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. 2011;6(6):e21032.
doi: 10.1371/journal.pone.0021032. Epub 2011 Jun 17.

Interactions of the Algicidal Bacterium Kordia Algicida With Diatoms: Regulated Protease Excretion for Specific Algal Lysis

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Free PMC article

Interactions of the Algicidal Bacterium Kordia Algicida With Diatoms: Regulated Protease Excretion for Specific Algal Lysis

Carsten Paul et al. PLoS One. .
Free PMC article

Abstract

Interactions of planktonic bacteria with primary producers such as diatoms have great impact on plankton population dynamics. Several studies described the detrimental effect of certain bacteria on diatoms but the biochemical nature and the regulation mechanism involved in the production of the active compounds remained often elusive. Here, we investigated the interactions of the algicidal bacterium Kordia algicida with the marine diatoms Skeletonema costatum, Thalassiosira weissflogii, Phaeodactylum tricornutum, and Chaetoceros didymus. Algicidal activity was only observed towards the first three of the tested diatom species while C. didymus proved to be not susceptible. The cell free filtrate and the >30 kDa fraction of stationary K. algicida cultures is fully active, suggesting a secreted algicidal principle. The active supernatant from bacterial cultures exhibited high protease activity and inhibition experiments proved that these enzymes are involved in the observed algicidal action of the bacteria. Protease mediated interactions are not controlled by the presence of the alga but dependent on the cell density of the K. algicida culture. We show that protease release is triggered by cell free bacterial filtrates suggesting a quorum sensing dependent excretion mechanism of the algicidal protein. The K. algicida / algae interactions in the plankton are thus host specific and under the control of previously unidentified factors.

Conflict of interest statement

Competing Interests: The authors have declared that no competing interests exist.

Figures

Figure 1
Figure 1. Screening for the susceptibility of four different diatom species to K. algicidafiltrate.
Mean values of in vivo fluorescence + SD (n = 4) displayed are measured after 39 h. Asterisks indicate significant differences between the respective control and treatment.
Figure 2
Figure 2. Effects of size fractionated S. costatum, K. algicida and co-culture cell free filtrates on the susceptible species S. costatum determined by in vivo fluorescence.
Values displayed are measured after 47 h and are mean values + SD (n = 5 for all but S. costatum where n = 4). Different letters indicate statistically significant differences.
Figure 3
Figure 3. Effect of the protease inhibitor PMSF on the inhibiting effect of K. algicida filtrates.
Values displayed are measured after 52 h and are mean values + SD (n = 5).
Figure 4
Figure 4. Left: Growth of S. costatum indicated as in vivo fluorescence in K. algicida filtrate conditioned for 24 h with S. costatum, C. didymus or seawater.
Right: Incubations with the same filtrates that were heat inactivated before the start of the experiment. Displayed are mean values + SD (n = 6) taken 70h after the start of the experiment. A statistically significant difference is indicates by different letters above the bar.
Figure 5
Figure 5. Protease release by K. algicida during the growth of a culture.
A) Protease release pattern of K. algicida under standard growth conditions. B) Protease release pattern of K. algicida with conditioned cell free filtrate added directly with the inoculation of the cultures. C) Protease release pattern of K. algicida with conditioned cell free filtrate added 16 h after the inoculation of the cultures. The line indicates bacterial growth measured as OD and the bars give the bacterial protease release rate. The arrows indicate the time of the addition of K. algicida conditioned cell free filtrate. Displayed are mean values + SD (n = 3).

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