172 Flexible Elemental Stoichiometry in Trichodesmium IMS101: Implications for Buoyancy‐Mediated Vertical Migration

Abstract

The cyanobacterium Trichodesmium is considered the most abundant and active nitrogen fixing plankton genus in tropical and subtropical marine waters. In the northern subtropical gyres of the Pacific and Atlantic Oceans, this organism is notable as a source of new production and as a potential biological shuttle for phosphorus to surface waters. We have conducted recent laboratory experiments revealing the remarkable stoichiometric flexibility exhibited by this diazotroph. These results indicate that Trichodesmium spp. are capable of viable growth with carbon to phosphorus ratios approximately 16 times Redfield stoichiometry. Such P‐sparing is clearly an adaptation to the oligotrophic environments from which these cultures were isolated. Building on this research, additional work was performed to determine the maximal temporal scale of vertical migration and the implicit dark period physiology. An experiment was designed to assess the physiological changes that would occur as cells of varying growth stages migrated below the euphotic zone into more phosphorus replete waters. At different stages of growth sub‐samples of phosphorus replete or phosphorus limited cells were transferred to dark conditions. For each dark bottle, chl‐a, particulate carbon, phosphorus and nitrogen, trichome count and fluorescence parameters were measured daily. Our experimental results indicate that the maximum duration of dark period survival (and hence vertical migration) is approximately three days during which period cells were able to fully recover from light depravation and resume typical growth rates. The knowledge of Trichodesmium bioenergetics gained from this research will serve to further constrain the potential role of Trichodesmium spp. in biogeochemical cycling.