AUTHOR=Tuo Sing-how , Mulholland Margaret R. , Taniuchi Yukiko , Chen Houng-Yung , Jane Wann-Neng , Lin Yen-Huei , Chen Yuh-ling Lee 

TITLE=Trichome Lengths of the Heterocystous N2-Fixing Cyanobacteria in the Tropical Marginal Seas of the Western North Pacific

JOURNAL=Frontiers in Marine Science

VOLUME=8

YEAR=2021

URL=https://www.frontiersin.org/journals/marine-science/articles/10.3389/fmars.2021.678607

DOI=10.3389/fmars.2021.678607

ISSN=2296-7745

ABSTRACT=<p><italic>Calothrix rhizosoleniae</italic> and <italic>Richelia intracellularis</italic> are heterocystous cyanobacteria found in the tropical oceans. <italic>C. rhizosoleniae</italic> commonly live epiphytically on diatom genera <italic>Chaetoceros</italic> (<italic>C-C</italic>) and <italic>Bacteriastrum</italic> (<italic>B-C</italic>) while <italic>R. intracellularis</italic> live endosymbiotically within <italic>Rhizosolenia</italic> (<italic>R-R</italic>), <italic>Guinardia</italic> (<italic>G-R</italic>), and <italic>Hemiaulus</italic> (<italic>H-R</italic>); although, they occasionally live freely (FL-<italic>C</italic> and FL-<italic>R</italic>). Both species have much shorter trichomes than the other marine filamentous cyanobacteria such as <italic>Trichodesmium</italic> spp. and <italic>Anabaena gerdii</italic>. We investigated the trichome lengths of <italic>C. rhizosoleniae</italic> and <italic>R. intracellularis</italic> in the South China Sea (SCS) and the Philippine Sea (PS) between 2006 and 2014. On average, <italic>H-R</italic> had the shortest trichome lengths (3.5 cells/trichome), followed by <italic>B-C</italic> and <italic>C-C</italic> (4.9–5.2 cells/trichome) and FL-<italic>C</italic> (5.9 cells/trichome), and <italic>R-R</italic>, <italic>G-R</italic>, and FL-<italic>R</italic> had the longest trichome lengths (7.4–8.3 cells/trichome). Field results showed the trichome lengths of <italic>C-C</italic> and <italic>B-C</italic> did not vary seasonally or regionally. However, FL-<italic>C</italic> and <italic>H-R</italic> from the SCS and during the cool season had longer trichomes, where/when the ambient nutrient concentrations were higher. <italic>R-R</italic>, <italic>G-R</italic>, and FL-<italic>R</italic> also showed regional and seasonal variations in trichome length. Ultrastructural analysis found no gas vesicles within the <italic>C. rhizosoleniae</italic> cells to assist in buoyancy regulation. Results suggest that the trichome lengths of <italic>C. rhizosoleniae</italic> and <italic>R. intracellularis</italic> might be regulated by their diatom hosts’ symbiotic styles and by ambient nutrients. Short trichome length might help <italic>C. rhizosoleniae</italic> and <italic>R. intracellularis</italic> to stay in the euphotic zone regardless as to whether they are free-living or symbiotic.</p>