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Effects of inorganic nitrogen on taxa‐specific cyanobacterial growth and nifH expression in a subtropical estuary
Author(s) -
Moisander Pia H.,
Paerl Hans W.,
Zehr Jonathan P.
Publication year - 2008
Publication title -
limnology and oceanography
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.7
H-Index - 197
eISSN - 1939-5590
pISSN - 0024-3590
DOI - 10.4319/lo.2008.53.6.2519
Subject(s) - cylindrospermopsis raciborskii , biology , anabaena , cyanobacteria , diazotroph , botany , nitrogen fixation , phytoplankton , ecology , nutrient , bacteria , genetics
The potentially toxic, diazotrophic filamentous cyanobacterium Cylindrospermopsis raciborskii has recently become a common component in the summer phytoplankton in the St. Johns River (SJR) estuary, Florida, where Anabaena spp. historically dominated. Using a microcosm nutrient enrichment experiment, we investigated the ability of C. raciborskii and Anabaena spp. to compete under a range of available NO 3 − and NH 4 + concentrations, to test the hypothesis that C. raciborskii benefits from increased dissolved inorganic nitrogen (DIN) availability. TaqMan quantitative polymerase chain reaction (PCR) probes were designed, tested, and applied to target the nifH gene in one C. raciborskii and two Anabaena spp. strains from the SJR. N limitation prevailed, as shown by increased N 2 ‐fixation rates if no N was added, increased chlorophyll a concentrations when DIN was added, and depletion of added DIN. Anabaena spp. and C. raciborskii showed rapid growth with no DIN additions and were the main taxa responsible for N 2 fixation. Abundances of C. raciborskii increased if NH 4 + was added, but nifH was expressed at low levels, suggesting growth was relying on NH 4 + . Anabaena spp. and C. raciborskii expressed nifH genes when NO 3 − or NH 4 + were present, but expression was higher with NO 3 − . The narB gene sequence was amplified from Anabaena spp. and C. raciborskii from the SJR, suggesting these taxa are capable of assimilating NO 3 − . However, even small NO 3 − additions blocked the growth of Anabaena spp. in the mixed phytoplankton community but not that of C. raciborskii . The results suggest that C. raciborskii in the SJR is a stronger competitor than Anabaena spp. when DIN is present.