The Energetics of Freshwater Algae; Energy Requirements for Biosynthesis and Volume Regulation

Abstract

SUMMARYFreshwater algae have an intracellular osmolarity in excess of that of the medium. The tendency for water to enter the cells is resisted, in cells with a mechanically functional cell wall, by the ability of the wall to withstand a turgor pressure equivalent to the intracellular osmolarit^^; in cells lacking such a mechanically functional wall the water which enters is expelled by a contractile vacuole (or its functional equivalent). A comparison of the energy costs of cell volume regulation by these two mechanisois has been carried out making energy costings based on the minimum thermodynamic requirement and on plausible mechanisms for the two processes (wall synthesis and contractile vacuoie function). This analysis yielded the following conclusions.(1) Wall synthesis is a 'growth' process, while contractile vacuole operation is a 'maintenance' process; other things being equal, a cell wall is a preferable mechanism of volume regulation in non-growing cells.(2) For a 5 ftm radius cell with a specific growth rate of 8 x 10"" s~' and a 10 /an radius cell with a rate of 4 x 10~* s~^ the energy input rate for contractile vacuole operation is considerably lower than for wall synthesis on the basis of minimum thermodynamic requirement; the plausible mechanism approach makes the two energy costs quite similar. Decreased growth rates favours the wall over the contractile vacuole mechanism.(3) Increased cell size (10 jwm radius rather than 5 /^na) has similar effects on the energy costs of both volume regulation mechanisms, provided a (cell organic weight)""'^' dependence of intrinsic growth rate is incorporated into the computations, (4) Increased intracellular osmolarity (at constant extracellular osmolarity) causes a directly proportional increase in energy input rate for the cell wall mechanism, and a more than proportional increase in energy cost for the contractile vacuole mechanism.(5) A non-spherical shape increases the energy cost (at constant cell volume) for both the cell wall and the contractile vacuole mechanisms.These conclusions may be used in the interpretation of such phenomena as the lower osmolarity of wall-less than walled cells and the tendency for the resting stages of normally wall-less and flagellate cells to have walls. It appears that the majority of freshwater algae have genetic capabilities to produce both walls and contractile vacuoles for cell volume regulation (e.g, the production of walled cysts by organisms which are normally flagellate, and the production of wall-less motile spores and gametes by normially walled organisms).

Coordination of plant cell division and expansion in a simple morphogenetic system

“…A value of 0.25 μm was estimated from transmission electron micrographs of Coleochaete cell walls (20). Measured turgor pressures can vary from 0.2 MPa to over 1 MPa (21,24,25). We have assumed a turgor pressure of 0.5 MPa.…”

Characterization and growth response to temperature and salinity of psychrophilic, halotolerant Chlamydomonas sp. ARC isolated from Chukchi Sea ice

“…A tough cell wall enables microorganisms to better withstand hypoosmotic shock (Hellebust 1985), which is suggested to be a reason why diatoms are so abundant in sea ice (Arrigo & Thomas 2004). However, for rapidly growing organisms, the metabolic costs of operating contractile vacuoles are much lower than those of synthesizing a stronger organic cell wall (Raven 1982), which suggests a strategy for fast-growing opportunistic chlorophytes in decaying sea ice.This research provides insight into the succession of microbial communities in areas seasonally covered by sea ice. High concentrations of Chlamydomonas sp.…”

A Linear Programming Approach for Modeling and Simulation of Growth and Lipid Accumulation of Phaeodactylum tricornutum

“…Table 1. Elemental composition of molecular components of biomass and yield coefficients for their synthesis based on ATP and NADPH requirements [28,29]. The compositions of the distinct molecular classes were derived from the molecular structures of the compounds.…”

“…Data given by Raven [29] concerning the ATP and NADPH requirement per carbon atom in different molecular components of microalgal cells were used to calculate yield coefficients for all molecules considered in the model except for pigments, RNA and DNA. In this manner, the yield coefficients for lipids, proteins and carbohydrates were calculated as inverse of the NADPH or ATP requirements for the synthesis of 1 g of the respective molecule.…”