Study of potential symbiotic bacteria isolated from a local isolated diatom, Halamphora sp.. Halamphora sp. AQ4 isolate:AQ4

Phytoplankton, such as microalgae, is known with the ability of 20 percent of carbon fixation, the function of converting carbon dioxide to carbon sources, which provides over 40 percent of primary productivity on the planet. Some bacterial species conduct chemo-exchange and signal transduction (e.g. Indole-3-acetic acid, IAA) with their specific host via continuous organic materials secretion. Those interactions occur in a micro-environment called phycosphere. Benthic diatom is a typical phycosphere-constructing microalga, by attaching to the surface and forming a biofilm which consists of diatom exopolysaccharides. These exopolysaccharides can also feed the surrounding microorganisms who move to the nutrient via chemotaxis at the same time. In this study, a locally isolated diatom (AQ4), Halamphora sp., is cultured for observing the population changing of its associated bacteria through 16S rDNA gene V3-V4 high-throughput sequencing. The top two dominant families in diatom stock were Alteromonadaceae and Puniceicoccaceae; while the major family changed to Rhizobiaceae and Pseudomonadaceae, Puniceicoccaceae and Rhodobacteraceae in the chloramphenicol (C) and Penicillin-streptomycin (PS) added media respectively. There were 40 bacterial isolates isolated from the diatom stock medium. After single colony 16S rDNA sequencing and NCBI sequence alignment (BLASTn), the information of bacterial isolates was revealed. The phylogenetic analysis was conducted using the V3-V6 region of these sequences. These isolates include two Rossellomorea, two Cytobacillus, three Sutcliffiella, two Staphylococcus, two Brachybacterium, seven Stutzerimonas, twelve Marinobacter, three Nitratireductor, one Thalassococcus and two Mameliella. Many references indicated that these bacteria isolated from the diatom medium were from marine or related to phytoplankton. For further investigations the ability of colonization in the phycosphere, chemotaxis to diatom secondary metabolites, biofilm formation ability, catalase activity, esterase activity and IAA production of each bacterial isolate were tested. Except for the catalase activity, one of Rossellomorea, Marinobacter and Stutzerimonas were more active or exhibited more obvious differences in the rest of assays. One of Staphylococcus (A4040) was the optimal isolate in catalase activity. Eventually, to validate the impacts of indole-3-acetic acid (IAA) on the diatom AQ4, different concentrations of IAA were added to the diatom culture medium. Indole-3-acetic acid (IAA) were found to promote diatom growth, with the optimal concentration being 20 microgram/mL. However, the concentration over 30 microgram/mL would result in a slight inhibition of diatom growth. To sum up, as the major group of bacterial isolates, Gammaproteobacteria (Stutzerimonas and Marinobacter) showed better activities much evident differences in the assays, with the exception of catalase activity. Additionally, the most abundant family in diatom stock, Alteromonadaceae, included Marinobacter. The results enhanced comprehension of the complex interactions between diatoms and bacteria within marine ecosystems.