Environment and Algal Nutrition - Seaweed Biology - page 66

The different processes involved in nutrient acquisition follow the terms as used
by Berges ( 1997 ). Accordingly, uptake is defined as removal of a given molecule or
ion from the environment and its physical transport into the algal cell. In
macroalgae and higher plants, a further distinction between transport into intercel-
lular spaces and transport into cells themselves may be useful (Redinbaugh and
Campbell 1991 ), but this difference is not considered here. The term assimilation is
reserved for the processes by which internalized inorganic molecules are used to
form small organic molecules, such as amino acids, and incorporation is defined as
the process by which nutrient-containing organic molecules are combined to form
macromolecules, such as proteins and nucleic acids, thus participating in growth.
These definitions are functional and pragmatic; for example, disappearance of
nitrate from the medium can be easily measured, and a separation of small organic
and inorganic molecules is relatively straightforward. Note, however, that the
enzymatic components of each process are not specified.
4.2 Meeting Nutritional Needs
Nutrient uptake mechanisms must be optimally evolved in order to meet the
nutritional needs of a given species in its environment. Algae use light energy to
fix carbon (C) and combine the carbon with elements such as nitrogen (N) and
phosphorus (P) at relatively constant stoichiometric ratios. C/N/P ratios of unicel-
lular algae are typically around 106:16:1, the so-called Redfield ratio (Redfield
et al. 1963 ), while the C/N/P ratios of benthic plants are larger and more variable,
with a median C/N/P ratio of 550:30:1, called the Atkinson ratio (Atkinson and
Smith 1983 ). Falkowski ( 2000 ) proposed that the origin of the constant Redfield
ratio is a result of a limitation intrinsic to protein synthesis. Atkinson and Smith
( 1983 ) proposed that the higher C/N ratio of benthic plants is related to a require-
ment of structural strength. Baird and Middleton ( 2004 ) and Raven and Kubler
( 2002 ) discussed the physical limits of nutrient uptake and metabolic rate, respec-
tively, in relation to light absorption. In addition, variability can be further
explained by different metabolic strategies and storage capacities (Fujita 1985 ;
Pedersen and Borum 1996 ).
Regarding uptake kinetics, the extracellular concentrations of many of the
chemical species that are required by intracellular assimilation processes are
often much lower than the half-saturation value of the transport and/or assimilating
enzyme for that chemical species, potentially greatly limiting the rate at which
nutrients could be assimilated and used in growth (e.g., Raven et al. 2008 ). While
this situation could, in the general case, be remedied by increasing the quantity of
the assimilatory enzyme, the mismatch between the external concentration and the
half-saturation value of the assimilatory enzyme can be extreme.