Cell quota

Bienfang, P.K. and Harrison, P.3., 1984. Co-variation of sinking rate and cell quota among nutrient replete marine phytoplankton. Mar. Ecol. Prog. Ser., 14 297-300. 

N2 fixers represent an important functional group that includes a variety of physiological types with unique nutrient requirements. Furthermore, N2 fixers have unique or elevated cell quotas for certain metals as a result of possessing the nitrogenase enzyme system which requires both molybdenum and iron. 

Figure 4.4 Modeled areas of diazotroph nutrient limitation. Nutrient-limitation patterns for the diazotrophs during summer months. Areas where all nutrient cell quotas are >97% of the maximum cell quota values are arbitrarily defined as nutrient-replete. Also shown is the percentage of total ocean area where each nutrient is limiting growth. From Moore et al. (2004) with permission.

Feeding and growth rates, egg production and hatching success, and fecal pellet production are affected by both quantity and quality of food supply (Abou Debs, 1984 Besiktepe and Dam, 2002 Frost, 1972 Tang and Taal, 2005). Changes in food quality can create a mismatch in stoichiometry between predator and prey (Sterner and Hessen, 1994) and effect all of the above parameters, as well as body N content, and ultimately zooplankton production and biomass. In fact the nutritional status of phytoplankton (N cell quotas), not the phytoplankton biomass, may be the major bottom-up determinant of zooplankton biomass (Hessen, 1992 Sterner and Hessen, 1994) except in periods of low food availability (Jonasdottir et al., 2002). 

Laboratory studies have suggested that there are three modes of transport for silicic acid (reviewed by Martin-Jezequel et al., 2000) first, silicic acid may be rapidly transported across the cell membrane, following surge uptake kinetics. This occurs primarily in Si-starved cells with cell quotas (Droop, 1968, 1973) near minimal values. Second, sdicic acid uptake can be controlled internally, presumably due to regulation ofsihcaprecipitation and deposition (e.g., Hildebrand et al., 1997). Third, silicic acid uptake may be controlled externally due to substrate hmitation. 

Assuming that x(t) remains positive, one has the following equation for the cell quota ... 

The growth rate increases with cell quota. The following form for the uptake rate appears in [G2], where Q has the range Qram — Q — gmax-... 

In Other words, p has the Monod form in S but the saturation value of the Monod function, p ,ax. decreases with cell quota g. Cunningham and Nisbet [CNl CN2] take p ,ax to be constant. Therefore, we assume that p is continuously differentiable in (S, g) for S > 0 and g > P and satisfies... 

In particular, p S,Q) >0 when S>0. Equation (2.2) requires that the uptake rate vanish in the absence of nutrient, increase with increasing nutrient, and decrease as the cell quota increases. 

The second level, or cell-quota theory, allows organisms to vary their content of nutrient Q (and hence their yield of biomass from assimilated nutrient). It is increasingly referred to as the DROOP model after one of its authors (Droop, 1968, 1983). In principle, the quota should be defined as the ratio of nutrient to biomass (Droop, 1979) and will here be understood as the population (atomic) ratio of the nutrient element to carbon. A simplified version of the theory and some deductions from it, is given in Box 1 (refer page 348). The key equation (ignoring physical transports) is ... 

Fig. 9.1 Cell-quota theory for control of photo-autotroph growth by internal nitrogen or phosphorus. Q is the cell quota for the nutrient, in atoms of the element per atom of organic carbon. kfj is the minimum value, or subsistence quota. /////1IU1X gives growth as a proportion of maximum rate. The function 10) multiplies nutrient uptake (which is also a function of ambient concentration) and brings it towards zero as Q tends towards (i llax. The third part of the diagram compares typical ranges of values of cellular N and P content and show how these contribute to variation in the cell N P ratio.

Finally, cell-quota theory treats the entire cellular content of a nutrient as being the pool controlling growth rate (under limiting conditions), and deals with multiple nutrient interactions empirically. At the third level of description, MECHANISTIC models aim to embody realistic accounts of the main biochemical processes and pools within cells. A recent example (Flynn Hipkin, 1999 Flynn, 2001) deals with nitrogen, phosphorus, silicon and iron as well as the carbon content of cells, photosynthesis, and the uptake competition between ammonium and nitrate. However, the model embodies many parameters and there is currently insufficient information to use it to distinguish between groups or species of phytoplankters. Its characteristic nutrient quota parameters are included in Table 9.3 except for those for silicon, which are cell-based. 

Droop, M.R. (1973) Some thoughts on nutrient limitation in algae. Journal of Phycology, 9, 264—272. Droop, M.R. (1979) On the definition of X and of Q in the Cell Quota model. Journal of Experimental Marine Biology and Ecology, 39, 203. 

Tett, P. and Droop, M.R. (1988) Cell quota models and planktonic primary production, in Handbook of Laboratory Model Systems for Microbial Ecosystems, vol. 2 (ed. J.W.T. Wimpenny), CRC Press, Florida, pp. 177-233. 

Droop, M.R. (2003) In defence of the cell quota model of micro-algal growth. Journal of Plankton Research 25, 103-107. 

Examples of Spirolide Composition and Cell Quota in Wild Cells and Cultures of... 

---