Throughout history there are sailor’s reports of seas that appear to glow, from horizon to horizon. These events are extremely poorly understood, as scientific enquiry into what drives these events has been limited to only a single chance encounter. From this single encounter, researchers discovered that the bioluminescence was being caused by high concentrations of Vibrio harveyi in association with the microalga Phaeocystis. However, until recently, this was the extent of our understanding of a process that can cause seas to glow for thousands of square kilometres. A recent study has attempted to shed light on the situation through the considerable power of remote sensing.
The authors searched through ship reports for potential records of this form of bioluminescence, then compared these reports against satellite images provided by US Department of Defence. This combination of in situ records and satellite imaging enabled the authors to gain considerable insight in the extent, duration and potential cause of the event. By culturing V. harveyi and measuring the wavelength emitted, it was possible to search specifically for this wavelength in the satellite images. Only a single occurrence was found in both the ship reports and the satellite images, from 25th January 1995 off the east coast of Somalia (Indian Ocean). The results were enhanced to avoid the confounding effect of the satellite used and other experimental noise.
By further interrogating oceanographic reports, the authors were able to link this region with a cold core eddy, which they believe formed an “accumulation zone”. As these zones are the preferred habitat for most phytoplankton species, the authors suggest this triggered a bloom in the local Phaeocystis population. Phaeocystis is renowned for its tendency to form massive polysaccharide gel matrices during blooms. It is believed that this large increase in extracellular polymers, caused an increase in DOC, and hence promoted the growth of V. harveyi.
Finally, a mathematical model was used to attempt to estimate the density of bacteria necessary to generate the result observed, by using a conservative estimate of the photon production per cell. They estimate the concentrations of V. harveyi required for the signal they received, would be approximately 1022 cells cm-3, however the authors clearly state that due to the number of estimations in this model, it should be regarded as highly approximate.
This study appears to have significantly advanced our knowledge of “milky sea” events through previously gathered data. While I appreciate the potential risks of performing enhancements to the data, the authors were extremely conservative in the alterations they made, with no clear adjustments that could increase the probability of Type 1 error. Considerable information is provided on the satellite imaging system in use, which does not provide further information on the results, but potentially allows readers with a good understanding of remote sensing to assess the reliability of the methods. The suggested model of a cold water eddy creating highly advantageous conditions for the growth of a specific phytoplankton species could to explain why this phenomenon is so rare, however this study demonstrates that through the use of remote sensing, it may be possible to understand this process better in the future.
Posted by Kristian McConville, November 2010
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