Microbial menagerie under the microscope

The marine science voyage to the Mertz Glacier region brought together researchers with a diversity of interests in marine microbial processes including the distribution, abundance, biodiversity and activity of phytoplankton and the effects of ocean acidification on organisms. My particular tasks were to ascertain the change in the concentration of cyanobacteria (blue-green algae) with temperature; to collect samples of a group of phytoplankton called Parmales; and to identify protists (see story below) that others collected as part of their sampling. As most of the organisms are very sensitive to elevated temperature I used a microscope set up in a 3°C cold room. To damp out the vibrations of the ship’s engines, the microscope was shock-mounted so the smallest organisms could be examined, counted and photographed.

In tropical and temperate oceanic waters some of the most abundant organisms are photosynthetic cyanobacteria, which occur at concentrations of 10–100 million cells per litre. These tiny organisms carry out most of the photosynthesis and play a critical role at the base of the food web. However their abundance declines dramatically with decreasing temperature across the Southern Ocean. Near the Antarctic coast where the water temperature is less than 2°C, their concentration drops to only about 1000 per litre. Their virtual absence in polar waters represents one of the fundamental differences in biodiversity between polar, temperate and tropical oceans. We discovered the quantitative relationship between water temperature and cyanobacterial concentration 20 years ago. On this voyage I repeated the counts to see if the relationship had changed, perhaps by different strains of cyanobacteria. It hadn’t.

Parmales are a group of minute phytoplankton, less than 5 µm in diameter, covered with patterned scales made from silica. They are especially abundant in polar and sub-polar waters but only poorly represented elsewhere in the global ocean. They are commonly found in the faecal material of grazers, which suggests they are heavily grazed and therefore important in polar food webs. Their high abundance is also indicated by their silica scales composing over 30% of the biogenic silica in some sediments close to the Antarctic coast. Although Beatrice Booth, from the University of Washington, and I formally described them as a new species of algae 25 years ago, they have only recently been characterised using molecular biological methods and their photosynthetic pigments. On this voyage I collected Parmales to study their life cycle, which has yet to be fully ascertained. These studies are continuing on cultures of living Parmales at the Australian Antarctic Division.

Perhaps the greatest surprise on the voyage was finding a massive phytoplankton bloom where the Mertz Glacier tongue was before it was dislodged by iceberg B09B. The bloom was apparently stimulated by iron being released into the sea by the melting of multi-year sea ice that had been dammed up by the glacier tongue. There was a huge drawdown of carbon dioxide in the vicinity of the bloom. Cameras deployed from the ship to see what organisms were living on the seafloor where the Mertz Glacier tongue used to be, revealed that phytoplankton from the bloom were dropping out and covering the bottom with a green fluff. Microscopy of the bloom revealed that the bloom was composed of the large diatoms Corethron, Rhizosolenia and Chaetoceros. These same organisms were found in samples of the fluff from the bottom. Interestingly, most of these diatoms still had their cellular contents, indicating that they had only recently fallen from the surface waters. So as well as providing the food for grazers at the surface, the phytoplankton were providing a fresh food supply for the bottom dwellers.


Australian National University

Single cell diversity

At the level of the single cell, what is a plant and what is an animal becomes blurred. Some phytoplankton graze on bacteria and other phytoplankton while some protozoa (animals) harbour and nurture chloroplasts (the photosynthetic organelles) from phytoplankton that they have consumed, and use the photosynthetic products themselves. These days it’s usual to lump marine phytoplankton and protozoa together and call them protists. In 2005 Fiona Scott and I published the book Antarctic Marine Protists in which we describe over 550 different species. These different species of microscopic organisms are as distinctive as the different species of plants and animals that you can see with the naked eye.