Main Re­search In­terests

My re­search fo­cuses on the biogeo­chem­istry and eco­logy of ni­tro­gen fix­a­tion in mar­ine eco­sys­tems, par­tic­u­larly the nu­tri­ent-poor open ocean and ve­get­ated coastal eco­sys­tems. In most of these eco­sys­tems, primary pro­ductiv­ity is lim­ited by the avail­ab­il­ity of ni­tro­gen (N) such as ni­trate and am­monium. Bio­lo­gical ni­tro­gen fix­a­tion, the en­zymatic re­duc­tion of in­ert N₂ gas to bioavail­able am­mo­nia, adds new ni­tro­gen, thereby sus­tain­ing pro­ductiv­ity and ex­ert­ing con­trol on the ex­port of or­ganic mat­ter, and hence at­mo­spheric car­bon di­ox­ide (CO₂). In my re­search, I aim to bet­ter un­der­stand the ex­tent of N₂ fix­a­tion activ­ity, to identify the re­spons­ible mi­croor­gan­isms and to re­veal their life­style as well as reg­u­lat­ory mech­an­isms. To ad­dress these top­ics, I use a com­bin­a­tion of geo­chem­ical, mo­lecu­lar, mi­cro­bi­o­lo­gical and single-cell ap­proaches in both field-go­ing and labor­at­ory-based re­search.

Nitrogen fixation in the open ocean

The ma­jor­ity of N₂ fix­a­tion in the open ocean is cur­rently at­trib­uted to cy­anobac­teria. However, mo­lecu­lar data in­dic­ates that non-cy­anobac­terial N₂-fix­ers are present in the open ocean, and N₂ fix­a­tion rates can be meas­ured in the (near) ab­sence of N₂-fix­ing cy­anobac­teria. One fo­cus of my re­search in the open ocean there­fore is to identify non-cy­anobac­terial N₂-fix­ers and to de­term­ine their activ­ity us­ing –om­ics ap­proaches and stable iso­tope in­cub­a­tions coupled to nano­scale Sec­ond­ary Ion Mass Spec­tro­metry (nanoSIMS). Fur­ther, I am in­ter­ested in learn­ing what para­met­ers reg­u­late the activ­ity of these novel as well as cy­anobac­terial N₂-fix­ers, with a par­tic­u­lar em­phasis on the role of phos­phorus and iron.

Nitrogen fixation in seagrass ecosystems

Many seagrasses world­wide flour­ish in nu­tri­ent-poor en­vir­on­ments, and N₂ fix­a­tion has been de­tec­ted in many of these eco­sys­tems. In Posidonia oceanica, a seagrass en­demic to the Medi­ter­ranean Sea, we have re­cently dis­covered an en­do­phytic bac­terium, Ca. Cel­erinatan­ti­mo­nas nep­tuna, which fixes N₂ in­side the roots and provides the fixed N to its plant host (Mohr et al. 2021) re­sem­bling the in­ter­ac­tion of ter­restrial rhizo­bia and their legume hosts. Based on mo­lecu­lar data, sim­ilar as­so­ci­ations may ex­ist between bac­teria and other seagrasses/​mac­ro­phytes. Hence, I am par­tic­u­larly in­ter­ested in deep­en­ing our un­der­stand­ing of seagrass-as­so­ci­ated N₂-fix­ers at both the eco­lo­gical and evol­u­tion­ary level.