Thursday 27 September
Earlier this week we met Dr Jess Melbourne-Thomas and PhD student Sarah Ugalde, both members of the Association of Polar Early Career Scientists (APECS). Today we meet the two other members of the group involved in this SIPEX voyage, PhD students Rob Johnson and Zhongnan (Molly) Jia.
Rob is investigating primary production (photosynthesis) and the health of phytoplankton living in the water column under Antarctic sea ice using an exciting bit of technology known as a Fast Repetition Rate Fluorometer or FRRF (affectionately referred to as ‘furf’).
When phytoplankton photosynthesise they absorb some of the sunlight passing through the sea ice, but they cannot absorb all the light that’s available to them. Some of this excess light is deflected by the cells and this is known as fluorescence.
To measure the capacity of phytoplankton to photosynthesise and, ultimately, their health, Rob is using the FRRF to measure this fluorescence. The instrument will be lowered through a hole in the sea ice to a depth of about 50m and as it descends its LED light will flash known quantities of light at different wavelengths (primarily red, green and blue) at phytoplankton as they float through the instrument.
The FRRF has two experimental chambers — a dark one and a light one. The light one measures the fluorescence of phytoplankton in their current state under the sea ice. In the dark chamber, water is pumped through at a constant rate and the phytoplankton are exposed to three seconds of darkness before they are irradiated with the LED light. This period of darkness ensures that most of the photosynthetic reaction centres (choloroplasts) in the cells are ‘empty’. Then, when they are irradiated by the LED they are able to absorb close to their maximum capacity of light.
Plugging this information into a set of mathematical equations, Rob will be able to deduce the phytoplankton’s ability to produce oxygen and to fix carbon from photosynthesis. He will also be able to compare the FRRF technique of measuring primary production with the more traditional and time-consuming technique that follows the uptake of a radioactive carbon isotope (14C) into the cell.
Unlike this traditional method, which takes about 12 hours to get one result, the FRRF will take measurements every half a second. For a 15 minute deployment in one direction, it will collect about 80 000 measurements.
Rob will also deploy the FRRF attached to a Conductivity Temperature and Depth (CTD) rosette — an oceanographic instrument that measures water properties up to about 3000m depth. Rob hopes that once the FRRF technology is proven, it will become a standard way of measuring primary production that can be easily accommodated on ships — much like the continuous plankton recorder (see the previous blog post).
Molly is investigating the diet and energy budget of larval krill and has joined this voyage to collect samples for her research. She is just over one year into her PhD, after completing an undergraduate degree at Xiamen University in China and a masters degree at the Institute of Marine and Antarctic Studies, University of Tasmania.
Larval krill are rarely collected as they develop in late winter and early spring, at a time when it is difficult to get to Antarctica. There are twelve stages in larval krill development but Molly is focusing on the later stages of this development — furcilia 3 to 6. These stages are about 1–2cm in size.
Molly will use three methods to study the diet of the larval krill she collects. The first will be genetic analysis of their stomach contents to get the DNA sequences of the organisms they eat. The second will be microscopy, which will allow her to identify broad groups, such as diatoms or copepods. The third will involve stable isotope analysis of carbon and nitrogen. This method uses ratios of naturally occurring 12C to 13C and 14N to 15N. These ratios change depending on the organism. For example, sea ice algae have a different ratio of 12C and 13C compared to phytoplankton in the water column, so the carbon isotope ratios in krill grazing on sea ice algae will reflect those of the sea ice algae. Together, this dietary information will enhance scientists’ knowledge about the trophic position krill occupy in the Southern Ocean ecosystem.
Molly will also study the growth rates of her larval krill. As krill grow they shed their exoskeleton and Molly will collect these moults, measure them and compare their length to that of the newly moulted krill. These measurements will be combined with metabolic measurements made by other scientists on this voyage, and will allow Molly to determine whether there are differences in the energy requirements of the various larval stages. As development is closely tied to environmental conditions, any changes in the sea ice environment as a result of climate change, for example, could have an impact on the ability of the larvae to acquire the energy (food) they need.