Physiological characteristics large standing stocks and unique positions of microbes in pelagic food webs

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The special roles of microbes in food webs and biogeochemical cycles are often explained by some of their physiological characteristics coupled with their large standing stocks. The physiological characteristics include: high turnover and growth rates, e.g. up to 5 day-1 for phytoplankton), 3 day-1 (bacteria) and 1 day-1 (protozoplankton) (White et al. 1991; Ducklow and Carlson 1992; Rose and Caron 2007; Bec et al. 2008); a wide range of C:N ratios, e.g. 6.6 to >12 (phytoplankton), 4-4.5 (bacteria), and 4.5-6 (protozooplankton); high nucleic acid content; and rapid adaptation of bacteria via gene transfer agents (Button and Robertson 2001; Canchaya et al. 2003; Casjens 2003; Lang and Beatty 2007). The standing stocks of phytoplankton, bacteria and protozooplankton in the euphotic zone of oceans are: 0.78, 0.35 and 0.16 Pg C (total: 1.3 Pg C), which is one order of magnitude greater than the standing stock of mesozooplankton, i.e. 0.1 Pg C (Le Quere et al. 2005).

To illustrate the large number of phytoplankton cells in oceans, Andersen (2008) imagined creating (in mind) a long string by placing side by side the 3.6 x 1025 phytoplankters (average diameter: 2 ^m) that exist at any one instant in the euphotic zone (average depth: 100 m). Using the Earth and the Moon as reference points, he calculated that the string of phytoplankton cells would extend from the Earth to the Moon and back (i.e. 770,000 km roundtrips) 100 billion times (ten billion times in Andersen 2008)! Doing a similar calculation for the 3.6 x 1028 bacteria (average diameter: 1 ^m) that exist at any one instant in the upper 200 m of the ocean (Whitman et al. 1998; about 10% of these are autotrophic cells) gives a string of cells that would extend from Earth to Moon and back 50 trillion times. The corresponding value for the whole ocean from surface to bottom (including the top 10 cm of marine sediments: 11.8 x 1028 bacteria) is 150 trillion times.

The special roles of microbes can be explained not only by their physiological characteristics and large standing stocks, but also (or alternatively) by their unique positions in pelagic food webs. To illustrate this point, Legendre and Rivkin (2008) examined the effect on bacterial activity of a progressive increase in the number of food-web links in a model (i.e. connections among compartments: food-web complexity). They found that increasing food-web complexity was accompanied by increase in the food-web channelling of organic carbon to bacteria, which led to increased bacterial respiration. For example, going from a model with 10 flows among trophic compartments to a model with 15 flows tripled bacterial respiration. In other words, while keeping constant the model parameters for bacteria (i.e. their physiological characteristics), the contribution of these organisms to heterotrophic community respiration increased with food-web complexity. Hence, the complexity of pelagic food webs and the accompanying feedbacks can account, at least in part, for the major role of bacteria in carbon cycling.

In order to illustrate quantitatively the points in the previous paragraphs, we use here a simple steady-state model that compares the effects of increasing physiological rates versus increasing the complexity of the food web, i.e. its internal linkages. Figure 1 shows the parameters (left side) and the modelled flows (right side) of a reference model and three variants. In all models, there is a flow of external resource into compartment A, a loss flow from each compartment, and flows among compartments. The "production" of compartments B, C, ... is passed to the next compartment (e.g. through prédation), were it is partly lost; this process is repeated until the food web has used up all the resource supplied. The three modified models illustrate the respective effects of increasing the rate of loss of A (by 50%), adding recycling of resource from B to A, and adding recycling of resource from all compartments to A.

MODEL PARAMETERS

MODELLED FLOWS

Reference model

Resource

Modified models Loss from A is increased by 50%

Resource

0.55

Recycling from B to A

Recycling from all compartments to A

Resource

330 J21 49

10.3

Resource

345 317 39 f

338 327 |35

"154

Fig. 1. Steady-state model that compares the effects of increasing physiological rates versus increasing the complexity of the food web, i.e. its internal linkages: reference model and three variants. Left side: model parameters; right side: modelled flows. Details are given in the text.

The left side of Fig. 1 shows the parameters of the reference and the three modified models. Because the models are in steady state, the sum of the output-flow parameters from any compartment is equal to 1.0. The right side of Fig. 1 shows the modelled flows for each model. Because the models are in steady state, the sum of output flows from a compartment is equal to sum of input flows into that compartment.

On the right side of Fig. 1, the external input of resource is the same in all models, i.e. 100 units of resource (in term of the chemical element considered, e.g. C, N, P). Because of recycling, the total input of resource into compartment A increases to 127 when there is recycling from B to A, and to 154 when there is recycling from all compartments to A. The loss from compartment A in the model with full recycling and in the model with increased loss is 46 and 45, respectively, showing that increasing the complexity of the food web (i.e. the number of internal linkages) has the same net effect on the modelled loss from A as increasing the physiological rate of loss by A. To better understand the results of the model, one can imagine that compartment A represents bacteria. In such a case, the loss from A would be bacterial respiration, and increasing the physiological rate of loss by A would be increasing the ratio of respiration to resource uptake by bacteria. Figure 1 (right side) also shows that the amount of resource transferred by compartment A to the remainder of the food web is much higher in the model with full recycling (i.e. 108) and than in the model with increased loss (i.e. 55), which indicates that increasing the complexity of the food web (i.e. its internal linkages) increases the overall activity of the whole food web.

