable to show this principle; the simplicity of the model becomes its own downfall, as it only shows that there is lost or wasted energy and matter as you go up trophic levels, not how they are recycled back to the bottom. For example, it does not show how seabird waste expelled through guano can return nitrogen back into its surrounding environment, which may be taken up again by algal species and renew the movement of nutrients through trophic levels (Gagnon et al., 2013). It also doesn’t show how the nutrients are ecologically recycled from organisms that died without being preyed on, whether it would be an apex predator or any other organism.

It is also challenging to explore the multifaceted nature of ecosystems with the model of trophic levels. Post (2002) puts this quite well: “the trophic level concept, however, is limited by the strict use of discrete trophic levels and its limited ability to capture the complex interactions and trophic omnivory that are prevalent in many ecosystems.” Some studies, such as Pauly and Christensen (1995), try to correct for this by taking the “average” trophic level of an organism to find an overall trophic level. However, this method subjects itself to the issues raised by Post (2002) regarding food webs, in that the model becomes “time consuming to construct,” “subjective in their resolution and scope,” and “holds all trophic links to be of equal importance.”

However, modelling through trophic levels still has a great amount of merit; it permits quantification and representation of energy and matter transfer through an ecosystem, giving scientists insights and explanations into the many biochemical phenomena that governs current and even historical ecosystems (Post, 2002; Trebilco et al., 2013). It also, through the simplification of predator-prey interactions in an ecosystem, allows scientists to better understand and define the function each organism has in its environment (Post, 2002). Because of these merits of trophic modelling, throughout the rest of this paper we will be employing it to explore the transfer of fatty acids, vitamins, and microbes throughout the ecosystem.

Fatty Acids and their Transfer Throughout Trophic Networks

Fatty acids (FAs) are acidic molecules composing lipids. These molecules are chains of hydrocarbons with a carboxylic acid attached on one end. There exists saturated fatty acids, consisting of solely single bonds, as well as monosaturated fatty acids and polyunsaturated fatty acids (PUFAs), consisting of one or more double bonds respectively (Figure 4) (Kaçar, 2019). PUFAs can include omega-3 fatty acids and omega-6 fatty acids (ω3-PUFA and ω6-PUFA) (Ruess & Müller-Navarra, 2019). FAs can usually be found in the form of phospholipids, which are integral to forming cell membranes of organisms, as well as triacylglycerol which also attaches three fatty acid molecules to a glycerol molecule and can be found in adipose tissues (Kaçar, 2019).

Fig. 4 Nomenclature, Chemical Structure and Examples of Fatty Acids. Note that the red arrows on the image on the left point towards the double bonds of the fatty acid to signify the unsaturation of the molecule. Also note that the “n-3” in the image on the right can be written as “ω3” too. Adapted from (Mráz, 2011; Mukumbo & Muchenje, 2016; Sampels, 2009).

These lipid components have three characteristics which allow them to be particularly useful for studying food webs. First of all, different types of species can modify fatty acids to elongate them (elongation) or to add more double bonds (desaturation) (Iverson, 2009). However, organisms from higher trophic levels (i.e. higher order predators) are less able to modify fatty acids any further. Therefore, if a type of fatty acid is synthesized by only one species (producer) and if that molecule is found in a predator, we can conclude that the predator’s diet consists of the corresponding producer. Second of all, fatty acids are usually not broken down like carbohydrates or proteins once they are consumed by a species (Iverson, 2009). Third of all, the fatty acids are stored in tissues and accumulated for a long time making it a reliable indicator of an organism’s diet (Iverson, 2009). These aspects allow fatty acids to serve as biomarkers for inter-species relationships mainly from the aquatic world where these biomolecules are most prevalent (Ruess & Müller-Navarra, 2019).

Circulation of Fatty Acids on Land and in Water

Indeed, when it comes to aquatic species, eukaryotic algae which include unicellular phytoplankton and macroalgae are among the main producers of ω3-PUFA and ω6-PUFA (Iverson, 2009; Ruess & Müller-Navarra, 2019). Algae are among the only species that can use enzymes that specifically synthesize long chain polyunsaturated fatty acids (LC-PUFAs) (Iverson, 2009). They do so by first converting Acetyl-CoA from the citric acid cycle to fatty acids using acetyl-CoA carboxylase as well as fatty acid synthase in the chloroplasts (Harwood, 2019). From there, they use enzymes called elongases and desaturases for elongation and desaturation of the produced FAs in the endoplasmic reticulum. One of the main LC-PUFAs algae produce are the docosahexaenoic acid (DHA) and eicosapentaenoic acid (EPA) starting from the α-linolenic acid (ALA), a simpler ω3-PUFA (Figure 5). LC-PUFAs that are then situated in the thylakoid membranes of algae work similarly to accessory pigments during the light reactions of photosynthesis (Ruess & Müller-Navarra, 2019).

