Environment and Algal Nutrition - Seaweed Biology - page 70

et al. 2010 ) were N-limited. Lapointe and Bedford ( 2010 ) suggested that the blooms
of the invasive Caulerpa brachypus f. parvifolia occurring at coral reefs off
southeast Florida (USA) were P-limited due to high biomass N:P ratio despite
relatively high ambient P.
Several studies have examined P-uptake kinetics in macroalgae (e.g.,
Wallentinus 1984 ; Hurd and Dring 1990 ; Gordillo et al. 2002 ; Runcie et al. 2004 ),
some of them comparing growth-related P requirements to uptake kinetics and
storage capacity (e.g., Bjornsater and Wheeler 1990 ; Lavery and McComb 1991 ).
Pedersen et al. ( 2010 ) found that tissue P-concentrations, the balance between
P-uptake and requirements, and the P-storage capacity varied systematically
among the three groups of algae (i.e., algae with fast, intermediate, and slow
growth).
4.2.4 Considering C as Just Another Nutrient
Traditionally, C has been considered as a special case when treated together with
nutrients, since the term nutrient is reserved mainly for N and P (e.g., Rees 2003 ),
and to a lesser extent S, Fe, Ca, and others, the latter set commonly being referred
to as micronutrients. This might be due to the majority of nutrient studies being
carried out in terrestrial plants, where C is taken from the air and the rest from soil
by roots in most cases. C is usually considered to be at saturating concentrations.
Although this is a pragmatic consideration for many studies on terrestrial plants
nutrition, the uptake and assimilation dynamics of C in aquatic plants share many
characteristics with other nutrients; i.e., it is essential for the cell, its utilization
needs photosynthetic energy, there are specific transporters in the cell surface, it
can be stored intracellularly and used later on when other restricting conditions
ease, and its availability can limit growth.
The vast majority of algal primary production involves the C 3 pathway (the
photosynthetic carbon reduction cycle or Calvin-Benson cycle) for DIC acquisi-
tion, fixing DIC directly via ribulose bisphosphate carboxylase oxygenase
(Rubisco). However, Rubisco has a relatively low affinity for CO 2 and, for most
species, it is consequently less than half saturated under current ambient CO 2 levels.
The biochemical properties of Rubisco imply that for autotrophs dependent on
diffusive CO 2 entry, the physiology of CO 2 assimilation is characterized by inher-
ent inefficiencies, such as significant inhibition of CO 2 fixation by oxygen, high
CO 2 compensation points, and low affinities for external CO 2 .
An alternative source of inorganic carbon available is HCO 3 .Thisisthemost
abundant form and can be converted to CO 2 to be used by Rubisco by means of
both spontaneous or catalyzed dehydration. Spontaneous HCO 3 dehydration
kinetics in seawater are described in Johnson ( 1982 ). The rate of spontaneous
CO 2 formation from HCO 3 dehydration is clearly below the photosynthetic