Abstract
Size dependent competition was examined in two marine centric diatoms, Coscinodiscus sp. and Thalassiosira rotula at various, and Si concentrations. The growth responses for both species to nutrient levels were evaluated using two forms of nitrogen (NH4 and NO3) and silicon in both monoculture and mixed culture conditions. Under single species culture, the impact of Si did not depend on N forms for both diatoms. The increase of NH4-N enhanced the growth of Coscinodiscus, but did not affect T. rotula. When NO3-N was the nitrogen source, cell densities of both species were significantly enhanced by increasing Si concentrations, but only T. rotula density was affected by increasing N concentration. When Coscinodiscus sp. and T. rotula grew in the same culture, Coscinodiscus sp. dominated in both N forms. The scale of the dominance of Coscinodiscus sp. over T. rotula increased with decreasing N and Si concentrations. In the competition experiment, when was the N source, both Coscinodiscus sp. and T. rotula were signifi- cantly affected by changes in N concentration, but only T. rotula was affected by Si. When NO3 was the N source, neither Coscinodiscus sp. nor T. rotula was affected by Si, but T. rotula was enhanced by N levels. Regardless the N form, the impact of Si on neither Coscinodiscus sp. nor T. rotula depended on N concentration. Our results indicate that large diatom species have a competitive advantage over small species, and both large and small species were sensitive to NH4-N limitation, but the small species was more sensitive to NO3-N limitation than the large species.
Highlights
The paradox of plankton refers to the apparent immunity of natural phytoplankton assemblages to the principle of competitive exclusion [1]
The objective of this study was to examine the possible influence of nitrogen source and silicon availability on competition for nitrogen between two species of marine diatom
The increase of NH4-N concentration significantly enhanced the abundance of the large species, but did not significantly affect the small species
Summary
The paradox of plankton refers to the apparent immunity of natural phytoplankton assemblages to the principle of competitive exclusion [1]. Many species of phytoplankton are able to co-exist despite competing for relatively few limiting resources [2]. Previous studies in this area of ecology have focussed on the needs of phytoplankton in general by examining the competitive interactions occurring between phytoplankton in different taxonomic divisions [3,4,5]. There has been little study of the interactions that occur within phytoplankton species that have similar resource requirements but directly compete for limiting resources. In the case of diatoms, all species require Si for growth and survival [6], but a non-limiting supply of Si alone appears to be insufficient to ensure the survival of diatom species. To adequately compete for Si, diatoms may require sufficient N for the formation of Si transporters [7,8]
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