Abstract
A number of studies have shown that the production of chemical defences is costly in terrestrial vascular plants. However, these studies do not necessarily reflect the costs of defence production in macroalgae, due to structural and functional differences between vascular plants and macroalgae. Using a specific culturing technique, we experimentally manipulated the defence production in the red alga Bonnemaisonia hamifera to examine if the defence is costly in terms of growth. Furthermore, we tested if the defence provides fitness benefits by reducing harmful bacterial colonisation of the alga. Costly defences should provide benefits to the producer in order to be maintained in natural populations, but such benefits through protection against harmful bacterial colonisation have rarely been documented in macroalgae. We found that algae with experimentally impaired defence production, but with an externally controlled epibacterial load, grew significantly better than algae with normal defence production. We also found that undefended algae exposed to a natural epibacterial load experienced a substantial reduction in growth and a 6-fold increase in cell bleaching, compared to controls. Thus, this study provides experimental evidence that chemical defence production in macroalgae is costly, but that the cost is outweighed by fitness benefits provided through protection against harmful bacterial colonisation.
Highlights
Organisms have evolved a variety of defence mechanisms to defend themselves against their natural enemies, and most theories about the evolution of defences assume that these mechanisms are costly [1,2,3]
After four weeks in culture where bacterial growth was inhibited by external addition of 1,1,3,3-tetrabromo-2-heptanone, algae with impaired defence metabolite production had grown on average 20% more than algae with normal metabolite production
We found that algae with impaired defence metabolite production were unable to control its epibacterial load
Summary
Organisms have evolved a variety of defence mechanisms to defend themselves against their natural enemies, and most theories about the evolution of defences assume that these mechanisms are costly [1,2,3]. Earlier studies provided mixed results [7], there is growing evidence that chemical defence production generally is costly for terrestrial vascular plants [6,8,9,10,11]. A significant cost of defence production was detected in D. pulchra germlings by culturing the alga with and without access to bromine. Such culturing manipulations offer a powerful tool to examine costs of chemical defences in macroalgae where poor mechanistic knowledge of chemical defence production restricts the use of the chemical elicitation or transgenic methods more commonly used for terrestrial plants [16,17]
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