Detection and characterization of nitrogen-fixing cyanobacterial harmful algal bloom communities using molecular approaches

Cyanobacterial (“blue-green algae”) harmful algal bloom species (cyanoHABs) are increasingly expanding into previously bloom free waters and can have detrimental impacts on aquatic systems in which they proliferate. This study uses both ecological and molecular approaches to characterize the composition and activity of the cyanoHAB communities present in the Neuse River Estuary (NRE), North Carolina and the freshwater lake system in northeastern Florida, USA. Molecular-based methods for detecting target species before they reach bloom proportions were also developed.

Blooms of non-nitrogen fixing cyanobacteria have been common in the upriver sections of the eutrophic NRE and N reductions to this system were mandated to decrease phytoplankton productivity. However, nitrogen-fixing (diazotrophic) cyanobacteria that can provide their own supply of N could potentially proliferate in N-limited waters. To assess the potential impact of these populations, the spatial and temporal distribution of diazotrophic cyanobacteria in the NRE was characterized based on nifH analysis, a gene involved in nitrogen fixation. Cyanobacterial nitrogen fixers were detected from the low salinity upper reaches of the NRE down to the mouth of the estuary, but were most commonly found in the mesohaline section where the estuary widens and residence time increases. There was a relatively low genetic diversity of nitrogen-fixing cyanoHAB species present, with the heterocystous cyanobacteria Anabaena sp. and Anabaenopsis sp. and a non-heterocystous species related to Lyngbya sp. most commonly identified. The widespread presence of these bloom-forming species indicates there is a community that could rapidly respond to changing nutrient conditions in the NRE.

The recent expansion of another cyanoHAB species, Cylindrospermopsis raciborskii, in many lakes and reservoirs throughout Florida is of concern due to its ability to produce the hepatotoxic cylindrospermopsin and the neurotoxic paralytic shellfish poisons. DNA-based methods were developed to specifically and rapidly detect C. raciborskii in environmental water samples. Using these methods, C. raciborskii was found in 90% of the lakes sampled and there was a high degree of genetic similarity in C. raciborskii populations within and between most of the lakes. C. raciborskii strains originating from different parts of the US and Brazil also were genetically very similar to the Florida strains, suggesting that these populations all originated from a common source relatively recently.

In addition to identifying the presence of C. raciborskii, molecular approaches were used to characterize diel patterns of nitrogen fixation in cultured isolates and natural communities of this species in order to further understand some of the environmental controls on its activity. While maximum rates were measured during the early afternoon, nighttime nitrogenase activity rates can be up to 70% of the maximum daytime rates. Both mRNA nifH transcripts and nitrogenase proteins were present in C. raciborskii cultures throughout the diel cycle and metabolic inhibitors were used to show that nifH was being transcribed before dawn and nitrogenase proteins synthesized just after dawn. Thus, molecular approaches were demonstrated to be useful in both detecting cyanoHABs and characterizing the nitrogen fixation activity of these populations.