The Atlantic Meridional Transect (AMT) programme (1995 - present) takes advantage of the biannual passage of the BAS research vessel James Clark Ross from the UK (50°N) to the Falkland Islands (50°S) to study basin scale patterns and dynamics of open-ocean planktonic ecosystems (more information can be found at http://www.amt-uk.org). An interest in open-ocean plankton has led to our understanding of the importance of various groups of cyanobacteria along the AMT transect: the dominance of small (< 0.002 mm) prochlorophytes in the subtropical gyres, the importance of small Synechococcus in equatorial and temperate waters and the vital nitrogen-fixing role of large (> 2 mm) colony-forming Trichodesmium in equatorial waters. Small cyanobacteria are usually detected through the collection of discrete water samples and flow cytometric analysis, where cell size and phycoerythrin fluorescence are used to identify the different groups. Trichodesmium is collected with nets (or buckets!) and microscopic examination of large water-volumes gives their abundance. A technique that detects both is highly advantageous, allowing targeted sampling and a better understanding of the ecology of marine cyanophytes.
During the 14th AMT cruise (April - June, 2004) we took advantage of the opportunity to use one of the new Cyclops fluorometers set to detect phycoerythrin (a pigment mostly found in cyanobacteria). Due to their insolubility in water, the cyanophyte pigments phycoerythrin and phycocyanin cannot be extracted or eluded with standard pigment analysis and thus our knowledge of the full pigment suite of open-ocean communities has been limited. The use of the Cyclops will provide us with a better understanding of the pigments, community structure and optical properties of the water-column.
Having sailed through the rough waters off the Falkland Islands ("roaring 40s") and into the South Atlantic Gyre we were able to attach the Cyclops fluorometer to our standard CTD package and gain real-time profiles of phycoerythrin and cyanophyte distribution. The appeal of using new technology on the AMT cruises is that the interdisciplinary nature of the cruise allows novel measurements to be related to other more traditional oceanographic measurements (e.g. chlorophyll a concentration, rates of carbon fixation, nutrient concentrations) as well as more specialised ones (e.g. Dimethylsulphide concentration).
Although the results are preliminary and still being validated (frozen and preserved samples to be analysed) several interesting results have come about from the use of the Cyclops. Phycoerythrin was highest in waters with high chlorophyll-a concentration, shallow nitraclines (defined as the 1 mM nitrate contour), and high rates of carbon fixation. Preliminary cell counts show that the source of the phycoerythrin changes with latitude: from Synechococcus and Trichodesmium in equatorial waters to Synechococcus and eukaryotic flagellates (Cryptomonads) in northern temperate waters.
Over the next few months, pigment analysis will allow us to compare phycoerythrin fluorescence to other phytoplankton pigments: previous knowledge of the distribution of such pigments indicates that phycoerythrin fluorescence shows a very similar distribution to the photoprotectant cyanophyte-related pigment, zeaxanthin. Analysis of preserved water samples may allow the phycoerythrin signal to be related to Trichodesmium abundance, as it was noticed during the cruise that when Trichodesmium was present in the water-column the Cyclops signal was highly spiky. Analysis of particle absorption samples and attempts to calibrate the fluorometer will allow us to estimate the concentration of phycoerythrin and its ratio to other phytoplankton pigments.
Many thanks to Turner Designs and RS Aqua (especially Charlotte Deeley) for the opportunity to use the Cyclops, which will become a regular feature on our CTD package during future cruises (AMT-15 sails September 2004!). I would also like to thank fellow AMT scientists for access to their preliminary data from the cruise which has aided in the interpretation of the Cyclops signal so far (Dr Mike Zubkov, Ms Jane Heywood and Ms Katie Chamberlain) and Jon Short and Dougal Mountifield (UKORS) for technical support.
Author: Dr. Alex Poulton
Institution: Southampton Oceanography Centre, Southampton, United Kingdom