Response of bacteria and phytoplankton from a subtropical front location Southern Ocean to micronutrient amendments ex-situ

Abstract

The effects of micronutrient amendments such as cobalt (Co), copper (Cu), iron (Fe) and their mixture (Co+Cu+Fe) on bacterial abundance, phytoplankton, bacterial community (BC) composition were examined at a subtropical front (STF) location in the Indian Sector of the Southern Ocean (ISSO) during the austral summer of 2012. From the 15-day on-board experiment it became evident that there was no significant increase in total chlorophyll a concentration or phytoplankton cell numbers in micronutrient enriched microcosms (MEM) compared to the control microcosms (CM) with no added micronutrients. Highest bacterial abundance was observed in the Fe-enriched microcosm within 5 days of incubation unlike in other microcosms. Interestingly, significant differences in the BC composition were observed between MEM and CM. In that an increase of ~10 unique phylotypes affiliated with Psychrobacter sp, Prochlorococcus sp, Burkholderia sp, Cytophaga−flavobactria cluster, Roseobacter sp, Bacillus sp and Vibrio sp was observed in the MEM. The phylotypes associated with Cyanobacteria preferentially responded to the Co and Fe additions, whereas the members of the Cytophaga−Flavobacteria cluster responded to the Cu, Fe and mixture amendments. Our results are useful to discern that availability of micronutrients, in particular of Fe, affects the bacterial abundance as well as BC composition, but not the phytoplankton growth/chlorophyll a concentration in the STF location. In addition, micronutrient amendments ex-situ appears to lead to predominance of only a few bacterial phylotypes. This shift in bacterial community composition might be due to preferential and/or versatile utilization of exogenously added micronutrients. Phylogenetic diversity of culturable bacterial populations from this sampling location was also assessed using 99 pure bacterial cultures. For this, a select set of biochemical characteristics was examined and numerical profiling was done before subjecting to 16S rRNA sequencing based identification. This effort yielded close to 20 clusters, and representative isolates from each cluster were subjected to molecular taxonomic analyses using 16S rRNA marker gene. From this analysis, ~90% of bacterial isolates were found to be affiliated with Firmicutes and ~10% to Gammaproteobacteria suggesting preponderance of only a few phylotypes in the deep chlorophyll maximum zone of this STF location