Interactions between phytoplankton and heterotrophic bacteria in freshwater

Jianhua Le, Fordham University


The relationship between autotrophic phytoplankton and heterotrophic planktonic bacteria is often called coupling because of a positive correlation between phytoplankton biomass or production and bacterioplankton abundance or production. The objectives of this dissertation are to examine how bacterial-phytoplankton coupling is affected by differences in algal composition, the concentration of dissolved organic carbon (DOC), inorganic nutrient concentration, and intensity of the grazing pressure. In this dissertation, I have found that bacterial-phytoplankton relationship is relatively insensitive to the supply of dissolved organic carbon (DOC). The additions of picophytoplankton (Synechococcus) and nanophytoplankton (Scenedesmus) do not significantly affect the coupling between phytoplankton and planktonic bacteria. However, different combinations of inorganic nutrients (N and P) are able to uncouple the bacterial-phytoplankton linkage. I have also designed a useful and simple Coupling Index (CI), which predicts that shifts in phytoplankton and bacterioplankton growth will be unrelated ($\Delta$bact/$\Delta$phyto $\to$ either +$\infty$ or $-\infty$) in systems with inorganic N:P (molar) ratios $<$ about 50. Grazing pressure from large zooplankters such as Daphnia can also unlink the bacterial-phytoplankton coupling by consuming a larger proportion of phytoplankton biomass than bacterial biomass. The altered relationship between phytoplankton and bacteria can also affect the pelagic ecosystem efficiency. The results suggest that different microbial components may have contrasting effects on the presumed enhanced efficiency provided by the microbial loop. I also proposed an automated procedure to measure total dissolved nitrogen (TDN). This procedure recovers 100% of several organic N forms in deionized water, and achieves over 90% recovery for organic-N-spiked natural water samples. This method will be a useful tool for routinely monitoring total dissolved nitrogen in freshwater ecosystems.

Subject Area

Ecology|Freshwater ecology|Microbiology

Recommended Citation

Le, Jianhua, "Interactions between phytoplankton and heterotrophic bacteria in freshwater" (1996). ETD Collection for Fordham University. AAI9628340.