Morpho-functionality of Carbon Metabolism in Seaweeds - Seaweed Biology - page 37

from organic compounds is allocated to fronds, while the rest is allocated to the
holdfast (Lawrence 1986 ). In the case of the kelp Postelsia palmaeformis , 63% of
the total organic carbon is allocated to holdfast and stipe, which is in line with a
gross morphology designed to resist the direct impact of waves (Lawrence and
McClintock 1988 ). In the southern kelp Lessonia nigrescens, carbon allocation
changed with increasing size and age: adult plants deposited higher proportion of
energy in the holdfast and stipes than young plants with important consequences for
population density and local demography (Westermeier and G´mez 1996 ).
Differential allocation of organic carbon along the thallus can also be functional
to withstand other environmental stressors (Wakefield and Murray 2009 ). The
optimal defense theory (ODT) has been proposed to understand the mechanisms
that control the interaction between algal allocation of organic compounds and the
action of, e.g., herbivores (Cronin and Hay 1996 ). For example, secondary
metabolites are normally allocated in the structures with high fitness value, proba-
bly where the investment in energy is higher (Pansch et al. 2008 ). In brown algae,
phlorotannins may function as deterrents for many grazers, but additionally, due to
their primary role as cell wall precursors, these compounds act also as cell-wall
hardening, conferring mechanical resistance and toughness (Lucas et al. 2000 ). In
the intertidal kelp Lessonia nigrescens , holdfast and stipes contain higher
concentrations of phlorotannins, and hence are better defended than transient fronds
(G ´ mez et al. 2005 ;G ´ mez and Huovinen 2010 , see also Chap. 8 by Iken).
Carbon fixation, biomass, and overall the sum of the morpho-functional pro-
cesses of seaweeds define much of the primary productivity and energy fluxes in the
coastal ecosystems (Mann 1973 ). Due to their size and patterns of substrate
occupation, seaweeds represent habitat for other organisms and also modify the
physical and chemical environment (Jackson 1998 ; Delille et al. 2000 ). In coastal
areas of cold-temperate and polar regions, seaweeds can account for

50% of the
total fixed carbon (Gattuso et al. 2006 ). The outcome of seaweed carbon metabo-
lism is transferred to the food web not only via direct consumption by herbivores
but also as secondary product via detritus in near shore (Duggins et al. 1989 ) and
abyssal areas (Wiencke and Fisher 1992 ).

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2.5.4 Concluding Remarks

Overall, carbon metabolism of seaweeds is highly versatile and has allowed these
organisms to thrive in all types of habitats and environmental conditions. However,
despite the considerable advances in our knowledge on mechanisms and pathways,
the ecological consequences of many photosynthetic adaptations are not well
understood. For example, carbon acquisition patterns in relation with morpho-
physiological processes, biomass allocation, reproduction, and development of
adult and early stages of seaweeds have been hitherto overlooked (Raven 2003 ).
The significance of endogenous processes regulating carbon metabolism, which has
been studied in few brown algae, is also a relevant topic that should be expanded to