Presenters

Sam WilsonPost-doc

University of Hawaii

Rachel FosterPost-doc

UCSC

Irina IlikchyanPost-doc

UCSC

Sasha TozziPost-docMBARI

Partnerships and Leveraged

Funding

Gordon and Betty Moore Foundation

The marine hydrogen cycleBackground: Dissolved H2 in the upper ocean

Cruises and Time Series: Hawaiian Ocean Time-series (HOT)

H2 production by diazotrophs

Future H2-related research

Published concentrations of dissolved H2 insurface seawater. The measurements arecolor-coded to distinguish between super-saturated (●) and under-saturatedconcentrations (●) with respect to atmosphericequilibrium. The low-latitude oceanic surfacewaters have an excess of H2 indicatingsustained H2 production in the mixed layer.

Dissolved H2 has been measured at Stn ALOHAsince June 2008 (HOT202). H2 concentrationsare super-saturated throughout the year. Theabove figure shows depth-integrated (0-100 m)measurements of H2 at Stn ALOHA with keyancillary parameters measured by the HawaiianOcean Time-series program(http://hahana.soest.hawaii.edu/hot).

Schematic diagram of the turnover of the dissolvedH2 pool at Stn ALOHA. The major input is biologicalreduction of dinitrogen (N2) indicated by thestoichiometry. Prominent N2-fixing diazotrophs atStn ALOHA include unicellular micro-organisms e.g.Group A (uncultivated), Group B represented byCrocosphaera spp., and filamentous organisms e.g.Trichodesmium.

Dave KarlProfessor

University of Hawaii

Jon ZehrProfessor

UCSC For all strains of N2-fixing cyanobacteriaanalyzed, H2 was produced on the same dielcycle as N2-fixation. Rates of H2 productionvaried over 2 orders of magnitude betweenTrichodesmium and Crocosphaera. The lowernet H2 production rate by Crocosphaerasuggests a greater reassimilation efficiency ofthe H2 produced by nitrogenase.

C. watsonii WH8501 was selected for additionalexperiments to better understand H2 cyclingwithin N2-fixing microorganisms. The aboveplots shows the diel variation in cell metabolismand gene expression for cells grown in SOmedia on a 12:12 L:D cycle at 26 oC and a lightintensity of 44 µmol photon m-2 sec-1.

The C. watsonii WH8501 cultures were grownin custom-built 2 L bioreactors whichcontinually measured O2, pH andphotosynthetic properties using benchtop fastrate repetition fluorometry sensors developedby Zbigniew Kolber. Replicate vessels alloweddiscrete measurements (e.g. DNA/RNA, chl a,PC/PN) to be conducted daily.

The H2 and N2-fixation correlationIf the dissolved H2 in the upper oceanoriginates from nitrogenase activity, then inthe absence of N2-fixation, the ocean shouldbecome a net sink for atmospheric H2.Supplying a fixed N source to a seawatersample should decrease the N2-fixation andtherefore draw down H2 concentrationsbelow atmospheric equilibrium.

Reduced gases in cyanobacterial metabolismThe oxidation of H2 provides energy, electrons and H+

for cell metabolism in a single step. The electronsfrom the hydrogenase enzyme are passed ontoplastoquinone. The PQ pool becomes partiallyoxidized as it transfers the electrons onto ferredoxinand this is evident in the Fv/Fm ratio highlighted in thefigure below.

Additional reduced gasesThe upper ocean is super-saturated in other tracegases e.g. CO, H2S, N2O, that are also connectedto cell metabolism. One such example is carbonmonoxide which is formed during the degradationof phycobilins, accessory pigments forcyanobacteria. The interplay between the reducedgases could have important implications for theredox chemistry of the cell.

Zbigniew KolberScientist

UCSC

N2-fixing microorganisms selected forH2 analysis included Trichodesmiumspp., shown on the left in colonyformation and single trichomes.(Image courtesy of Rachel Foster)

Another target diazotroph wasCrocosphaera watsonii, a unicellularcyanobacterium with a cell diameterof 3-5 µm that fixes N2 at night.(Image courtesy of Rachel Foster)

N2-fixation was measuredusing an online acetylenereduction assay. The flowthrough system allowed asample to be analyzedevery 10 mins using theautomated system.

H2 production wasmeasured using a reducedgas analyzer configuredfor continual automatedanalysis. Measurementswere made every 10 minswith a detection limit of3 pmol H2 µmol chl a-1 h-1.