Allelopathic properties of some marine benthic diatoms on marine bacteria

Allelopathy is a chemically mediated interaction that occurs widely among marine plants. Since various algal-bacteria interactions are common in nature, the potential of the extracellular product (ECP) from marine benthic diatoms as a source of allelochemicals against some marine bacteria was investigated. It is hypothesized in this study that: first, the production of extracellular products is a source of allelochemicals, which regulates certain algal bacteria consortia. Second, the production of ECP and bioactive compounds is associated with the condition of the environment where diatoms are growing. The amount of available nutrients, and thus the age of culture, has an influence on the amount of ECP, as well as on the allelopathic activity by diatoms. Five marine benthic diatoms were grown in axenic culture under laboratory conditions. Two independent tests, the disc method and the Optical Density Test, were employed to measure the inhibitory or stimulatory properties of the diatom exudates on marine bacterial growth. Allelopathic properties of benthic diatoms in culture at various growth phases (exponential (to late exponential) and stationary) were tested to assess the effect of the age of culture on the ECP produced by diatoms as a possible source of allelochemicals. Results from this study showed that stimulation and inhibition of bacterial growth are related to the growth phase of the diatom culture, as well as to concentration of exudates in diatom filtrates. Filtrates from all the test diatoms growing at the exponential growth phase exhibited no visible inhibition towards the six test bacteria. Zones of inhibition, which were evident only during late exponential (for T. fasciculata) and stationary growth phases, ranged from 0.5-2.00 mm. N. perminuta S2 showed inhibition against all the test bacteria, and inhibition was measured repeatedly against bacteria strains B1, B2, B4 and B6 (mean zone of inhibition (ZI):0.96 mm, n=42). Likewise, N. perminuta S1 and T. fasciculata inhibited all the test bacteria (mean ZI: 0.88 mm, n=22 and 0.82 mm, n=17, respectively). However, results between replicate experiments varied. Pleurosigma sp. showed evidence of inhibition, though results of inhibition were not consistent against certain bacteria (i.e. B1, B2, B5 and B6) (mean ZI: 0.81 mm, n=16), while Amphora sp. did not demonstrate any signs of inhibition for both exponential and stationary growth phases. In each zone of inhibition (or stimulation) measured, the area inhibited/stimulated by test discs with diatom cells was significantly different (p less than or equal to 0.05) from the control discs. By comparison to the disc method, results from the optical Density Test showed less inhibition of the six test bacteria in diatom filtrate growing at the exponential growth phase. Filtrates obtained from diatoms growing in the stationary growth phase, however, when shown to have an inhibitory and stimulatory effect on the growth of some test bacteria. N. perminuta S1 and S2 growing at stationary phase exhibited negative effects on the growth of almost all of the test bacteria, relative to the other diatoms used in this study. Results from statistical analysis showed a significant decrease in the growth rate and generation time of the test bacteria in 100 percent filtrate of these diatoms compared to the control and other filtrate concentrations (growth rate: p=0.015, generation time: p=0.019 at alpha=0.05). In most cases, the 100 percent diatom filtrate was observed to be the most effective concentration in inhibiting the growth of the different bacterial strains. Enhanced bacterial growth was observed in the filtrate of diatom T. fasciculata growing at stationary phase, especially at 25-50 percent concentrations. The growth rate and generation time of the test bacteria at 25 percent filtrate concentrations were significantly higher than bacterial growth in the control medium, and in 75-100 percent filtrate concentrations (growth rate: p=0.000, generation time: p=0.001 at alpha=0.05). Considerations on the types of bioassays used were also given emphasis to explain the observed relationships between diatoms and bacteria compared to other research done, in which measurable signs of allelopathy could differ and be limited by methodology. In general, the complexity of various kinds of interactions involving allelochemicals, was to a degree observed in this study. Further studies are recommended in order to address specific questions pertaining allelopathy.