Graduate Thesis Or Dissertation
 

The release of total and dissolved organic carbon from macroalgae and phytoplankton

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  • Many phytoplankton and macroalgae release dissolved organic carbon (DOC). Many macroalgae also produce and store secondary metabolites which in some case have been shown to deter their grazers. The metabolites may be released into the surrounding seawater and could inhibit primary production. On the other hand, some phytoplankton can use DOC as a supplemental nutrient. In the first part of my experimental work, I tested the toxic or stimulatory effect of released materials from macroalgae on phytoplankton growth. Two out of six macroalgal culture media showed a weak inhibitory effect on phytoplankton growth, but the effect was far less than expected. Data from these experiments provided preliminary estimates of DOC release rates of both macroalgae and phytoplankton. The release rates of phytoplankton were nearly a hundred times higher than those of macroalgae. The DOC release seemed to be positively correlated with the surface area to volume (SA:V) ratio. This result implies that the release of DOC may be dominated by a passive diffusion. In the second part of my experimental work, I tested whether the DOC release rates differed with growth conditions. In the macroalgal experiment, the release rates were not different between batch and continuous cultures. Extremely high DOC release in the continuous culture reported by Sieburth (1969) seemed to be a result of adverse experimental conditions. In the phytoplankton experiment, the release rates seemed to be positively correlated with growth rate. The highest release rate was observed during the exponential growth phase. This result and reports in the literature suggest that both quantitative and qualitative differences in phytoplankton DOC release are a function of growth phase. For example, phytoplankton may produce and release more nitrogen-rich low-molecular-weight organic carbon during the exponential growth phase and more carbon-rich high-molecular-weight organic carbon during the stationary growth phase. DOC is a main food source for heterotrophic microbes. Both quantitative and qualitative variations in phytoplankton DOC release are likely to have a large influence on the microbial loop. More detailed information on the composition of the released materials and mechanism involved in the release is needed to understand these processes and the role they play in the carbon cycle in coastal ecosystems.
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file details KawasakiNobuyuki2000.pdf 2017-08-19 公开 下载