Phycoerythrin (PE) spectral diversity was investigated in eastern tropical Australian waters and around New Caledonian and Fijian archipelagos. Colony sorting of filamentous cyanobacteria revealed slight differences in the PE excitation spectrum of Trichodesmium thiebautii and T. erythraeum. Spectra of PE from Katagnymene spiralis and Richelia intracellularis were examined for the first time. PE spectra of filamentous cyanobacteria (Trichodesmium, Katagnymene, and Richelia) showed a broader phycoerythrobilin (PEB) band than those of Synechococcus. The influence of PE Trichodesmium on the global spectrum of PE in natural waters was clearly visible at various stations. The PEB band was large at the surface and narrower at increased depth, suggesting a shift of the cyanobacterial community from a dominance of diazotrophic filamentous cyanobacteria to small Synechococcus. Size fractionation of water samples confirmed this. A good linear relationship was observed between PE concentration in the .10-mm cellular-size fraction and the abundance of filamentous cyanobacteria expressed by either trichome numbers, total trichome surface area, or total trichome volume. PE in the .10-mm fraction is a useful tool for rapidly quantifying filamentous cyanobacteria. Neither diel variations nor photoacclimation significantly influenced the PE fluorescence excitation spectra in T. thiebautii and T. erythraeum. Using this method, we identified green colonies of filamentous cyanobacteria in deep waters (50-120 m) of the Coral Sea with a novel high-phycourobilin PE. While morphologically similar to Trichodesmium, it possesses distinctive photosynthetic responses and could be a new species.

Contributions of filamentous and picoplanktonic cyanobacteria to the 26 phytoplankton community structure were examined in New Caledonian waters during the 2001-27 2003 El Niño period at three stations. Morphometric characteristics of diazotrophic filamentous 28 cyanobacteria are given as well as the seasonal and inter-annual variations of their surface areas 29 and integrated abundances. Trichodesmium tenue and T. thiebautii were the dominant species 30 followed by T. erythraeum, altogether accounting for more than 51-80 % of the biomass of the 31 free-living filamentous cyanobacteria. Katagnymene spp. accounted for a smaller percentage 32 (<13.8 % at ocean stations, <3.6 % in the lagoon). R. intracellularis biomass was relatively small 33 (<1 % of total surface area and volume of Trichodesmium trichomes) with the highest 34 2 ! concentration observed in summer (735 trichomes l -1 ). Colonies of unidentified cyanobacteria 35 composed of spherical cells accounted on average for <1 % of the Trichodesmium biomass, with 36 values exceeding 4000 cells l -1

The photosynthetic cyanobacterium Trichodesmium is widely distributed in the surface low latitude ocean where it contributes significantly to N 2 fixation and primary productivity. Previous studies found nifH genes and intact Trichodesmium colonies in the sunlight-deprived meso-and bathypelagic layers of the ocean (200-4 000 m depth). Yet, the ability of Trichodesmium to fix N 2 in the dark ocean has not been explored. We performed 15 N 2 incubations in sediment traps at 170, 270 and 1 000 m at two locations in the South Pacific. Sinking Trichodesmium colonies fixed N 2 at similar rates than previously observed in the surface ocean (36-214 fmol N cell -1 d -1 ). This activity accounted for 40 ± 28 % of the bulk N 2 fixation rates measured in the traps, indicating that other diazotrophs were also active in the mesopelagic zone. Accordingly, cDNA nifH amplicon sequencing revealed that while Trichodesmium accounted for most of the expressed nifH genes in the traps, other diazotrophs such as Chlorobium and Deltaproteobacteria were also active. Laboratory experiments simulating mesopelagic conditions confirmed that increasing hydrostatic pressure and decreasing temperature reduced but did not completely inhibit N 2 fixation in Trichodesmium. Finally, using a cell metabolism model we predict that Trichodesmium uses photosynthesis-derived stored carbon to sustain N 2 fixation while sinking into the mesopelagic. We conclude that sinking Trichodesmium provides ammonium, dissolved organic matter and biomass to mesopelagic prokaryotes.

A powerful eruption within the Hunga Tonga-Hunga Ha’apai (HTHH) volcano (20.64°S, 175.19°W) in the Kingdom of Tonga, occurred on 15 January 2022. The volcanic blast was enormous, leading many scientists to investigate the full impact and magnitude of this event via satellite observations. In this study, we describe a new ocean color signature from a discolored water patch created by the HTHH eruption using NASA and CMEMS products of satellite-derived biological and optical properties. Elevated surface chlorophyll-a concentration (Chl-a) between 0.15 to 2.7 mg.m-3 was not associated with phytoplankton growth, but to basalt-andesitic ash material expelled by the volcano and into the ocean, which resulted in erroneous Chl-a estimates. Distribution of the patch over time was aligned with CMEMS ocean currents for 19 days. The gradual decrease of light attenuation or diffuse attenuation coefficient for downward irradiance at 490 nm, Kd(490), was interpreted as due to the sinking of ash particles with time. It is suggested that due to high porosity of 30-40%, a density close to that of seawater, ash particles stay suspended in the water column for more than 10 days with sustained high values of satellite-derived Chl-a, Kd(490), and particulate backscattering coefficient at 443 nm. The high attenuation of light due to ash, reducing the penetration depth to less than 10 meters during the first period after the eruption may have had implications on ecological processes and biogeochemical cycles in Tongan waters.