Future Directions in Bioluminescence Research

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ABSTRACT

What We Don’t Know About Bioluminescence

Steven Haddock

Monterey Bay Aquarium Research Institute, 7700 Sandholdt Rd., Moss Landing, CA 95039

There have been many advances in our understanding of marine bioluminescence. For example, we know quite a bit about certain model systems, such as cultured dinoflagellates and bacteria. Photoprotein and luciferase genes have been cloned and inserted into heterologous systems, new bioluminescent species have been identified, and instruments to measure luminescence have been deployed across the globe. In spite of these breakthroughs, however, there are still fundamental gaps our knowledge.

It might be argued that the gaps in this field are no greater than those in other comparable fields. Other effector systems have persistent questions, and general oceanographic and ecological models are in constant states of revision. However, there are several aspects of bioluminescence which make it especially difficult to study. Among these: (1) organisms must be collected in good condition to exhibit proper behavior, (2) luminous creatures are often found in remote or inaccessible areas of the ocean, (3) basic ecological relationships, which might shed light on roles of luminescence, are not well-known for many marine creatures, (4) many luminous organisms can not be maintained in the lab (5) luminescence is not easily quantified by remote instrumentation. Some of the most exciting progress in bioluminescence research is coming about as a result of recent technological advances, which overcome these historical limitations. Interesting topics for further work are outlined below.

Chemistry
Although the chemistry of certain bioluminescent species is well known, in most cases we do not have basic knowledge of the reactions that are involved. This applies to some extent to scyphomedusae, polychaetes, siphonophores, urochordates, amphipods, and other common marine phyla. Radiolaria have calcium-activated photoproteins which use the same luciferin as cnidaria, yet the molecular roots of this evolutionary convergence are obscured. Even in the photoprotein aequorin, which has been studied for over 30 years, the source of the bioluminescent substrate has not been discovered. The molecular roots of multiple colors of luminescence, and the diversity of luciferins employed by crustacea cannot be explained.

Cellular Events
Within a cell, certain events must occur for light to be produced. Many excellent researchers have uncovered elements of this process in dinoflagellates, yet the basic events which occur between membrane deformation and light emission are not clearly understood. With recent advances in cellular techniques, headway is being made along these fronts.

Roles of Luminescence
Perhaps the most vexing unanswered questions are related to what functions luminescence plays. In some cases, elegant explanations for luminescence have been discovered, but nothing can satisfactorily explain why it is so widespread, indiscriminately crossing taxonomic lines. Why do some organisms invest a large amount of resources, while others seem to thrive without it. The actual rates of bioluminescence flashes in the sea have only been measured a few times, so it is difficult to speculate about how much luminescence an organism encounters in its natural environment. This will clearly continue to be an area fostering much fruitful study.

Biological Extent
The evolutionary origins are generally poorly understood. Lacking a fossil record, we must reconstruct the rise of luminescence through molecular investigations, which are only now beginning. Related to this is the unpredictable taxonomic spread of luminescence. For example, certain species of the dinoflatellate Ceratium are bioluminescent, and others aren't. Nearly all ctenophores and siphonophores are luminous, but a few species seem never to be. Up to now, there has been no reliable means for predicting a priori whether a species is likely to be luminous, but with increasing genetic information, and information about dietary links, we are in a better position to discover the basic rules governing the phylogenetic distribution of luminescence.

We can also expect continued advances in our knowledge of which species are luminous. Although the most abundant luminescent organisms are well known, the last decade has seen the discovery of new luminous species, orders and even a phylum. What bioluminescent organisms will be found in the future among the nanoplankton and the deep-sea invertebrates?

Oceanographic Extent
Although several researchers have been adding to data on the geographical and vertical distribution of bioluminescence, and new technology is being applied to the problem there is currently no way to get a synoptic view of the oceanic-scale distribution of bioluminescence. Luminescence beyond the very surface is not well seen by aerial or satellite images, nor is it well predicted by readily measured oceanographic parameters. It is clearly not distributed homogeneously in either a vertical or horizontal sense. Through integrated programs of study, bringing together physical and biological oceanographers as well as modelers, prediction of the distribution of bioluminescence will begin to be realized.

Summary
To date there have been many fundamental breakthroughs in understanding of bioluminescence, but some of the most fascinating and important questions are only now beginning to be addressed. The years to come should provide interesting advances in how bioluminescence is made, what triggers its expression, why it exists, which organisms can do it, and how to predict and measure how they are distributed.

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