Fig. 5 “Major Pathways in Synthesis of Long-Chain Polyunsaturated Fatty Acids (LC-PUFA) in Eukaryotic Organisms”. (Ruess & Müller-Navarra, 2019) The pathway for algae is represented in green and that of higher animals is represented in light blue. “+C2” indicates elongation. “Carboxyl dd” and “methyl dd” indicate desaturation. LA; linolenic Acid. ARA; arachidonic acid (20:4ω6). ALA; α-linolenic acid. EPA; eicosapentaenoic acid (20:5ω3). DHA; docosahexaenoic acid (22:6ω3). Adapted from (Ruess & Müller-Navarra, 2019).

In the animal kingdom, LC-PUFAs are essential in many biological functions since they aid with the selective permeability and viscosity of cell membranes as well as cell growth (Ruess & Müller-Navarra, 2019). EPA and DHA are the main FAs present in nervous tissues. Additionally, ARA (arachidonic acid) and EPA are used to synthesize hormones like eicosanoids for cell growth and inflammation responses (Ruess & Müller-Navarra, 2019). Unlike algae, animals from higher trophic levels (consumers) are less efficient at synthesizing ω3-LC-PUFAs de novo due to their inability at integrating a double bond between the methyl end and the ninth carbon in the fatty acid chain (Iverson, 2009). However, these organisms can mainly convert ALA (a smaller ω6-PUFA) into another type of LC-PUFA called ARA using delta-6-desaturases (Figure 5). Due to the necessity in PUFAs, such as ω3-LC-PUFAs, consumers acquire these nutrients primarily through diet (Ruess & Müller-Navarra, 2019). In the aquatic ecosystems, phytoplankton is consumed by zooplankton which is then ingested by higher order consumers (Figure 6). For instance, using FAs as trophic biomarkers, scientists concluded that larval herring and the fin whale’s diets consist of copepods as they are rich in 20:1 and 22:1 FAs which were found in the fish’s tissues (Iverson, 2009). Another study found that jellyfish was present in the diets of ocean sunfish and leatherback turtles due to the presence of trans-6-hexadecenoic acid (Iverson, 2009). If PUFAs, especially LC-PUFAs, are essential to both animals in water and on land, how are they present in terrestrial food webs?

Fig. 6 Aquatic and Terrestrial Trophic Networks for Polyunsaturated Fatty Acids (PUFAs). ALA; α-linolenic acid. EPA; eicosapentaenoic acid. DHA; docosahexaenoic acid. Adapted from (Ruess & Müller-Navarra, 2019).

Most researchers agree that the terrestrial animals gain LC-PUFAs through the ingestion of aquatic animals (Ruess & Müller-Navarra, 2019). Figure 6 illustrates the transfer of EPA and DHA from algae to other species through food chains. In fact, many land animals such as bears and eagles can consume aquatic species namely crustaceans and fish which enable this transfer of fatty acids from one habitat to another (Ruess & Müller-Navarra, 2019). Nonetheless, vascular plants are the main terrestrial organisms capable of producing PUFAS. These plants provide ALA and LA to higher order land species which can convert these FAs into LC-PUFAs to some extent (Figure 6) (Ruess & Müller-Navarra, 2019). For example, cattle and chickens are capable of converting ALA from plants to EPA and DHA and this reaction occurs in their liver tissues (Gladyshev et al., 2015).

As previously mentioned, there are limited sources of PUFAs, particularly ω3-PUFAs, for animals including humans. As such, fish such as salmon, trout and shellfish as well as PUFA-rich fish oils derived from oily fish liver tissues are consumed for their accessibility and abundance in these nutrients (Huang et al., 2018; Qiu et al., 2020). However, aquaculture and fish oil production are not sustainable options, since fish populations are decreasing due to overexploitation and climate change (Qiu et al., 2020). Qiu et al. (2020) mention the studies of the genome responsible for synthesizing LC-PUFAs. By analyzing the genes associated with the production of LC-PUFAs, scientists are attempting at making a transgenic production of the nutrients using oilseed crops and oleaginous microorganisms, which would help in alleviating this pressure on fish populations.

