Although mat-forming cyanobacteria dominate many freshwater ecosystems in the Arctic and Antarctic, their optimal temperature for growth (Topt) is usually much higher than the temperature range of their native habitat. The present study compared the temperature dependence of growth, pigment composition and absorbance, photosynthesis and photosynthate partitioning for two strains of cyanobacteria with contrasting Topt values; Phormidium subfuscum, isolated from McMurdo Ice Shelf, Antarctica, and Phormidium tenue, collected from the Kuparuk River in the tundra region of northern Alaska. Phormidium subfuscum grew between 5 and 20°C with a Topt of 15°C whereas P. tenue showed detectable growth from 10 to 40°C and a Topt of 30°C. Light utilization efficiency, photosynthetic capacity and the irradiance at the onset of light saturation increased with increasing temperature up to Topt in both strains. The cellular concentrations of chlorophyll a (Chl a) and carotenoid (CAR) and the in vivo absorbance maxima for Chl a, CAR, C-phycocyanin and allophycocyanin changed little for P. subfuscum but all these variables increased across the temperature range up to Topt for P. tenue. Neither P. subfuscum nor P. tenue showed changes in relative carbon allocation with varying temperature, suggesting that gross biochemical alterations are not a characteristic of temperature acclimation in these cyanobacteria. We conclude that the eurythermal cyanobacterium P. tenue optimizes growth over a wide range of temperatures by adjusting its light-capturing as well as carbon fixation characteristics, whereas stenothermal P. subfuscum relies on changes in carbon fixation without concomitant shifts in pigment content.

The effects of three conditions likely to affect soluble carbohydrate pools, namely drought, expression of barley sucrose: fructan 6-fructosyl transferase (6-SFT, EC 2.4.1.10) and the establishment of the arbuscular mycorrhizal symbiosis with Glomus mosseae were studied in a multifactorial experiment using tobacco (Nicotiana tabacum). Tobacco, a plant naturally unable to form fructan, accumulated fructan in leaves, and to a larger extent in the roots, when transformed with 6-SFT. Under drought conditions, growth was considerably reduced, but neither expression of 6-SFT nor mycorrhiza formation had an effect on growth rate. However, in response to drought, carbon partitioning was significantly altered towards accumulation of soluble sugars. In plants exposed to drought, pools of sucrose were greater than those of unstressed plants, particularly in their roots. In the transgenic plants expressing 6-SFT, there were also increased contents of the products of 6-SFT, namely fructan, most probably because of the increased availability of the substrate, sucrose. These effects were the same in the presence or absence of mycorrhiza. Hexoses (glucose and fructose) also increased in response to drought, primarily in the leaves. This effect of drought was little affected by the expression of 6-SFT, except that it slightly enhanced drought-induced glucose accumulation in roots. However, the presence of mycorrhiza led to a considerable reduction in drought-induced accumulation of hexoses in the leaves. The content of the fungal disaccharide trehalose was greatly increased in the roots of all mycorrhizal plants upon exposure to drought, particularly in some of the transgenic plants expressing 6-SFT.