Comparative mechanisms of photosynthetic carbon acquisition in Hizikia fusiforme under submersed and emersed conditions

The economic seaweed Hizikia fusiforme (Harv.) Okamura (Sargassaceae, Phaeophyta) usually experiences periodical exposures to air at low tide. Photosynthetic carbon acquisition mechanisms were comparatively studied under submersed and emersed conditions in order to establish a general understanding of its photosynthetic characteristics associated with tidal cycles. When submersed in seawater, H fusiforme was capable of acquiring HCO3- as a source of inorganic carbon (Ci) to drive photosynthesis, while emersed and exposed to air, it used atmospheric CO2 for photosynthesis. The pH changes surrounding the H fusiforme fronds had less influence on the photosynthetic rates under emersed condition than under submersed condition. When the pH was as high as 10.0, emersed H fusiforme could photosynthesize efficiently, but the submersed alga exhibited very poor photosynthesis. Extracellular carbonic anhydrase (CA) played an important role in the photosynthetic acquisitions of exogenous Ci in water as well as in air. Both the concentrations of dissolved inorganic carbon in general seawater and CO2 in air were demonstrated to limit the photosynthesis of H fusiforme, which was sensitive to O-2. It appeared that the exogenous carbon acquisition system, being dependent of external CA activity, operates in a way not enough to raise intracellular CO2 level to prevent photorespiration. The inability of H fusiforme to achieve its maximum photosynthetic rate at the current ambient Ci levels under both submersed and emersed conditions suggested that the yield of aquaculture for this economic species would respond profitably to future increases in CO2 concentration in the sea and air.

Impacts of anthropogenic activity on the recent evolution of the Huanghe (Yellow) River delta

Hydrological statistical data, remote sensing images, and bathymetric charts were used to study the recent evolution of the Huanghe (Yellow) River delta under human-induced interventions. It was clear that water and sediment discharge from the Huanghe River had dropped rapidly since 1970, particularly after 1986. The water and sediment discharges for the period of 1986-2000 were found to have been reduced to only 29.2% and 31.2% of those in the period of 1950-69. This was caused by human factors in the upper and middle reaches of the Huanghe River, including water diversion, damming and reservoir construction, and water and soil conservation. Based on the results from visual interpretation of processed Landsat (MSS or TMJETM+) images dated from 1976 to 2001 and two digital elevation models generated from bathymetric charts surveyed in 1976 and 1992, we found that human-induced reduction of water and sediment discharge led to coastline retrogradation, with the maximum mean recession rate of -0.51 km yr-1 over the period of 1976-98, and seabed erosion beyond the -20 m isobath between 1976 and 1992. Other impacts of human activities on the recent evolution of the Huanghe River delta, including tidal flats shrinking, artificial coastline increasing, land surface sinking and so on, were also analyzed. We found that: (i) the whole delta, including subaerial and subaqueous, has turned from a highly constructive period to a destructive phase; (ii) channelization and dredging were two of the main causes of delta destruction; (iii) land loss in the Huanghe River delta caused by submersion will be increased in the near future; (iv) the Huanghe River delta was becoming more fragile and susceptible to natural hazards.