Abstract
Calothrix rhizosoleniae and Richelia intracellularis are heterocystous cyanobacteria found in the tropical oceans. C. rhizosoleniae commonly live epiphytically on diatom genera Chaetoceros (C-C) and Bacteriastrum (B-C) while R. intracellularis live endosymbiotically within Rhizosolenia (R-R), Guinardia (G-R), and Hemiaulus (H-R); although, they occasionally live freely (FL-C and FL-R). Both species have much shorter trichomes than the other marine filamentous cyanobacteria such as Trichodesmium spp. and Anabaena gerdii. We investigated the trichome lengths of C. rhizosoleniae and R. intracellularis in the South China Sea (SCS) and the Philippine Sea (PS) between 2006 and 2014. On average, H-R had the shortest trichome lengths (3.5 cells/trichome), followed by B-C and C-C (4.9–5.2 cells/trichome) and FL-C (5.9 cells/trichome), and R-R, G-R, and FL-R had the longest trichome lengths (7.4–8.3 cells/trichome). Field results showed the trichome lengths of C-C and B-C did not vary seasonally or regionally. However, FL-C and H-R from the SCS and during the cool season had longer trichomes, where/when the ambient nutrient concentrations were higher. R-R, G-R, and FL-R also showed regional and seasonal variations in trichome length. Ultrastructural analysis found no gas vesicles within the C. rhizosoleniae cells to assist in buoyancy regulation. Results suggest that the trichome lengths of C. rhizosoleniae and R. intracellularis might be regulated by their diatom hosts’ symbiotic styles and by ambient nutrients. Short trichome length might help C. rhizosoleniae and R. intracellularis to stay in the euphotic zone regardless as to whether they are free-living or symbiotic.
Highlights
Marine cyanobacterial dinitrogen (N2) fixation plays a substantial role in global biogeochemical nitrogen (N) and carbon cycling
We investigated the trichome lengths of C. rhizosoleniae and R. intracellularis in the South China Sea (SCS) and the Philippine Sea (PS) between 2006 and 2014
Our results suggest that trichome length varies based on the nature of the diatom diazotroph associations/assemblages (DDAs), and this should be considered in future models
Summary
Marine cyanobacterial dinitrogen (N2) fixation plays a substantial role in global biogeochemical nitrogen (N) and carbon cycling. Calothrix rhizosoleniae (hereafter Calothrix, unless noted) and Richelia intracellularis (hereafter Richelia) are commonly observed in tropical and subtropical ocean regions and marginal seas (Kimor et al, 1992; Carpenter et al, 1999; Gómez et al, 2005; Bar Zeev et al, 2008; Tuo et al, 2017) Both Calothrix and Richelia are most often observed in symbiotic relationships with certain diatom species and only occasionally observed freeliving (FL) outside of a host (Karsten, 1907; Gómez et al, 2005; White et al, 2007; Foster et al, 2011; Tuo et al, 2017). These diatom diazotroph associations/assemblages (DDAs) are thought to contribute substantial fixed N to oceanic regions where they occur (Mague et al, 1974; Carpenter et al, 1999; Bar Zeev et al, 2008; Grosse et al, 2010)
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