Empirical relationship between nifH gene abundance and diazotroph cell concentration in the North Pacific Subtropical Gyre.

Cyanobacterial N2 -fixing microorganisms (diazotrophs) play a critical role in nitrogen and carbon cycling in the oceans; hence, accurate measurements of diazotroph abundance are imperative for understanding ocean biogeochemistry. Marine diazotroph abundances are often assessed using qPCR of the nifH gene, a sensitive, taxa-specific, and time/cost-efficient method. However, the validity of nifH abundance as a proxy for cell concentration has recently been questioned. Here, we compare nifH gene abundances to cell counts for four diazotroph taxa (Trichodesmium, Crocosphaera, Richelia, and Calothrix) on two cruises to the North Pacific Subtropical Gyre, one of the largest habitats for marine diazotrophs. nifH:cell relationships were strong and significant for Crocosphaera, Richelia, and Calothrix (nifH:cell 1.51-2.58; R2  = 0.89-0.96) but were not significant for Trichodesmium, despite previous studies reporting significant nifH:cell relationships for this organism. Limited available data suggest that empirical nifH:cell can vary among studies but that relationships are usually significantly linear and >1:1. Our study indicates that nifH gene abundance, while not a direct measure of cells, is a useful quantitative proxy for diazotroph abundance.

Diel variability of bulk optical properties associated with the growth and division of small phytoplankton in the North Pacific Subtropical Gyre.

Cross-platform observing systems are requisite to capturing the temporal and spatial dynamics of particles in the ocean. We present simultaneous observations of bulk optical properties, including the particulate beam attenuation (cp) and backscattering (bbp) coefficients, and particle size distributions collected in the North Pacific Subtropical Gyre. Clear and coherent diel cycles are observed in all bulk and size-fractionated optical proxies for particle biomass. We show evidence linking diurnal increases in cp and bbp to daytime particle growth and division of cells, with particles <7µm driving the daily cycle of particle production and loss within the mixed layer. Flow cytometry data reveal the nitrogen-fixing cyanobacterium Crocosphaera (∼4-7µm) to be an important driver of cp at the time of sampling, whereas Prochlorococcus dynamics (∼0.5µm) were essential to reproducing temporal variability in bbp. This study is a step towards improved characterization of the particle size range represented by in situ bulk optical properties and a better understanding of the mechanisms that drive variability in particle production in the oligotrophic open ocean.

SeaFlow data v1, high-resolution abundance, size and biomass of small phytoplankton in the North Pacific.

SeaFlow is an underway flow cytometer that provides continuous shipboard observations of the abundance and optical properties of small phytoplankton (<5 µm in equivalent spherical diameter, ESD). Here we present data sets consisting of SeaFlow-based cell abundance, forward light scatter, and pigment fluorescence of individual cells, as well as derived estimates of ESD and cellular carbon content of picophytoplankton, which includes the cyanobacteria Prochlorococcus, Synechococcus and small-sized Crocosphaera (<5 µm ESD), and picophytoplankton and nanophytoplankton (2-5 µm ESD). Data were collected in surface waters (≈5 m depth) from 27 oceanographic cruises carried out in the Northeast Pacific Ocean between 2010 and 2018. Thirteen cruises provide high spatial resolution (≈1 km) measurements across 32,500 km of the Northeast Pacific Ocean and 14 near-monthly cruises beginning in 2015 provide seasonal distributions at the long-term sampling site (Station ALOHA) of the Hawaii Ocean Time-Series. These data sets expand our knowledge of the current spatial and temporal distributions of picophytoplankton in the surface ocean.

Kilauea lava fuels phytoplankton bloom in the North Pacific Ocean.

From June to August 2018, the eruption of Kilauea 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. We hypothesize that the high nitrate was caused by buoyant plumes of nutrient-rich deep waters created by the substantial input of lava into the ocean. This large-scale ocean fertilization was therefore a unique perturbation event that revealed how marine ecosystems respond to exogenous inputs of nutrients.

Molecular evidence of iron limitation and availability in the global diazotroph Trichodesmium.

The activity of the N(2)-fixing cyanobacterial genus Trichodesmium is critical to the global nitrogen (N) and carbon (C) cycles. Although iron (Fe) has been shown to be an important element limiting the growth and N(2) fixation of Trichodesmium, there have been no specific data demonstrating the in situ affect of Fe on Trichodesmium. We surveyed Trichodesmium populations from the Atlantic and Pacific Oceans for Fe limitation using a novel quantitative reverse transcriptase-PCR (qRT-PCR) method monitoring the expression of an Fe limitation-induced gene, isiB. Here we report the first molecular evidence of in situ Fe limitation of Trichodesmium N(2) fixation, which was evident in samples from the Pacific Ocean, whereas limitation appeared minimal to nonexistent in Atlantic Ocean samples. As our method is Trichodesmium clade specific, we were also able to determine that representatives from the Trichodesmium tenue clade were the most biologically active group of Trichodesmium in the majority of our samples, which speaks to their dominance in open ocean regimes. Furthermore, comparisons of our field expression and chemical data with laboratory studies suggest that the majority of dissolved Fe in the open ocean is available to Trichodesmium colonies regardless of Fe complexation.

COMPARISON OF CULTURED TRICHODESMIUM (CYANOPHYCEAE) WITH SPECIES CHARACTERIZED FROM THE FIELD(1).

The filamentous, colonial cyanobacterium Trichodesmium has six well-described species, but many more names. Traditional classification was based on field samples using morphological characteristics such as cell width and length, gas vesicle distribution, and colony morphology. We used the Woods Hole Trichodesmium culture collection to identify 21 cultured strains to species using cell morphology; phycobiliprotein absorption spectra; and sequences of the 16S rRNA gene, the 16S-23S internal transcribed spacer (ITS), and the heterocyst differentiation gene hetR. We compared our results to previous studies of field specimens and found similar clades, though not all phylogenetic groups were represented in culture. Our culture collection represented two of the four major clades of Trichodesmium: clade I, made up of Trichodesmium thiebautii Gomont, Trichodesmium tenue Wille, Katagnymene spiralis Lemmerm., and Trichodesmium hildebrandtii Gomont; and clade III, consisting of Trichodesmium erythraeum Ehrenb. and Trichodesmium contortum Wille. These clades were genetically coherent with similar phycobiliprotein composition, but morphologically diverse. In the continual revision of cyanobacterial taxonomy, genetic and biochemical information is useful and informative complements to morphology for the development of a functional classification scheme.