Aluminum (Al) is the most abundant metal in the earth's crust, but its availability depends on soil pH. Despite this abundance, Al is not considered an essential element and so far no experimental evidence has been put forward for a biological role. In plants and other organisms, Al can have a beneficial or toxic effect, depending on factors such as, the concentration of the metal, the chemical form of Al, growth conditions and plant species. Here we review recent advances in the study of Al in plants at physiological, biochemical and molecular levels, focusing mainly on the beneficial effect of aluminum in plants (stimulation of root growth, increased nutrient uptake, the increase in enzyme activity, etc.). In addition, we discuss the possible mechanisms involved in improving the growth of plants cultivated in soils with acid pH, as well as mechanisms of tolerance to the toxic effect of aluminum.

Aluminum, a Friend or Foe of Higher Plants in Acid Soils

The changing irradiance environment: consequences for marine macrophyte physiology, productivity and ecology

The Pearl River Estuary is a subtropical estuary and the second largest in China based on discharge volume from the Pearl River. Processes in the estuary vary spatially and temporally (wet vs dry season). In the dry season at the head of the estuary, hypoxic and nearly anoxic conditions occur and NH4 reaches > 600 mu M, NO3 is similar to 300 mu M and nitrite is similar to 60 mu M indicating that nitrification and denitrification may be important dry season processes in the region extending 40 km upstream of the Humen outlet. There are very few biological studies conducted in this upper section of the estuary in either the dry or wet seasons and hence there is a need for further research in this region of the river. In the wet season, the salinity wedge extends to the Hongqimen outlet and oxygen is low (35-80% saturation). Nitrate is similar to 100 mu M, silicate similar to 140 mu M: and phosphate is relatively low at similar to 0.5 mu M, yielding an N:P ratio up to similar to 200:1 in summer. Nutrients decrease in the lower estuary and primary productivity may become potentially P-limited. Eutrophication is not as severe as one would expect from the nutrient inputs from the Pearl River and from Hong Kong's sewage discharge. This estuary shows a remarkable capacity to cope with excessive nutrients. Physical processes such as river discharge, tidal flushing, turbulent dispersion, wind-induced mixing, and estuarine circulation play an important role in controlling the production and accumulation of algal blooms and the potential occurrence of hypoxia. Superimposed on the physical processes of the estuary are the chemical and biological processes involved in the production of the bloom. For example, the 100N:1P ratio indicates that P potentially limits the amount of algal biomass (and potential biological oxygen demand) in summer. While extended periods of hypoxia are rare in Hong Kong waters, episodic events have been reported to occur during late summer due to factors such as low wind, high rainfall and river discharge which result in strong density stratification that significantly dampens vertical mixing processes. Nutrient loads are likely to change over the next several decades and monitoring programs are essential to detect the response of the ecosystem due to the future changes in nutrient loading and the ratio of nutrients. (c) 2008 Elsevier Ltd. All rights reserved.

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Physical–biological coupling in the Pearl River Estuary

A techno-economic analysis of industrial production of marine microalgae as a source of EPA and DHA-rich raw material for aquafeed: Research challenges and possibilities

