Abstract
The nitrogen fixation rates of planktonic and intertidal benthic cyanobacteria were investigated in Sanya Bay from 2003 to 2005. Trichodesmium thiebautii was the dominant species of planktonic cyanobacteria during our study. Significant seasonal and spatial variations in Trichodesmium spp. abundance were observed (P < 0.01). The highest Trichodesmium concentrations occurred during intermonsoon periods and in the outer region of Sanya Bay (Outer Bay stations). At fixed station S03 the abundance of T. thiebautii ranged from 1.14 x 10(3) to 2060 x 10(3) trichomes m(-2), with an annual mean of 273 x 10(3) trichomes m(-2). The average nitrogen fixation rate per colony of T thiebautii was 0.27 nmol N h(-1) colony(-1) and it did not show any obvious seasonal variations. Nitrogen fixation by planktonic cyanobacteria was highest in the Outer Bay stations, where the estimated amount of new nitrogen introduced by Trichodesmium contributed 0.03 to 1.63% of the total primary production and up to 11.64% of the new production. Statistical results showed that significant seasonal and spatial variations of nitrogen fixation rates were found among the intertidal communities. The main benthic nitrogen-fixing cyanobacteria were identified as members of the genera Anabaena, Calothrix, Lyngbya, Nostoc and Oscillatoria. The highest nitrogen fixation rate was found in microbial mats and the lowest in reefs and rocky sediments. All the benthic communities studied presented their highest nitrogen fixation activity in summer, with an average nitrogen fixation rate of 33.31 mu mol N h(-1) m(-2), whereas the lowest nitrogen activity was detected in winter, with an average nitrogen fixation rate of 5.66 mu mol N h(-1) m(-2). A Pearson correlation analysis indicated that the nitrogen fixation rate of three types of intertidal communities was significantly positively correlated to seawater temperature (P < 0.05), whereas only the nitrogen fixation rate of the reefs and rock communities was significantly negatively correlated to seawater salinity (P < 0.05).
Highlights
Cyanobacteria are important contributors to open water and benthic oceanic primary production through photosynthesis and nitrogen (N) fixation (Hoffmann, 1999; Lugomela et al, 2002; Karl et al, 2002; Omoregie et al, 2004)
SUMMARY: The nitrogen fixation rates of planktonic and intertidal benthic cyanobacteria were investigated in Sanya Bay from 2003 to 2005
Trichodesmium thiebautii was the dominant species of planktonic cyanobacteria during our study
Summary
Cyanobacteria are important contributors to open water and benthic oceanic primary production through photosynthesis and nitrogen (N) fixation (Hoffmann, 1999; Lugomela et al, 2002; Karl et al, 2002; Omoregie et al, 2004). Trichodesmium, equipped with buoyancy-regulating gas vesicles and nitrogen fixation enzymes, is considered to be well adapted to tropical and subtropical oligotrophic oceans (Capone et al, 1997), where it contributes a major fraction of new nitrogen to oligotrophic surface waters (Karl et al, 1997; Lugomela et al, 2002). In near-shore environments, there is usually a large diversity of benthic cyanobacteria (Zehr et al, 1995; Arnaud et al, 2003). They are found in many locations, such as within coral skeletons, in association with algae in soft sediments between colonies, on rock, and as epiphytes on seagrasses and other macrophytes (Dong et al, 2002b; Dong et al, 2006). Up to 50% of plants’ nitrogen requirements may come from nitrogen fixation within the rhizosphere sediments (Patriquin, 1972; Capone and Taylor, 1980; O’Donohue et al, 1991)
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