UCYN-A/haptophyte symbioses dominate N fixation in the Southern California Current System.

The availability of fixed nitrogen (N) is an important factor limiting biological productivity in the oceans. In coastal waters, high dissolved inorganic N concentrations were historically thought to inhibit dinitrogen (N) fixation, however, recent N fixation measurements and the presence of the N-fixing UCYN-A/haptophyte symbiosis in nearshore waters challenge this paradigm. We characterized the contribution of UCYN-A symbioses to nearshore N fixation in the Southern California Current System (SCCS) by measuring bulk community and single-cell N fixation rates, as well as diazotroph community composition and abundance.

Critical Role of Light in the Growth and Activity of the Marine N-Fixing UCYN-A Symbiosis.

The unicellular N-fixing cyanobacteria UCYN-A live in symbiosis with haptophytes in the lineage. Maintaining N-fixing symbioses between two unicellular partners requires tight coordination of multiple biological processes including cell growth and division and, in the case of the UCYN-A symbiosis, N fixation of the symbiont and photosynthesis of the host. In this system, it is thought that the host photosynthesis supports the high energetic cost of N fixation, and both processes occur during the light period.

Unusual marine cyanobacteria/haptophyte symbiosis relies on N fixation even in N-rich environments.

The microbial fixation of N is the largest source of biologically available nitrogen (N) to the oceans. However, it is the most energetically expensive N-acquisition process and is believed inhibited when less energetically expensive forms, like dissolved inorganic N (DIN), are available. Curiously, the cosmopolitan N-fixing UCYN-A/haptophyte symbiosis grows in DIN-replete waters, but the sensitivity of their N fixation to DIN is unknown.

Kīlauea lava fuels phytoplankton bloom in the North Pacific Ocean.

From June to August 2018, the eruption of Kīlauea volcano on the island of Hawai'i injected millions of cubic meters of molten lava into the nutrient-poor waters of the North Pacific Subtropical Gyre. The lava-impacted seawater was characterized by high concentrations of metals and nutrients that stimulated phytoplankton growth, resulting in an extensive plume of chlorophyll a that was detectable by satellite. Chemical and molecular evidence revealed that this biological response hinged on unexpectedly high concentrations of nitrate, despite the negligible quantities of nitrogen in basaltic lava.

Distributions and Abundances of Sublineages of the N-Fixing Cyanobacterium Atelocyanobacterium thalassa (UCYN-A) in the New Caledonian Coral Lagoon.

Nitrogen (N) fixation is a major source of nitrogen that supports primary production in the vast oligotrophic areas of the world's oceans. The Western Tropical South Pacific has recently been identified as a hotspot for N fixation. In the Noumea lagoon (New Caledonia), high abundances of the unicellular N-fixing cyanobacteria group A (UCYN-A), coupled with daytime N fixation rates associated with the <10 μm size fraction, suggest UCYN-A may be an important diazotroph (N-fixer) in this region.

Distinct ecological niches of marine symbiotic N -fixing cyanobacterium Candidatus Atelocyanobacterium thalassa sublineages.

A recently described symbiosis between the metabolically streamlined nitrogen-fixing cyanobacterium UCYN-A and a single-celled eukaryote prymnesiophyte alga is widely distributed throughout tropical and subtropical marine waters, and is thought to contribute significantly to nitrogen fixation in these regions. Several UCYN-A sublineages have been defined based on UCYN-A nitrogenase (nifH) sequences. Due to the low abundances of UCYN-A in the global oceans, currently existing molecular techniques are limited for detecting and quantifying these organisms.