ARS | Publication request: Measuring the response of individual algal cells to nutrient and atrazine mixtures within biofilms

Submitted to: Microbial Ecology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: December 4, 2011
Publication Date: N/A

Interpretive Summary: Multiple pollutants such as nutrients and atrazine often enter aquatic systems together in agricultural runoff. Yet it is not clear how pollutant mixtures of potential growth stimulators and growth inhibitors alter algae in receiving waters. We grew algal biofilms on ceramic tiles, exposed them to nutrients, atrazine, and mixtures of nutrients and atrazine, and measured the impact of individual and combined pollutants on algae community composition and metabolic activity. We applied an emerging technology, synchrotron infrared microspectroscopy, to isolate the biochemical responses of single cells / species within the biofilm to further examine the complex whole community level results. We found that at the assemblage level, mixtures generally increased algal biovolume, decreased chlorophyll, and had no effect on productivity. Cellular biochemical changes in all four species were generally similar to cells experiencing low stress conditions, but the magnitude of species responses differed among treatments. In general, mixture responses were similar to nutrient only exposures. Our results show that individual microbial species in close proximity in a biofilm can have unique responses to pollutants and that the makeup of the community exposed should have a strong influence in the overall impact of nutrient and atrazine mixtures.

Technical Abstract: Pollutant effects on biofilm physiology are difficult to assess due to differential susceptibility of species and difficulty separating individual species for analysis. Also,measuring whole assemblage responses such as metabolism can mask species-specific responses, as some species may decrease and others increase metabolic activity. Physiological responses can add information to compositional data, and may be a more sensitive indicator of effect. It is difficult, however, to separate individual species for biochemical analyses. Agricultural runoff often contains multiple pollutants that may alter algal assemblages in receiving waters. It is unclear how mixtures containing potential algal growth stimulators and inhibitors (e.g., nutrients and herbicides) alter algal assemblage structure and function. Inresearch presented here, algal biofilms were exposed to nutrients, atrazine, and their mixtures, and assemblage level structural and functional changes were measured. Synchrotron infrared microspectroscopy (IMS) was used to isolate the biochemical changes within individual cells from a dominant species of a green alga (Mougeotia sp.), a diatom (Navicula sp.), and a cyanobacterium (Hapalosiphon sp.). At the assemblage level, mixtures generally increased algal biovolume, decreased chlorophyll a (Chl a), and had no effect on metabolism or ammonium uptake. Navicula had a strong negative response to atrazine initially, but later was more affected by nutrients. Hapalosiphon responded positively to both atrazine and nutrients, and Mougeotia did not exhibit any biochemical trends. Generally, biochemical changes in each species were similar to cells experiencing low stress conditions, with increased relative protein and decreased relative lipid. IMS provided direct evidencethat individual species in a natural biofilm can have unique responses to atrazine, nutrients, and mixtures. Results suggest that the initial benthiccommunity composition should have a strong influence on the overall impact of agricultural pollutants.