Vitamins’ Role and Traffic in Trophic Networks

Traffic of vitamins through trophic networks is particularly important. Vitamins also figure amongst essential nutritional biomolecules which most animal species are unable to synthesize or do not synthesize in sufficient amounts (Drouin et al., 2011). Their role is essential for proper cell function and development (Vijayalakshmy et al., 2018). Vitamins are divided into two major groups. The first ones being lipid soluble molecules such as vitamin A, D and E which are stored in tissues. The second ones are water-soluble molecules such as vitamin C and the B vitamins which cannot be stored in tissues. Therefore, many species of animals need a constant supply of them (Vijayalakshmy et al., 2018). Exchanges of these biomolecules in the food chain are provided by mutualistic relationships between producer and demander as well as via classical predator pray interactions (Ruess & Müller-Navarra, 2019).

Vitamin A

To begin with, vitamin A is required to prevent night blindness. It is also necessary for optimal growth and reproduction in the animal as well as maintenance of normal pressure in the cerebrospinal fluid (Green & Fascetti, 2016; Rubin & De Ritter, 1954). There are two different types of ways in which a particular living organism can meet its daily requirement of vitamin A. One way would be through preformed vitamin A which must be administered directly though the animal’s diet. The other way would be to acquire them through the processing of provitamin A carotenoids which are the precursors of the vitamin A molecule (Green & Fascetti, 2016; Rubin & De Ritter, 1954). Photoautotrophic organisms namely plants, algae and photosynthetic bacteria are the only organisms who can synthesize in their chloroplast and chromoplasts carotenoids (provitamin A) and implement them in the food chain for other animals to consume them  (Huang et al., 2017; Rubin & De Ritter, 1954; Ruess & Müller-Navarra, 2019; Saini et al., 2015). Provitamin A is not yet in its active form without being transformed into retinol by the living organism who consumed it. The process of activating is done by metabolizing intracellularly the provitamin A which then becomes vitamin A. An example of Provitamin A metabolism is illustrated in Figure 7. The ability of Vitamin A precursors to form the vitamin itself is referred to as provitamin A activity (Green & Fascetti, 2016). The process depends solely on the action of the enzyme carotene dioxygenase and on whether the precursor has at least one unmodified β-ionone ring present in the molecule (Green & Fascetti, 2016; Saini et al., 2015).

Fig. 7 Example of the Provitamin A Metabolism. The first arrow indicates the reaction of β-carotene central cleavage by 15, 14’ dioxygenase. The first step results into two retinal molecules. Both molecules are then reduced through enzyme retinal reductase. The final product is two retinol molecules. Adapted from (Keijer et al., 2005).

There exist more than 600 naturally occurring carotenoids in the wilderness (Green & Fascetti, 2016). β-carotene is one example of a carotenoid that has a 100% provitamin A activity as it contains two unsubstituted β-ionone rings as -opposed to β-cryptoxanthin which only has one unsubstituted β-ionone rings. Another type of naturally occurring carotenoids is Lycopene which does not have any β-ionone rings. This means the molecule does not have provitamin A activity. For each of these carotenoid molecules, there will be a different yield of retinol molecules. In the case of β-carotene, the yield would be two retinol molecules due to the two β-ionone rings. For the β-cryptoxanthin, the yield would be one retinol molecule due to its single β-ionone rings. The Lycopene is one of many vitamin A molecules which doesn’t yield any retinol molecule due to its lack of β-ionone ring (Green & Fascetti, 2016; Saini et al., 2015).

Now, the proportion of each carotenoid in nature depends on the production of carotenoids of each autotrophic species. Figure 8 illustrates the different ratios of carotenoids that each phytoplankton produces. Through the table we notice that β-carotene is mainly produced by all phytoplankton which makes it the primary source of vitamin A in most organisms (Green & Fascetti, 2016; Huang et al., 2017).

Fig. 8 Distribution of Carotenoids in Phytoplankton. Adapted from (Huang et al., 2017).

