Seaweed Responses to Temperature - Seaweed Biology - page 52

and the Joint Genome Institute ( http://www.jgi.doe.gov/ ) is presently sequencing
the complete nuclear genome of Porphyra umbilicalis , which is a large eukaryotic
genome (haploid genome 5-270 mb; reviewed in Gantt et al. 2010 ). However,
notably large expressed sequence tag (EST) projects on the red alga Porphyra
yezoensis (Nikaido et al. 2000 ; Asamizu et al. 2003 ), but also smaller EST projects
on, e.g., the red alga Chondrus crispus (Coll ´ n et al. 2006 ), the brown alga
Laminaria digitata (Roeder et al. 2005 ) and the green alga Ulva linza (Stanley
et al. 2005 ), allow the utilization of powerful tools of functional genomics.
3.4 Disruptive Temperature Stress and Thermal Tolerance
Heat stress and cold stress (including freezing) cause damage to seaweeds and are
referred to “disruptive stress” sensu Davison and Pearson ( 1996 ) due to adverse
conditions beyond phenotypic temperature acclimation to suboptimal temperatures.
The timescale of disruptive temperature stress is particularly relevant. Organisms may
cope temporarily (timescale of hours) with strong temperature stress and subsequently
recover from damage at optimal conditions (Eggert et al. 2003b ). But on a longer
timescale (timescale of weeks) and/or increasingly stressful conditions, the organisms
experience progressively more impaired cellular processes until the minimum
and maximum temperatures for survival are reached. The degree of physiological
dysfunction becomes very severe at these cardinal temperatures and cell death
ultimately follows. The individual temperature tolerance of seaweed species defines
the minimum and maximum temperatures for survival. Upper and lower temperature
limits for survival with biogeographical implications are typically assessed using
incubation periods of 2-8 weeks and a recovery phase of 2 weeks at optimal
temperatures (e.g., Wiencke et al. 1994 ; Eggert et al. 2003a ).
Short-term thermal stress is severest for benthic algae in the high intertidal,
where factors causing desiccation and/or critical temperatures determine the upper
limit for growth (Davison and Pearson 1996 ). Seaweeds that grow in shallow tide
pools or are even exposed to air during tidal emersion may regularly experience
abrupt temperature changes of 10-20 C (Helmuth and Hofmann 2001 ). Also,
freezing is an important stress for polar and cold-temperate intertidal algal
communities (Pearson et al. 2000 ). Tolerance ranges (i.e., the range between
upper and lower lethal temperatures) have been found to be broader in intertidal
seaweeds occupying the upper shore than in species from the subtidal (Einav et al.
1995 ; Stengel and Dring 1997 ; Martone et al. 2010 ). Short-term thermal stress on
the organism level seems ecologically less relevant for seaweeds growing in the
subtidal where algae are virtually always submerged.
Seaweeds growing in the center of their geographic distribution show seasonal
variation in biomass that is typically not directly controlled by seasonal temperature
stress, but most importantly by the variability of the light regime and the availabil-
ity of nutrients (Kain 1989 ; Wiencke et al. 2009 ). However, thermal stress may
limit seaweed growth in all populations growing near distribution boundaries that