Buoyancy Regulation and the Potential for Vertical Migration in the Oceanic Cyanobacterium Trichodesmium

Abstract

Diel protein and carbohydrate content in Trichodesmium thiebautii was measured to evaluate the relationship to buoyancy status. Carbohydrate:protein ratio was the best predictor of buoyancy and fit a cosine curve with increasing values during the day and decreasing values at night in cycles that paralleled observed diel buoyancy patterns. This ratio also increased in short-term experiments as a function of light and increased in parallel with decreasing positive buoyancy. We used changes in this ratio to estimate the potential for vertical migration. Whereas limited vertical excursions in the upper 70 m are possible, deeper migrations appear unlikely unless respiration rates decrease significantly. N:P ratios in sinking and floating colonies were used to test for the P acquisition at depth (vertical migration). We noted that pooled N:P ratios were not significantly different between sinking and ascending colonies (N:P = 65.6 and 66.3, respectively) collected along the northern Australian coast, much like published results from north of Hawaii. Highly significant differences (p <0.0001) were observed in the western Gulf of Mexico between sinking and ascending colonies (N:P = 87.0 and 43.5, respectively) and provide the best direct evidence to date of vertical migration for P acquisition. Our physiological data on compositional changes during buoyancy reversals suggest a complex relationship between light and nutrients. It appears likely that light and P metabolism interact to regulate the vertical extent of migrations, with deep vertical migration regulated by P metabolism superimposed on a mixed-layer light-driven migration. The variability in N:P ratios suggests that care should be taken in assuming buoyancy reversals always result in P acquisition in this oceanic cyanobacterium.

Programmed cell death in the marine cyanobacterium Trichodesmium mediates carbon and nitrogen export

“…Although artificially expedited using dense batch cultures (Figure 1), bloom formation in our water column possessed notable similarities to natural blooms observed in tropical oceans, where surface slicks of dense Trichodesmium aggregations develop under calm conditions with sea surface temperatures of ± 26 1C, high solar irradiance and high concentrations of Fe and P (Karl et al, 2002;Le Borgne, 2008, 2010;Bergman et al, 2012). In these oceanic blooms, the vertical ascent of Trichodesmium cells due to positive buoyancy (Villareal and Carpenter, 2003;Le Borgne, 2008, 2010) occurs simultaneously with rapid growth rates, both processes accounting for the hastened, ultra-dense accumulation of cells at the surface Le Borgne, 2008, 2010;Bergman et al, 2012).…”

Polyphosphate in Trichodesmium from the low‐phosphorus Sargasso Sea

“…It has been hypothesized that this organism may migrate to the phosphocline, store P as polyP, and return to the surface in a process termed P-mining (Karl et al 1992;Villareal and Carpenter 2003). Although this could result in elevated polyP, the phosphocline ranged from 160 to 300 m on this transect, which is deeper than Trichodesmium is likely to migrate (White et al 2006a), and is deeper than Trichodesmium is typically observed in this region (Orcutt et al 2001;Davis and McGillicuddy 2006).…”

Metatranscriptomics of N2-fixing cyanobacteria in the Amazon River plume

“…Two of the most abundant transcripts in the low-salinity offshore, transitional and oceanic Trichodesmium transcriptomes were from gas vesicle protein genes adjacent to each other in the genome. Gas vesicles provide buoyancy to return to surface waters after Trichodesmium sinks to depth, possibly to acquire phosphorus (Villareal and Carpenter, 2003). Gas vesicles are important for remaining in the photic zone.…”