Interactions between intertidal macroalgae and ammonium-oxidizing microbes in the presence of ammonium, a shared resource

Background/Question/Methods

Marine macroalgae are often nitrogen-limited, and as a consequence exhibit increased growth rates following nitrogen enhancement by adjacent animals; additionally, an abundant nitrogen-based marine microbial assemblage is associated with nitrogen-excreting animals. North America’s Pacific Northwest (PNW) intertidal and shallow subtidal zones with high animal biomass show increased regenerated nitrogen, ammonium (versus upwelled nitrate). If animals play a quantitative role as nitrogen recyclers in shallow marine systems, then algal and microbial communities dependent upon ammonium excretion are intimately linked to animal community abundance. Microbial nitrification could pose a competitive force on algae via removal of ammonium from the water column. By utilizing ammonium, nitrifying microbes could facilitate algal productivity by retaining nitrogen onshore as nitrate and nitrite in high-energy areas where advection is expected. In this work, we query the range of possible interactions between microbial activities on intertidal algae in the presence of ammonium, a shared resource, in laboratory mesocosm experiments. We varied microbial abundance via novel use of BioBalls for high surface area settlement substrate, and we varied ammonium abundance in through pulsed additions of aqueous 1.5M NH₄Cl. Prionitis sternbergii (hereafter Prionitis), a common intertidal rhodophyte, was maintained in experimental mesocosms for 27 days. Response variables measured were algal growth rate, nitrogen concentrations (ammonia, nitrite, nitrate) in the mesocosm water column, and stable isotopes of nitrogen in algal tissue. A corresponding 5-hour ¹³C and ¹⁵N tracer enrichment experiment was performed in tidepools containing Prionitis but with and without microbe-laden BioBalls to explicitly query nitrogen movement. 

Results/Conclusions

Ammonium addition was positively correlated with water column ammonia concentration and algal growth rate, and elevated microbial communities altered algal nitrogen uptake and water chemistry. Microbe-laden BioBalls influenced %N in algal tissue. Percent nitrogen content in Prionitis tissue was positively correlated with ammonium addition, but this result varied by microbial abundance: Prionitis %N was higher in the presence of increased microbes. Water column nitrite concentrations in the presence of added ammonium and increased microbes increased monotonically throughout the 27-day experimental period. This indicates development of a nitrifying (ammonium oxidizing) microbial community in the presence of abundant ammonium resource and settlement substrate. In the tidepool tracer enrichment experiment, we measured increased uptake of ¹³C and ¹⁵N in the presence of microbe-laden BioBalls. We infer that microbes facilitated algal photosynthesis. This work reveals linkages between algal growth and microbial activity that could play a key role in broader patterns of coastal productivity.