The ecological interpretation of the model is as follows. Compartment A receives input from both outside the food web (external resource) and within (recycling), it loses (i.e. remineralises or excretes) part of the resource it acquires, and it transfers the remainder to the food web. The resource could be an inorganic or an organic nutrient. The chemical element modelled could be C, N, P, etc. Compartment A corresponds to microbial compartments (autotrophic and/or heterotrophic) in real pelagic food webs. The model results have both ecological and biogeochemical significance, as explained in the following paragraphs.

The food-web resources can be dissolved or particulate. The threshold between the two size categories depends on the retention characteristics of the filters used to separate the filtrate from the particles, i.e. it is generally 0.7, 0.2 or 0.1 ^m. In addition, researchers often distinguish between small and large particles. Here again, the threshold between the two size categories depends on the porosity of the filter used to separate the two types of particles, i.e. it is generally 2 or 5 ^m.

The origin of dissolved resources is both external and internal to the food web. External resources are both inorganic and organic nutrients, from deep waters (upwelling, deep convection, ventilation) and continents (flowing fresh waters, i.e. rivers and groundwater, and atmosphere). Internal resources are inorganic and organic nutrients produced by autotrophs (phytoplankton exudation) and hetero-trophs (recycling). The food-web use of dissolved resources is quite specialised, because specific metabolic rates (i.e. rates per unit biomass or size) are generally a direct function of size and surface to volume ratio. Given that microbes are small and have high turnover and growth rates, they (especially phytoplankton and bacteria) outcompete larger organisms in the uptake of dissolved resources.

In the food web, autotrophs and heterotrophs both contribute to the production of dissolved organic matter (DOM), but with different mechanisms. (1) Phytoplankton release part of their photosynthate as dissolved organic carbon (DOC). This is called exudation. (2) The organic materials consumed by heterotrophs often have a C:N ratio equal to or higher than their own. Hence, in order to maintain their internal stoichiometric balance, heterotrophs can release the excess elements as CO2, NH3 and DOM (or take up inorganic nutrients). This is called respiration or excretion, and more generally recycling (physiologists often use the term "excretion" for the release of both CO2 and organic compounds). (3) The viral lysis of bacteria, phytoplankton and protozoa causes the release of cellular material in the surrounding medium. Respiration, excretion and loss due to viral lysis are called remineralisation.

The origin of particulate resources is also both external and internal to the food web. The external resources include organic particles from continents. The internal resources are organic particles produced by the food web, such as phytoplankton cells, detritus and marine snow (e.g. faecal pellets, and transparent exopolymeric particles, TEP). The food-web use of particulate resources by microbes may include competition with larger organisms. On the one hand, protozoa generally consume small particles that are below the size that can be effectively ingested by metazoa (mostly bacteria and small phytoplankton), i.e. there is no competition between protozoa and metazoa. As an important exception to this, some hetero-trophic dinoflagellates readily ingest diatoms and thus effectively compete with larger zooplankton (e.g. reviews of Sherr and Sherr 1994, 2008). In addition, the main grazers of mesozooplankton faecal pellets may be protozoa (Poulsen and Iversen 2008), and not copepods as usually thought (Iversen and Poulsen 2007). Bacteria indirectly compete with metazoa for large particles (phytoplankton, detritus and marine snow) in a unique way. Bacteria break down particles with exoenzymes (hence, making them unavailable to larger organisms), and take up the released DOM. Thus, heterotrophic microbes continually modify their environment to optimize the availability of essential resources while limiting the access of these same resources for other components of the pelagic food web.

Table 1. Relations between the size of food-web resources (origin: external and internal), the source of internal resources within the food web, and the use of resources by microbes versus metazoa.

Size of food-web resources

Source within the food web

Microbes versus metazoa

Small particles (<2, or <5 ^m) Large particles (>2, or >5 ^m)

Phytoplankton and heterotrophs (i.e. from the bottom and the top of the food web)

Mostly phytoplankton (i.e. from the bottom of the food web) Phytoplankton and heterotrophs (i.e. from the bottom and the top of the food web)_

Microbes outcompete metazoa in using dissolved resources

Microbes use small particles without competing with metazoa Microbes compete with metazoa for large particles

Table 1 summarises the relations between the size of food-web resources (their origin is both external and internal), the source of internal resources within the food web, and the use of resources by microbes versus metazoa. Because the internal resources come from the whole food web, it is convenient to divide the sources between the bottom of the food web, i.e. phytoplankton, and the top, i.e. metazoa, the heterotrophic microbes occupying an intermediate position. That unique position provides microbes with access to resources from both the bottom and the top of the food web. As a consequence, microbes use almost all the dissolved resources, and a significant share of the particulate resources, and they monopolise (or dominate the use of) external and internal resources. In other words, microbes not only utilise most of the resources, but they also prevent metazoa from accessing them, i.e. they "corner the market" of resources.

It follows from the previous paragraphs that because they dominate the use of resources, microbes play key roles in food webs. Autotrophic microbes (phytoplankton) used dissolved inorganic resources, i.e. in coastal waters they compete with benthic autotrophs, and in the coastal and open ocean they compete with heterotrophic bacteria. Heterotrophic microbes use both dissolved and particulate resources. Concerning the inorganic and organic resources, bacteria compete with phytoplankton for inorganic nutrients, and are the dominant users of dissolved organic matter (there are a few reports indicating that some planktonic protozoa can also use DOM, e.g., Laybourn-Parry et al. 1997, but this is not of ecological significance). The small organic particles that are not efficiently grazed by metazoa are consumed by protozoa. The large organic particles are the object of a competition between microbes (bacteria and protozoa) and metazoa.

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