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Another look at green Trichodesmium colonies

Neveux et al. (2006) provide an important contribution on spectral diversity of phycoerythrins and diazotrophic abundance in tropical waters. As a minor component of the paper, Neveux et al. (2006) put forward two hypotheses about the spectral differences found between Trichodes-mium theibautii and the ''green'' Trichodesmium colonies: (1) the green colonies are T. theibautii photoacclimated to low light, or (2) the green colonies may be a new species. In this commentary, we offer an alternative third hypothesis that was not considered by Neveux et al. (2006): the green Trichodesmium colonies may have been in senescence. We participated in the 1999 cruise aboard the R/V Ewing (Neveux et al. 2006) and genetically characterized Tricho-desmium colonies using three high-resolution molecular methods. The methods included whole genomic DNA fingerprinting using primers based on the highly iterated palindromic (HIP) repeat sequence (Orcutt et al. 2002). This polymerase chain reaction (PCR) technique allows for cyanobacterial species and strains to be distinguished and has been used to discriminate between species of Tricho-desmium (Orcutt et al. 2002). Our technique uses a whole-cell approach where the entire colony is used in the PCR, and there is no DNA extraction step. The gel in Fig. 1 represents one entire PCR run using base-pair extended HIP primers (HIP-GC and HIP-CA) where all the samples were collected on the same day at Sta. 32 (one of the stations described in Neveux et al. 2006). All colonies were prepared the same way and included the same green Trichodesmium as described in Neveux et al. (2006). Of the five green tuft and two green puff colonies examined, only the green puff colonies produced DNA amplicons. We found that the green Trichodesmium colonies that produced DNA amplicons were identical to other T. theibautii collected on the same day on the cruise (Fig. 1). Since all the other samples produced DNA amplicons, there was no indication that the PCR was inhibited by environmental factors or any of the reagents used in the reaction. The lack of DNA amplicons from the green tuft colonies suggests degenerated DNA, which is an indication that the colonies may have been in senescence. The conclusion above was supported by the fact that the green colonies had no measurable nitrogenase activity (J. Burns and D. Capone, pers. comm.). The green colonies were collected between 50 and 120 m in the Coral Sea, and the presence of nitrate may have inhibited the …