Increasing atmospheric CO 2 concentration affects calcification in most planktonic calcifiers. Both reduced or stimulated calcification under high CO 2 have been reported in the widespread coccolithophore Emiliania huxleyi. This might affect the response of cells to photosynthetically active radiation (PAR; 400-700 nm) and ultraviolet radiation (UVR; 280-400 nm) by altering the thickness of the coccolith layer. Here we show that in the absence of UVR, the calcification rates in E. huxleyi decrease under lowered pH levels (pH NBS of 7.9 and 7.6; pCO 2 of 81 and 178 Pa or 804 and 1759 ppmv, respectively) leading to thinned coccolith layers, whereas photosynthetic carbon fixation was slightly enhanced at pH 7.9 but remained unaffected at pH 7.6. Exposure to UVR (UV-A 19.5 W m 22 , UV-B 0.67 W m 22 ) in addition to PAR (88.5 W m 22 ), however, results in significant inhibition of both photosynthesis and calcification, and these rates are further inhibited with increasing acidification. The combined effects of UVR and seawater acidification resulted in the inhibition of calcification rates by 96% and 99% and that of photosynthesis by 6% and 15%, at pH 7.9 and 7.6, respectively. This differential inhibition of calcification and photosynthesis leads to significant reduction of the ratio of calcification to photosynthesis. Seawater acidification enhanced the transmission of harmful UVR by about 26% through a reduction of the coccolith layer of 31%. Our data indicate that the effect of a high-CO 2 and low-pH ocean on E. huxleyi (because of reduced calcification associated with changes in the carbonate system) enhances the detrimental effects of UVR on the main pelagic calcifier.

We studied the physiological response of phytoplankton to the interacting effects of 3 factors affected by global climate change: CO 2 , nutrient loading and irradiance. Treatments had a high and low level for each factor: CO 2 was bubbled at 1000 ppm by volume versus present atmo spheric values; high nutrient treatments had a combination of inorganic and organic nutrients; and light treatments were obtained by covering the tanks with a single or double layer of screen. We measured esterase activity, oxidative stress (ROS), cell death, DNA damage, photosynthetic efficiency and 14 C assimilation as particulate or dissolved organic material (POC and DOC respectively). Conditions simulating future global change scenarios showed similar chlorophyllnormalized primary productivity as present conditions. The main effect driving phytoplankton physiology was the downregulation of the photosynthetic apparatus by elevated CO 2 , which decreased esterase activity, ROS, cell death and DNA damage. Nutrient concentration and light acted as additional modulators, upregulating or contributing to downregulation. The percentage of DO 14 C extracellular release (PER) was low (0 to 27%), significantly lower under ultraviolet radiation (UVR) than under photosynthetically active radiation (PAR), and acted mainly to reequilibrate the internal balance when cells grown under UVR were exposed to PAR. PER was almost 3 times lower under high CO 2 , confirming a higher resource use efficiency of phytoplankton under future CO 2 concentrations.

The assimilation of N-NO requires more energy than that of N-NH . This becomes relevant when energy is limiting and may impinge differently on cell energy budget depending on depth, time of the day and season. We hypothesize that N-limited and energy-limited cells of the oceanic cyanobacterium Synechococcus sp. differ in their response to the N source with respect to growth, elemental stoichiometry and carbon allocation. Under N limitation, cells retained almost absolute homeostasis of elemental and organic composition, and the use of NH did not stimulate growth. When energy was limiting, however, Synechococcus grew faster in NH than in NO and had higher C (20%), N (38%) and S (30%) cell quotas. Furthermore, more C was allocated to protein, whereas the carbohydrate and lipid pool size did not change appreciably. Energy limitation also led to a higher photosynthetic rate relative to N limitation. We interpret these results as an indication that, under energy limitation, the use of the least expensive N source allowed a spillover of the energy saved from N assimilation to the assimilation of other nutrients. The change in elemental stoichiometry influenced C allocation, inducing an increase in cell protein, which resulted in a stimulation of photosynthesis and growth.

The maturation pattern of sexual reproduction in Hizikia fusiformis (Harvey) Okamura (Sargassaceae, Phaeaophyta) was examined in 2003 at Yunao Bay, Nanao Island, Shantou, China. Maturation began in mid-April (seawater temperature 19-21 • C), reached the peak in mid-May (maturation rate ca. 70%, and seawater temperature 23.5-25 • C) and finished in lateJune (seawater temperature 27.5-30 • C). The Hizikia plants continued to gain the length from the beginning of maturation season to reach a maximum mean length of 34.8 cm in mid-May, after which the mean length was reduced drastically due to the senescence and rupture of the larger plants in size. The major portion of the mature plants belonged to the larger plants between April and May, but to the smaller ones in June. It is suggested that the plant must achieve a critical size before reproductive maturation occurred. There was a positive relationship between the number of receptacles (NR), as well as the reproductive allocation (RA), and the plant size of Hizikia population, with the recorded