Moreover, the way each species fulfills its requirement of vitamin A is influenced by its metabolism of the β-carotene molecule. We notice that in the case of the omnivores, the efficiency of conversion of β-carotene to vitamin A seems to be at its peak due to the vast variety of their diets which make β-carotene less prevalent and therefore the organism needs to have the best conversion efficiency to assure its requirements of vitamin A.  In the case of the herbivores, they are less efficient than the omnivores due to the sheer amount of β-carotene present in their diets (photoautotrophic organisms). For the carnivores, their efficiency is way lower due to the fact that their diets is already rich in preformed vitamin A. Thus, in most cases,  they don’t need to convert β-carotene to obtain their vitamin A requirements (Green & Fascetti, 2016). In short, species can obtain vitamin A by metabolizing provitamin A provided by autotrophic organisms or by directly ingesting preformed vitamin A. However, each species will meet its needs for vitamins A differently since animals have different carotenoids metabolism (Green & Fascetti, 2016).

B-Group Vitamins

To continue, the B-group vitamins are essential in various metabolic and reproductive activities. Thiamin (B1) is a co-factor for carbohydrates metabolism (Vijayalakshmy et al., 2018). This molecule contributes to the conversion of blood sugar into ATP by being a cofactor of the two enzymes: dehydrogenases and decarboxylases (Ruess & Müller-Navarra, 2019). The active form of the Thiamin molecule is ThDP and is made up of a thiazole and a pyrimidine element. The two elements are initially synthesized distinctively. Thiamine-phosphate synthase then forms ThMP by combining the two moieties. ThMP is then phosphorylated to give a ThDP. Figure 9 shows this synthesis process. This process is handled by plants, fungi and yeast (Du et al., 2011; Yoshii et al., 2019).

Fig. 9 The Biosynthesis of Thiamin in Bacteria. The thiazole moiety of thiamin is derived from an oxidative condensation of 1-deoxy-D-xylulose 5-phosphate (DXP) (a), cysteine (b), and glycine or tyrosine (c). When the thiazole and pyrimidine moieties are formed, ThiE will coupled them to be thiamin monophosphate and followed by a phosphorylation step to give ThDP (d). Adapted from (Du et al., 2011).

Folic acid (B9) is involved in catalyzing nucleotide syntheses and therefore contributes to DNA repair and replication. Autotrophic organisms are responsible for the synthesis of this vitamin (Ruess & Müller-Navarra, 2019).

Cobalamin (B12) is another typical product of bacteria and archaea since plants cannot synthesize this molecule. In fact, the molecule is transferred into animal or plant tissues through symbiotic interactions. In the aquatic environment, the phytoplankton are the organisms responsible for the implementation of this specific vitamin into the ecosystem’s food chain. This vitamin is required in the synthesis of amino acids and of DNA. It is also necessary in providing carbon-based free radicals for reactions that remove the non-acid hydrogen atoms (Ruess & Müller-Navarra, 2019). The synthesis of cobalamin consists of approximately 30 enzymatic steps handled by bacterial organisms (Moore & Warren, 2012; Yoshii et al., 2019). In essence, it is clear that the biosynthesis of these molecules is of great importance in trophic networks since they occupy important roles in cells. Likewise, the majority of these biosynthesis are initiated by autotrophic organisms, mainly bacteria.

Essential Nutrients and Microbial Flow through a Trophic Network

Whilst the transfer of essential nutrients from one trophic level to the next can be intuited from the widely accepted saying “you are what you eat,” we can analyze the composition of a predator at a slightly less microscopic level by studying its microbiota and the influence that its prey’s microbiota has on its composition. Of course, an animal’s microbiota varies according to a wide variety of other factors, including but limited to age, genetics, infections, habitat, and more (Hasan & Yang, 2019; Wen & Duffy, 2017), but we will limit ourselves to the microbial interactions from one trophic level to the next.

Before proceeding, we must emphasize that this article remains focused on the transfer of essential nutrients. Indeed, an animal’s microbiota is crucially important in this function. Most obviously, the digestion, and therefore the absorption, of nutrients is largely determined by an animal’s gut microbiota (Hooper et al., 1998). Moreover, the microbiota can come to play more indirect functions in how essential nutrients are transferred. For instance, unlike humans, cat skin cells do not produce vitamin D in response to UV light (Morris, 1999). They are thus dependent on their diet to obtain this vital hormone. As a result, when a cat eats a mouse, it is reliant on its prey’s skin and fur microbiota to be properly functioning as it is this in these areas that a mouse synthesizes vitamin D (Mallya et al., 2016).

Through such examples, we can begin to understand the importance of broadening our scope of investigation when it comes to research in the domain of essential nutrient flow through a trophic network. Whilst research in this field of microbiota transfer remains quite new, the literature nonetheless points to their being a strong link between an animal’s microbiota and that of its prey within a trophic network (Dion-Phénix et al., 2021; Kennedy et al., 2020).

