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are set by lethal temperature limits. Notably these boundary populations are
threatened by global warming, an issue that is addressed in Chap. 18 by Bartsch
et al. Heat stress may limit seaweed growth in tropical regions in summer when
high temperatures exceed the upper thermal tolerance limit of seaweeds, which is
<
35 C in many strictly tropical seaweeds. Pakker et al. ( 1995 ) describe experimen-
tally determined upper tolerance limits of tropical Caribbean seaweeds at 30 Cor
33 C, i.e., very close to the local summer temperatures of 30 C. Populations of the
fucoid seaweed Sargassum lapazeanum from the Gulf of California also showed
maximal mortality rates coinciding with highest values of seawater temperatures in
summer (29-30 C, Rivera and Scrosati 2006 ). On shallow reef flats in the southern
Red Sea, temperatures may even exceed 34 C for prolonged periods in summer and
seaweeds strongly decrease in biomass or disappear during the hot season
(Ateweberhan et al. 2005 ). The tropical green macroalga Cladophora submarina
is restricted in the western Atlantic to the Caribbean and Bermuda. Its northward
extension is prevented by lethal, low winter temperatures
15 C (Cambridge et al.
1987 ). This species may experience severe cold stress very close to the lethal limits
for 1-2 weeks during cold winters at the Florida Keys, when temperatures in the
shallow bays may fall below 15 C caused by intrusion of polar air masses.
On the cellular level, heat stress is known to affect membrane-associated
processes as high temperatures cause fluidization of membranes and finally disin-
tegration of the lipid bilayer (Los and Murata 2004 ). Also, protein stability and
function are impaired and cause decreased enzyme activities or even enzyme
inactivation. Moreover, membrane and protein damage trigger the production of
reactive oxygen species, which in turn inhibit the de novo proteins synthesis
(Larkindale et al. 2005 ). Specifically with respect to photosynthesis, there are
three major heat-sensitive sites: photosystem II with the oxygen evolving complex,
ATP generating ATP synthase, and enzymes of the Calvin-Benson cycle
(Allakhverdiev et al. 2008 ). The primary event of cold stress is the formation of
lipid gel phases in cell membranes. When a model membrane enters a phase-
separated state in which gel and liquid-crystalline phases coexist, the membrane
becomes permeable to small electrolytes. This permeability results in the disruption
of ion gradients, across the membrane, that are essential for the maintenance of
cellular activities (Nishida and Murata 1996 ). Changes in the ultrastructure of
chloroplasts were detected in the green macroalga Valonia utricularis when exposed
to cold stress (Eggert 2002 ). Chloroplasts were less dense packed in the warm-
temperate Mediterranean isolate and became particularly disorganized and more
swollen in the tropical Indian Ocean isolate when algae were grown at 15 and
18 C, respectively (Fig. 3.3 ). Disorganization of chloroplasts reduces light-
harvesting capacity and ultimately photosynthetic activity (Ciamporova and
Trginova 1996 ). As temperature decreases to freezing temperatures, ice forms in
the intercellular spaces (Tomashow 1998 ). The accumulation of ice would result in
the physical disruption of the cells. However, freezing injury results primarily from
cellular dehydration rather than from direct mechanical damage by ice crystals.
Freeze-induced dehydration causes denaturation of proteins, precipitation of various
molecules, and multiple forms of membrane lesions (Steponkus and Webb 1992 ).
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