Bioluminescence: What is  its role to marine snow? 

Bacteria use a variety of mechanisms to inhabit a large diversity of environments. One of these organisms is the bioluminescent bacteria which have the ability to produce light via quorum sensing. These organisms occur in a large diversity of environments including marine snow and in the photophores of fish. It has long been hypothesised that the bacteria living on the snow glow to mark the presence of a food particle for other organisms in order to get into their guts. This is termed the bait hypothesis. So far, this hypothesis has only been supported by showing that bacteria survive passage through fish guts. Once in the gut of fish, bioluminescent bacteria gain a nutritious environment for growth and a faster moving vector for dispersal.

 The objective of this study was to test the proposed steps of the bait hypothesis:

1.        The visual attraction of zooplankton to bacterial bioluminescence

2.       Promotion of glow in zooplankton ingesting bacteria (using planktonic brine shrimps as a surrogate for zooplankton)

3.        Attraction of fish to glowing prey

4.        Survival of bacteria in the guts of both zooplankton and fish.

Zarubin's experiment was set up in a sea water tank, this was used to examine if the luminescence of Photobacterium leiognathi attracts Decapod, Mysid and Copepod zooplankton. Zarubin et al observed significant changes in the distribution of Decapods and Mysids but not in Copepods. The brine shrimp Artemia salina (the surrogate to test the promotion of glow) became luminescent after swimming in a culture of P. leiognathi. Using long-exposure photographs, the luminescence in the guts of Artemia were clearly visible, with additional glow produced by bacteria attached externally. Similarly, non glowing individual marine Mysids, Anisomysis marisrubri started to glow after contacting P. leiognathi.

Zarubin and co found that glowing A. salina increased risk of predation by the fish Apogon annularis in dark. Almost all glowing Artemia were consumed compared with rare predation of non glowing specimens, the authors stated that this occurred only when the prey drifted by chance directly toward  the predator. Glow was detected in faeces of Artemia that fed on the bacteria which indicated that the luminescent bacteria survived the passage through the guts.

This study provides experimental evidence for some steps of the bait hypothesis. But the author’s did not differentiate between external and internal glow on the surrogates. The authors argue that both sources occur in nature, but cells attached externally are not following the proposed steps of the bait hypothesis. Therefore these results should not be used to answer steps 1 and 2 of the bait hypothesis in my opinoun. Zarubin et al used Artemia as a surrogate for zooplankton prey in this experiment because it is readily eaten by fishes, easy to handle, and lacks evasive behaviour. But I do not agree with this choice of organism because it is a freshwater organism that is being used as a model for an experiment which relates to deep sea ecology and therefore is not an appropriate choice. The authors argue that the luminesce observed in A. marisrubri had the same intensity as Artemia but they have not made it clear why A. marisrubri was not chosen in the first place. Although the experiments with P. leiognathi did not fully simulate in situ conditions I think this study provides a good basis for experimental testing of the bait hypothesis. However, experiments with real marine snow are needed to test this claim explicitly.

 

I chose to review this study due to my interest in bioluminescent bacteria. I also have a large curiosity in  marine snow ecology and how these bacteria are affecting it.

 

http://www.pnas.org/content/109/3/853.full