C. van den Hoek

Temperature responses of tropical to warm temperate seaweeds. I. Absence of ecotypic differentiation in amphi-Atlantic tropical-Canary Islands species

Thirty-two isolates of four amphi-Atlantic seaweed species with a tropical to subtropical distribution (Dictyopteris delicatula, Ceratodictyon intricatum, Ernodesmis verticillata and Lophocladia trichoclados) have been investigated for their temperature responses. Long (8 week) incubation times at 15–18°C caused damage or death in all isolates. Lower temperatures were tolerated only for shorter incubation times or caused severe damage. High temperatures in the range 30–35°C were tolerated. For L. trichoclados growth response curves were also determined which showed growth at temperatures ranging from 18/20 to 30°C, with optimum growth rates at 25–30°C. Crossing experiments with isolates of L. trichoclados showed that no reproductive barrier exists between isolates from opposite sides of the Atlantic. Comparison of upper and lower tolerance limits and growth response curves showed no indication of ecotypic differentiation between isolates from different localities on eastern and western Atlantic coasts, de...

Evolution of temperature responses in the Cladophora vagabunda complex and the C-albida/sericea complex (Chlorophyta)

Differentiation in temperature responses (survival and growth) was investigated among isolates of two tropical to temperate green algal lineages: the Cladophora vagabunda complex and the C. albida/sericea complex. The results were analysed in relation to published data on 18S rRNA and ITS sequence divergence, which have shown that the overall degree of genetic divergence is similar in the two lineages but that very different patterns of radiation have occurred. In the C. vagabunda complex, the two main clades in the well-resolved phylogenetic tree differed mainly in their tolerance to low temperatures. Within-clade variation was no stronger in the Atlantic/Pacific than in the all-Pacific clade. In the C. albida/sericea complex, six distinctive ITS types indicated early radiation. Although distinctive differences were found between some of these types, the thermal responses of others were very similar, indicating physiological stasis. In both lineages there was evidence for some adaptation to local temperature regimes but phylogenetic constraints were generally more important. Isolates with the same ITS sequences showed similar temperature responses even though collected from different climate zones. Evidence was found for a physiological trade-off between growth at high and at low temperatures in the C. albida/sericea complex, whereas, in the C. vagabunda complex, one clade showed more eurythermal growth responses than the other. In the C. vagabunda complex, which is the ancestral lineage of the C. albida/sericea complex, major differentiation was found in cold tolerance but not in heat tolerance, whereas the reverse pattern was found in the derived C. albida/sericea complex. These findings suggest that an acquisition of cold tolerance preceded the loss of heat tolerance during adaptation to colder climates.

Growth and Reproductive Phenology of Nine Intertidal Algae on the Murman Coast of the Barents Sea

Monthly replicate samples of 30 plants of each of nine species were collected in the intertidal zone in the Dalnezelenetskaya Inlet, Murman coast, Barents Sea. Germlings and new shoots emerged in great quantities in March when temperature was near 0 degrees C and daylength 10-13 h. All nine intertidal species on the Murman coast exhibit a period of rapid vegetative growth in early spring, at temperatures far below the optimum temperatures in at least five of them for which experimental evidence from the literature is available. The increasing light level in early spring is probably responsible for this growth. In six of them (Ulvaria obscura, Monostroma grevillei, Chorda filum, Chordaria flagelliformis, Stictyosiphon tortilis, Dumontia contorta) summer peaks of reproduction are apparently responsible for the summer decline of the thalli. The last five of these six have heteromorphic life histories in which microthallus stages are thought to ensure approximate synchronization of early spring recruitment (for instance by short day induction of microthallus development at autumnal temperatures, as in D. contorta and M. grevillei). In one species (Membranoptera alata), with an isomorphic diplohaplontic life history, early spring recruitment is thought to be ensured by the release of tetraspores and carpospores in winter. Earlier expectations that southern species would shift their reproduction to the summer near their northern distribution boundary were not confirmed. The general conclusion is that each of the nine species has approximately the same seasonal cycle of growth and reproduction in various parts of its geographic range, with only minor shifts from spring towards summer (but precise data on seasonal cycles elsewhere are scarce). This suggests that there may be subtle differences in temperatures and photoperiods triggering the onset of reproduction; this aspect needs further experimental testing of strains from the Murman coast.