Preservation of organic matter in estuarine and coastal areas is an important process in the global carbon cycle. This paper presents bulk δ 13 C and C/N of organic matter from source to sink in the Pearl River catchment, delta and estuary, and discusses the applicability of δ 13 C and C/N as indicators for sources of organic matter in deltaic and estuarine sediments. In addition to the 91 surface sediment samples, other materials collected in this study cover the main sources of organic material to estuarine sediment. These are: terrestrial organic matter (TOM), including plants and soil samples from the catchment; estuarine and marine suspended particulate organic carbon (POC) from both summer and winter. Results show that the average δ 13 C of estuarine surface sediment increases from −25.0 ± 1.3‰ in the freshwater environment to −21.0 ± 0.2‰ in the marine environment, with C/N decreasing from 15.2 ± 3.3 to 6.8 ± 0.2. In the source areas, C 3 plants have lower δ 13 C than C 4 plants (−29.0 ± 1.8‰ and −13.1 ± 0.5‰ respectively). δ 13 C increases from −28.3 ± 0.8‰ in the forest soil to around −24.1‰ in both riverbank soil and mangrove soil due to increasing proportion of C 4 grasses. The δ 13 C POC increases from −27.6 ± 0.8‰ in the freshwater areas to −22.4 ± 0.5‰ in the marine-brackish-water areas in winter, and ranges between −24.0‰ in freshwater areas and −25.4‰ in brackish-water areas in summer. Comparison of the δ 13 C and C/N between the sources and sink indicates a weakening TOM and freshwater POC input in the surface sedimentary organic matter seawards, and a strengthening contribution from the marine organic matter. Thus we suggest that bulk organic δ 13 C and C/N analysis can be used to indicate sources of sedimentary organic matter in estuarine environments. Organic carbon in surface sediments derived from anthropogenic sources such as human waste and organic pollutants from industrial and agricultural activities accounts for less than 10% of the total organic carbon (TOC). Although results also indicate elevated δ 13 C of sedimentary organic matter due to some agricultural products such as sugarcane, C 3 plants are still the dominant vegetation type in this area, and the bulk organic δ 13 C and C/N is still an effective indicator for sources of organic matter in estuarine sediments.

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Bulk organic δ13C and C/N as indicators for sediment sources in the Pearl River delta and estuary, southern China

Seasonal variation in zooplankton composition and grazing impact on phytoplankton standing stock in the Pearl River Estuary, China

Variation of phytoplankton biomass and primary production in Daya Bay during spring and summer.

The status of the ecological environment and a proposed protection strategy in Sanya Bay, Hainan Island, China.

This paper examines spatial and temporal variations of mesozooplankton abundance, biomass and community structure during three cruises of July 2002 (summer), January 2003 (winter), and April 2003 (spring) in the Pearl River estuary, China. Zooplankton abundance and biomass fluctuated widely and showed distinct heterogeneity in the Pearl River estuary. A total of 154 species were identified during three surveys. The number of zooplankton species richness was strongly linked to salinity. Hierarchical cluster analysis identified three zooplankton groups during this study. Estuarine, neritic and pelagic groups corresponded to the upper, middle and lower reaches in the Pearl River estuary. The difference among groups could be mainly ascribed to changes in the relative contributions of the dominant species. The fluctuations in the zooplankton abundance, biomass and community structure were determined by the interactive effects of freshwater inflow, tidal and coastal currents, chlorophyll a, salinity and temperature. Significant spatial variability in the distribution of zooplankton species, abundance and biomass can be ascribed to the virtual presence of a horizontal gradient in salinity. (c) 2005 Elsevier Ltd. All rights reserved.

Spatial and temporal variations of mesozooplankton in the Pearl River estuary, China

The distribution, species, biomass, productivity of main seagrasses and their suffered main threats are introduced and discussed in this paper based on the field surveys in the offshore areas of Guangdong, Guangxi and Hainan provinces. The seagrass beds distributed mainly in Liusha Bay, Donghai Island and Hailing Island of Guangdong Province; Hepu and Pearl Bay of Guangxi Zhuang Autonomous Region; Li’an Bay, Xincun Bay, Longwan Bay and Sanya Bay of Hainan Province and so on. The total area of seagrass beds in the coastal sea of South China is about 2400 ha. There are about 8 species, such as Halophila ovalis, Halodule uninervis, Halophila beccarii, Zostera japonica, Enhalus acoroides, Thalassia hemperichii, Ruppia martime and Cymodocea rotundata. These seagrasses have high biomass and productivity. However, they are subjected to some threats resulting from artificial and natural factors, especially anthropogenic causes, such as culture of shrimps and fishes, shellfish collection, fishing by means of poisons, electric power and explosives as well as trawling, artificial pollution, dredging channels, and typhoons natural disaster.

Yehui Tan

Papers

Seasonal variation in zooplankton composition and grazing impact on phytoplankton standing stock in the Pearl River Estuary, China

Variation of phytoplankton biomass and primary production in Daya Bay during spring and summer.

The status of the ecological environment and a proposed protection strategy in Sanya Bay, Hainan Island, China.

Spatial and temporal variations of mesozooplankton in the Pearl River estuary, China

Main seagrass beds and threats to their habitats in the coastal sea of South China