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Chemically reduced nitrogen forms are increasing in aquatic systems and beginning to reach concentrations not previously measured. Despite this, little research has examined the potential of reduced nitrogen forms to encourage excess nitrogen storage and promote algal bloom longevity compared to oxidised forms.

A 2‐week field, pulse‐application experiment was carried out using 1,100‐L plastic limnocorrals to examine cyanobacterial community response to three nitrogen forms, including nitrate, ammonium, and urea (added as 600 µg N/L). Cell pigments and counts were used to calculate cell‐specific pigment concentrations, and cell‐associated microcystin concentrations were also measured to examine toxin response to a shift in nitrogen source.

Results showed that, upon nitrogen introduction, extracellular nitrogen quickly decreased in accordance with an increase in cellular phycocyanin 72 hr after fertilisation. Ammonium and urea treatments had more phycocyanin/cell than nitrate or control treatments at 72 hr. After 72 hr, phycocyanin content quickly decreased, consistent with the use of nitrogen from phycobiliproteins. Despite the decrease in light‐harvesting pigments, the total number of cyanobacterial cells increased in the ammonium and urea treatments after 2 weeks. Cyanobacterial particulate toxin (microcystin) quotas were not affected by nitrogen additions.

Results show that reduced nitrogen forms encourage greater nitrogen storage as pigments and increase bloom longevity compared to oxidised forms.

Findings support previous studies that suggest reduced nitrogen forms encourage greater cell density and algal bloom persistence. Results further point to excess nitrogen storage as another mechanism that allows cyanobacteria to dominate freshwater systems despite variable environmental conditions.

 

Bacteria play key roles in the function and diversity of aquatic systems, but aside from study of specific bloom systems, little is known about the diversity or biogeography of bacteria associated with harmful cyanobacterial blooms (cyanoHABs). CyanoHAB species are known to shape bacterial community composition and to rely on functions provided by the associated bacteria, leading to the hypothesized cyanoHAB interactome, a coevolved community of synergistic and interacting bacteria species, each necessary for the success of the others. Here, we surveyed the microbiome associated withMicrocystis aeruginosaduring blooms in 12 lakes spanning four continents as an initial test of the hypothesizedMicrocystisinteractome. We predicted that microbiome composition and functional potential would be similar across blooms globally. Our results, as revealed by 16S rRNA sequence similarity, indicate thatM. aeruginosais cosmopolitan in lakes across a 280° longitudinal and 90° latitudinal gradient. The microbiome communities were represented by a wide range of operational taxonomic units and relative abundances. Highly abundant taxa were more related and shared across most sites and did not vary with geographic distance, thus, likeMicrocystis, revealing no evidence for dispersal limitation. High phylogenetic relatedness, both within and across lakes, indicates that microbiome bacteria with similar functional potential were associated with all blooms. WhileMicrocystisand the microbiome bacteria shared many genes, whole‐community metagenomic analysis revealed a suite of biochemical pathways that could be considered complementary. Our results demonstrate a high degree of similarity across globalMicrocystisblooms, thereby providing initial support for the hypothesizedMicrocystisinteractome.

 

Abstract Advances in sequencing technology have resulted in the expectation that genomic studies will become more representative of organismal diversity. To test this expectation, we explored species representation of nonhuman eukaryotes in the Sequence Read Archive. Though species richness has been increasing steadily, species evenness is decreasing over time. Moreover, the top 1% most studied organisms increasingly represent a larger proportion of total experiments, demonstrating growing bias in favor of a small minority of species. To better understand molecular processes and patterns, genomic studies should reverse current trends by adopting more comparative approaches.