A novel cohabitation between two diazotrophic cyanobacteria in the oligotrophic ocean

Abstract

The cyanobacterial genus Trichodesmium is biogeochemically significant because of its dual role in nitrogen and carbon fixation in the oligotrophic ocean. Trichodesmium species form colonies that can be easily enriched from the water column and used for shipboard rate measurements to estimate their contribution to oceanic carbon and nitrogen budgets. During a July 2010 cruise near the Hawaiian Islands in the oligotrophic North Pacific Subtropical Gyre, a specific morphology of Trichodesmium puff-form colonies were examined under epifluorescent microscopy and found to harbor a colonial endobiont, morphologically identified as the heterocystous diazotrophic cyanobacterium Calothrix. Using unialgal enrichments obtained from this cruise, we show that these Calothrix-like heterocystous cyanobionts (hetDA for 'Trichodesmium-associated heterocystous diazotroph') fix nitrogen on a diurnal cycle (maximally in the middle of the light cycle with a detectable minimum in the dark). Gene sequencing of nifH from the enrichments revealed that this genus was likely not quantified using currently described quantitative PCR (qPCR) primers. Guided by the sequence from the isolate, new hetDA-specific primers were designed and subsequent qPCR of environmental samples detected this diazotroph from surface water to a depth of 150 m, reaching densities up to B9 Â 10 3 l À 1 . Based on phylogenetic relatedness of nifH and 16S rRNA gene sequences, it is predicted that the distribution of this cyanobiont is not limited to subtropical North Pacific but likely reaches to the South Pacific and Atlantic Oceans. Therefore, this previously unrecognized cohabitation, if it reaches beyond the oligotrophic North Pacific, could potentially influence Trichodesmium-derived nitrogen fixation budgets in the world ocean.

The Trichodesmium microbiome can modulate host N2 fixation

“…b). Although Trichodesmium has co‐occurred with a N 2 fixing epibiont in the North Pacific Ocean (Momper et al ; Gradoville et al ), herein Trichodesmium colonies did not show evidence of non‐ Trichodesmium nif genes (Frischkorn et al ), indicating that Trichodesmium was the only diazotrophic organism within these samples. The magnitude and direction of microbiome‐induced changes in host N 2 fixation ranged from significantly decreased at the northernmost station to significantly increased at the southernmost station, despite similarities in TDP between proximal stations (Fig.…”

Microbial diversity within the Trichodesmium holobiont

“…Within the colony, Trichodesmium are only a member of a complex holobiont, a host-microbiome system, where Trichodesmium filaments host high concentrations of tightly attached heterotrophic bacterial epibionts (Herbst and Overbeck, 1978;Paerl et al, 1989;Zehr, 1995;Dyhrman et al, 2002;Hewson et al, 2009). In addition, other cyanobacteria have been observed in association with Trichodesmium colonies (Hewson et al, 2009;Hynes et al, 2009;Hmelo et al, 2012;Momper et al, 2015). Studies have suggested that the metabolism of Trichodesmium and heterotrophic bacterial epibionts is tightly interconnected and that epibionts might play an important role in the cycling of iron and phosphorus within the colony (Achilles et al, 2003;Roe et al, 2012;Van Mooy et al, 2012), with concomitant impacts on ocean biogeochemistry.…”

Temporal Variability of Trichodesmium spp. and Diatom-Diazotroph Assemblages in the North Pacific Subtropical Gyre

“…If we adjusted our heterocyst counts by a factor of 5 to include gene copies in vegetative cells, qPCR based abundance are on average more than a 100 × higher than cell-based estimates. Other potential explanations for this disparity include (1) the potential for nonspecificity of het-1 and het-2 primers and probes as described in Foster et al (2007), (2) omission of heterocysts that may have been embedded in Trichodesmium colonies and hence not readily visible via microscopy (Momper et al, 2015), and (3) amplification of moribund DNA that would not have been detected by epifluorescence microscopy. The lack of agreement between gene and cell based abundances is a topic that clearly needs further study.…”