SEASONAL-VARIATION IN RATES OF HETEROTROPHIC NITROGEN-FIXATION (ACETYLENE-REDUCTION) IN ZOSTERA-NOLTII MEADOWS AND UNCOLONIZED SEDIMENTS OFTHE BASSIN DARCACHON, SOUTH-WEST FRANCE
Dt. Welsh et al., SEASONAL-VARIATION IN RATES OF HETEROTROPHIC NITROGEN-FIXATION (ACETYLENE-REDUCTION) IN ZOSTERA-NOLTII MEADOWS AND UNCOLONIZED SEDIMENTS OFTHE BASSIN DARCACHON, SOUTH-WEST FRANCE, Hydrobiologia, 329(1-3), 1996, pp. 161-174
Nitrogen fixation (acetylene reduction) rates were measured over an an
nual cycle in meadows of the seagrass Z. noltii and uncolonised sedime
nts of the Bassin d'Arcachon, south-west France, using both slurry and
whole core techniques. Measured rates using the slurry technique in Z
. noltii colonised sediments were consistently higher than those deter
mined in isolated cores. This was probably due to the release of labil
e organic carbon sources during preparation of the slurries. Thus, in
colonised sediments the whole core technique may provide a more accura
te estimate of in situ activity. Acetylene reduction rates measured by
the whole core technique in colonised sediments were 1.8 to 4-fold gr
eater, dependent upon the season, in the light compared with those mea
sured in the dark, indicating that organic carbon released by the plan
t roots during photosynthesis was an important factor regulating nitro
gen fixation. In contrast acetylene reduction rates in uncolonised sed
iments were independent of light. Addition of sodium molybdate, a spec
ific inhibitor of sulphate reduction inhibited acetylene reduction act
ivity in Z. noltii colonised sediments by >80% as measured by both slu
rry and whole core techniques irrespective of the light regime, throug
hout the year inferring that sulphate reducing bacteria (SRB) were the
dominant component of the nitrogen fixing microflora. A mutualistic r
elationship between Z. noltii and nitrogen fixing SRB in the rhizosphe
re, based on the exchange of organic carbon and fixed nitrogen is prop
osed. In uncolonised sediments sodium molybdate initially severely inh
ibited acetylene reduction rates, but the fever of this inhibition dec
lined over the course of the year. These data indicate that the nitrog
en fixing SRB associated with the Zostera roots and rhizomes were prog
ressively replaced by an aerobic population of nitrogen fixers associa
ted with the decomposition of this recalcitrant high C:N ratio organic
matter. Acetylene and sulphate reduction rates in the seagrass beds s
howed distinct summer maxima which correlated with a reduced availabil
ity of NH4+ in the sediment and the growth cycle of Z. noltii in the B
assin. Overall, these data indicate that acetylene reduction (nitrogen
fixation) activity in the rhizosphere of Z. noltii was regulated both
by release of organic carbon from the plant roots and maintenance of
low ammonium concentrations in the root zone due to efficient ammonium
assimilation. Nitrogen fixation rates determined from acetylene reduc
tion rates measured by the whole core technique ranged from 0.1 to 7.3
mg N m(-2) d(-1) in the Z. noltii beds and between 0.02 and 3.7 mg N
m(-2) d(-1) in uncolonised sediments, dependent upon the season. Nitro
gen fixation in the rhizosphere of Z. noltii was calculated to contrib
ute between 0.4 and 1.1 g N m(-2) y(-1) or between 6.3 and 12% of the
annual fixed nitrogen requirement of the plants. Heterotrophic nitroge
n fixation therefore represents a substantial local input of fixed nit
rogen to the sediments of this shallow coastal lagoon and contributes
to the overall productivity of Z. noltii in this ecosystem.