135.
Larval settlement in the marine polychaete
Hydroides elegans (Haswell) is induced by certain bacteria in marine biofilms. The exact nature of the settlement cue that larvae of
H. elegans receive from these bacteria remains unknown. In this study, we revealed some properties of the bacterially derived larval
settlement cue by investigating the larval settlement inductive activity of two bacterial strains after various treatments.
These two bacterial strains,
Roseobacter sp. and an α-subclass
Proteobacteria, are highly inductive to larval settlement of
H. elegans. The larvae responded similarly to
Roseobacter and
Proteobacteria in all the larval settlement bioassays, suggesting that the larval settlement-inducing substances produced by these bacteria
may share common characteristics. First of all, the larvae did not settle in the seawater conditioned by the bacteria attached
as a film or by the bacteria that were freely suspended in seawater. The results suggest that the putative larval settlement
cue is not released into seawater and, therefore, should be associated with the surface of the bacteria. Secondly, formaldehyde
treatment entirely eliminated the larval settlement induction activity of the bacterial films, and streptomycin treatment
reduced the percentage of larval settlement on the bacterial films in a concentration-dependent manner. Since both treatments
can kill bacteria with little damage to the surface chemistry of bacterial cells, the decline in larval settlement is suggested
be due to a reduction of the viable bacterial population in the bacterial films. In fact, the reduction of larval settlement
in the streptomycin treatments coincided with the decrease in viable bacterial populations in broth cultures containing respective
concentrations of streptomycin. These results suggest that the viability of
Roseobacter and
Proteobacteria is important to their settlement induction effect. Since the larval settlement induction activity of the bacterial strains
appears to correlate with their viability, we suggest that the putative larval settlement cue is derived from a metabolic
pathway in the bacteria and that the cue is exported to and concentrated at the extracellular polymer matrix of the bacterial
cell, at which the larvae establish contact with the bacteria. The larval settlement cue may be highly susceptible to degradation
so that a metabolically active bacterial film is needed to maintain the putative cue at a concentration that surpasses the
threshold for induction of larval settlement.
Received: 14 October 1998 / Accepted: 5 September 2000
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