Marine microbial ecology
Introduction
This program studies the microscopic life in the Southern Ocean: phytoplankton, protozoa, bacteria and viruses.
These organisms comprise most of the living matter in the sea, and most organic matter flows within this group of organisms, called the microbial loop. Only a small proportion flows on to higher organisms (such as krill, fish and whales).
Phytoplankton are single-celled algae. Like all plants, they use carbon dioxide and light to produce food in the process of photosynthesis. Most range in size from 1 - 100 micrometres (1 micrometer = 1/1000 millimetre), although some needle-shaped species reach over 2 mm in length. They are the base of the oceanic food web: all other oceanic organisms depend on them directly or indirectly for food.
Phytoplankton also have major effects on the world's climate. By absorbing carbon dioxide, they contribute to the uptake of CO2 from the atmosphere, thereby reducing the global Greenhouse Effect. The Southern Ocean is one of the world's important 'sinks' for carbon dioxide. Some phytoplankton also produce chemicals which when released to the atmosphere form aerosol particles that can trigger the formation of clouds.
Protozoa are single celled animals that are consumers of phytoplankton, bacteria and organic matter. Their respiration releases much of the carbon dioxide that was incorporated by the phytoplankton. However they also help remove CO2 from the atmosphere by repackaging food into faecal pellets that sink into the deep ocean. Phytoplankton and protozoa together are called Protists. Each litre of surface seawater contains about a million protists.
Bacteria are abundant in the Southern Ocean. They break down particulate matter, releasing nutrients for use by other organisms and releasing CO2. They also take up dissolved organic matter, converting it to cell mass, and making it available to grazers.
Viruses are the most abundant biological agents in seawater. They infect phytoplankton, protozoa and bacteria and may be responsible for up to 50% of deaths of marine bacteria. Bursting cells release their contents into the water, where they fuel bacterial growth. As each virus infects a particular species of microbe, viruses may be important in controlling the abundance and composition of microbial communities in Antarctic waters.
Programs
The Australian Antarctic Division programs focus on:
Identifying the components of Southern Ocean protist populations
An atlas with photographs and full descriptions of more than 450 Antarctic protists is being compiled as an aid to the identification of these organisms.
Determining the distribution and abundance of protists in the Southern Ocean
The abundance and types of phytoplankton present are being determined using their photosynthetic pigments, by light and electron microscopy, and using electronic particle counters. Controls on population abundance are being studied in relation to oceanography, nutrients and grazers. This information is vital in developing global carbon budgets.
Determining the response of protist populations to enhanced ultraviolet radiation due to the Antarctic ozone hole
Protists in the upper waters of the ocean are exposed to increased ultraviolet radiation (UV) at the time of their most rapid growth. Different species vary in their susceptibility to UV, and changes of species composition have been observed in Antarctic communities. This changes the abundance and nature of food available to higher organisms and the extent of CO2 uptake in surface waters.
Determining the feeding rates and preferences of protozoa
Measurements are being made of the rates of protozoan feeding in the Southern Ocean, as well as studies of their feeding preferences in the lab. Protozoan preference for particles of particular sizes and "flavours" affects their role in the microbial loop.
These programs link with external programs that measure phytoplankton production, carbon dioxide uptake, carbon sedimentation, nutrient concentrations, oceanography etc. (links to be added) to build an integrated picture of the smallest, but the most important, components of the marine food web.
For more information, see: bio@aad.gov.au
See more information on the Australian Antarctic Division Biology program
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