The Blue Tit, the Caterpillar, and the Leaf

As aforementioned, microbiota is often associated with many factors unrelated to diet, which likely was a catalyst for a study published earlier this year by Dion-Phénix et al. (2021) which analyzed the microbiota dependence between predators and prey when controlling for geographical factors. The trophic network that the authors studied was composed of the blue tit, caterpillars, and the leaves of downy and holm oaks. In this network, the bluet tit is a predator of the caterpillar, and the caterpillar is a consumer of the leaf.

The results of the study were, if basing ourselves off the trophic levels of the studied organisms, not surprising. The DNA sequences of the microbiota of all three showed significant overlap, however the greatest overlap was noticed between the blue tit and the caterpillar, as well as the caterpillar and the downy and holm oak leaf. This suggests that just like total energy, microbiota diversity is progressively lost as it travels through a trophic network and studying organisms with adjacent trophic levels is important for observing the least possible microbial variation. A crucial difference with energy transfer through trophic levels however is, as hypothesized by Dion-Phénix et al. (2021), that microbiota transfer could potentially occur horizontally rather than simply vertically, meaning that a potentially important part of the microbiota overlap could be caused by another organism which was excluded from the trophic network, may that be from another insect or maybe even from another bird or common predator.

When it comes to the loss of significant part of the microbiota between trophic levels, the absolute cause is by no means definitive, though we can certainly maintain that it is likely that much of the microbiota is killed by the acidic digestive system of the organism which consumes its original host (Paula et al., 2015) and/or that the foreign microbiota is outcompeted by the local one. There are, to our knowledge, no studies having quantified the impact of the various potential causes across species.

All in all, this paper supports the model in which we can partially analyze microbiota as being transferrable cross-species. Thus, just like the nutrients which flow through a trophic network, the properties relating to microbiota that govern nutrient flow may also be, at the very least, somewhat transferable.

The Grey House Spider and the Fly: “You are what you eat, sometimes…”

Another study conducted last year by Kennedy et al. (2020) found similar results as the one conducted by Dion-Phénix et al. (2021), however the former focused also on measuring the microbiota of the predator at various intervals of time post having eaten its prey. Indeed, this additional factor may give us a more nuanced view of the extent to which microbiota is transferable from one trophic level to the next. In the given study, the researchers fed both grasshoppers, Gryllodes, and flies, Drosophila, to Badumna longinqua spiders, also known as grey house spiders. As expected, there was significant increase in overlap in the species gut-microbiota 24 hours post feeding, and it even continued to increase up to 72 hours afterwards reaching approximately 20% increased overlap. The researchers nonetheless continued to measure the overlap up to 648 hours, or 27 days, afterwards and found that at that time, it had dropped down to only a 5% increased overlap. In light of these results, we can start to build a more dynamic theory of microbial acquisition, or “transient” theory (Kennedy et al., 2020). As an organism, such as spider in the case of the aforementioned study, preys upon another organism, it acquires some of similarities with its prey’s microbiome, however this acquisition is somewhat temporary and wanes over time. Consequently, if this microbial shift is to provide an evolutionary advantage, it would be necessary to consume the given prey on a regular basis. We can therefore also affirm that the current evidence points to a certain “baseline” microbiota that nonetheless has the ability to be quite malleable. It remains to be seen whether these changes could become more permanent if an organism were to be exposed to a specific microbiota on a reoccurring basis.

Conclusion

In conclusion, we discussed some of the essential nutrients’ traffic and roles throughout trophic levels.  We elaborated on the different fatty acids found throughout the plant and animal kingdoms along with their roles, more specifically in the forming of the cell membranes. We explained how certain fatty acid characteristics could give us an insight on the role of a particular species in the food web and their contribution into trophic networks. Moreover, some examples of lipid-soluble and water-soluble vitamins such as vitamin A and B-group vitamins respectively were developed as a way to explain their importance in the trophic networks. Their traffic in the food webs is initiated by their biosynthesis in autotrophic organisms. Finally, we defined the important implication of animal microbiota throughout trophic networks. By analyzing the transfer of nutrients and microorganisms from one species to another, we can deeply understand inter-species relationships as well as the origins and importance of nutritious elements. As such, scientists can use this knowledge for bioengineering applications that include omega-3 fatty acid rich fish oils or oilseed crops.

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