Nutrient gradient

Presumably, microenvironments that are extensively structured have more potential to establish and sustain electrochemical and nutrient gradients, and thus are able to support higher biotic diversity. Furthermore, high... 

Hollow-fiber bioreactors constitute an optimized production system where it is possible to achieve higher cell concentrations (107 to 108 cells/ ml), and the product concentration can reach a level of 0.7-2.3 g/L, which is similar to what can be obtained with ascitic fluid (Hendriksen and Leeuw, 1998). This system can operate for over 3 months without affecting cell viability, but presents problems with mass transport, and the formation of nutrient gradients, which require specific solutions (Kretzmer, 2002). 

Cavender-Bares, K. K., Karl, D. M., and Chisholm, S. W. (2001). Nutrient gradients in the western North Atlantic Ocean Relationship to microbial community structure and comparison to patterns in the Pacific Ocean. Deep-Sea Research 148(11), 2373—2395. 

Lee, K. S., Short, F. T., and Burdick, D. M. (2004). Development of a nutrient pollution indicator using the seagrass, Zostera marina, along nutrient gradients in three New England estuaries. Aquat Bot. 78, 197-216. 

In the preceding chapter we saw how the chemostat could be modified to account for a new phenomenon - the presence of an inhibitor. In this chapter we extend the idea behind the simple chemostat to a new apparatus in order to model a property of ecological systems that it is not possible to model in the simple chemostat. The idea is to capture the essentials of the new phenomenon without destroying the tractability of the chemostat either as a mathematical model or as an experimental one. A very simple situation will be described here a more complicated one -with a less explicit (in the sense of less computable) analysis - will be discussed in the next chapter. Just as the chemostat is a basic model for competition in the simplest situation, the apparatus here shows promise of being the model for competition along a nutrient gradient. 

The well mixed hypothesis for the chemostat does not allow a nutrient gradient to be generated. A basic tenet is that the nutrient concentration is the same everywhere hence any advantage in nutrient consumption is present everywhere. The model that incorporates a true gradient would be one involving partial differential equations a new variable, space, must be accommodated. Systems of nonlinear partial differential equations are difficult mathematical objects to understand and analyze. Even numerical solutions pose added and significant difficulties. Moreover, even if an experimental gradient is devised, measurements that do not disturb the local environment take on new difficulties. 

A piece of laboratory apparatus was devised by Lovitt and Wimpenny [LWl LW2 WL] for experiments along a nutrient gradient. It is a concatenation of chemostats in which the adjacent vessels are connected in both directions. Output occurs at the first and last vessels, and those in... 

In the previous chapter the gradostat was introduced as a model of competition along a nutrient gradient. The case of two competitors and two vessels with Michaelis-Menten uptake functions was explored in considerable detail. In this chapter the restriction to two vessels and to Michaelis-Menten uptake will be removed, and a much more general version of the gradostat will be introduced. The results in the previous chapter were obtained by a mixture of dynamical systems techniques and specific computations that established the uniqueness and stability of the coexistence rest point. When the number of vessels is increased and the restriction to Michaelis-Menten uptake functions is relaxed, these computations are inconclusive. It turns out that unstable positive rest points are possible and that non-uniqueness of the coexistence rest point cannot be excluded. The main result of this chapter is that coexistence of two microbial populations in a gradostat is possible in the sense that the concentration of each population in each vessel approaches a positive equilibrium value. The main difference with the previous chapter is that we cannot exclude the possibility of more than one coexistence rest point. 

The gradostat was an attempt to create a nutrient gradient in a piece of laboratory apparatus it was discussed in detail in Chapters 5 and 6. An alternative to the gradostat is to remove the well mixed hypothesis -that is, to think of the chemostat but without mixing the vessel. If the... 

SW2] H. Smith and P. Waltman (1991), The gradostat A model of competition along a nutrient gradient, Microbial Ecology 22 207-26. 

Alveolata - Ciliophora (ciliates), Dinophyta (dinoflagellates) Am, H, Hm 16 — 1 + 12 1 phagotrophy, vertical migration may exploit nutrient gradients, toxicity... 

Jeppesen, E., Jensen, J.P., Spndergaard, M., Lauridsen, T. and Landkildehus, F. (2000) Trophic structure, species richness and biodiversity in Danish lakes changes along a nutrient gradient, Freshwater Biology, 45, pp. 201-218. 

McCormick, P.V., Rawlik, P.S., Lurding, K., Smith, E.P. and Sklar, F.H. (1996) Periphyton-water quality relationships along a nutrient gradient in the northern Florida Everglades. Journal of the North American Benthological Society 15, 433-449. 

DeBusk, W. F. 1996. Organic matter turnover along a nutrient gradient in the Everglades. Ph.D. Dissertation. University of Florida, Gainesville, FL. 

Miao, S. L. and E. H. Sklar. 1998. Biomass and nutrient allocation of sawgrass and cattail along a nutrient gradient in the Elorida Everglades. Wetland Ecol. Manage. 5 245-263. 

As an interesting alternative, a sensor package for the simultaneous determination of nanomolar concentrations of nitrite, nitrate, and ammonia in seawater by fluorescence detection is presented in Figure 7.48. Field experiments carried out demonstrated the capability of this sensor to determine horizontal nutrient gradients in oligotrophic surface waters [179]. 

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