Nutrient Cycling in Port Phillip Bay; The Role of Sedimentary Protozoa in Nitrification and Denitrification

A project undertaken at the School of Applied Sciences, RMIT University, Melbourne, and supervised by Dr. Jeff Shimeta

The nitrogen cycle is a critical factor determining biological productivity and water quality of coastal marine areas. Port Phillip Bay, Victoria, receives large inputs of nitrogen from treated sewage and agricultural and urban runoff. Nonetheless, high rates and efficiencies of the coupled processes of nitrification and denitrification (carried out by bacteria in the sediments) maintain low levels of nitrogen in the water and good water quality. Factors influencing the rates of nitrification and denitrification in coastal sediments, however, are poorly understood. Consequently, the reasons for Port Phillip Bay's high efficiency of denitrification, and the great variation of this efficiency among different parts of the bay, are unknown. Considering the likelihood of environmental changes in coastal systems in the near future (for example, climate change, continued development and increasing nutrient inputs, environmental disturbances such as dredging), better understanding of the variables affecting the nitrogen cycle, coastal productivity, and water quality are urgently needed.

Protozoan microbes in the sediments (ciliates, flagellates, and amoebae) interact closely with bacteria and may play key roles in the nitrification-denitrification process. Protozoa can be extremely abundant in sediments (up to hundreds of thousands of cells per cubic centimetre); they excrete ammonium and enhance the flux of sedimentary porewater (which could enhance denitrification); and they ingest nitrifying bacteria (which could reduce denitrification).

We are investigating the influences of protozoan abundances and species composition on measured nitrification and denitrification rates in the laboratory by manipulating protozoa in samples of sediment from Port Phillip Bay. We will also compare the protozoan communities and their impacts on nitrification and denitrification between sites in the bay that differ greatly in their denitrification efficiency. The relative areas of these types of habitat determine the overall water quality in the bay, and it is important to know what factors (such as protozoa) contribute to the differing efficiencies of denitrification. The results of this research should yield not only an improved understanding of the ecological processes affecting denitrification, they may also reveal components of the sedimentary community that are important to manage for maintaining the water quality and environmental health of Port Phillip Bay.

Outcomes from this project included the following:
  1. We found no evidence of an influence of protozoa on nitrification or denitrification in the sediments. This conclusion was reached from experiments using several methodological approaches. We are continuing our research by investigating protozoan-bacterial interactions in alternative experimental systems. Our negative results in sediments suggest investigations of other processes to explain the high denitrification efficiency in Port Phillip Bay, such as the role of burrowing invertebrates that irrigate the sediments.
  2. During the course of our experiments we discovered several problems inherent in an approach commonly applied in studies of protozoan-bacterial interactions in the fields of aquatic ecology, waste-water and sewage treatment processes, and soil ecology. We are working to publish these findings with the aim of improving the experimental methods used in these fields.
  3. This project contributed to the training of one honours student and two research assistants.
Publication in preparation
Shimeta, J. and P. Cook. Inappropriateness of eukaryotic inhibitors for studies of protozoan-bacterial interactions in marine sediments. For submission to Limnology and Oceanography Methods.

 

Figures 1 & 2. Ciliated protozoans isolated from the denitrification zone of Port Phillip Bay sediment.

Figure 3. Sediment from Port Phillip Bay being incubated for the measurement of bacterial denitrification rates.