163
human population and the collapse of many wild marine fisheries (UN-FAO 2018),
marine aquaculture (or mariculture) is an emergent area globally which can supply
needed food resources. Marine N 2 fixation could potentially play a direct role in
promoting macroalgal, fish and multi-trophic mariculture, particularly in more oligotrophic marine areas, by subsidizing nutrient demand within the system. Marine
N 2 -fixers, such as Trichodesmium and the symbiont UCYN-A may hold important
clues for bioengineering N 2 -fixing plants.
Another area of biotechnology which can potentially exploit marine N 2 -fixers is
the area of bioremediation. Petroleum hydrocarbon releases are a perennial problem
in coastal waters (Howarth 1989). Oil discharges in nutrient poor waters often
require fertilization to promote biodegradation (Pritchard et al. 1992). Research in
the wake of the 2010 massive release of hydrocarbons during Deep Water Horizon/
Macondo Well event in the Gulf of Mexico (GOM) noted the enrichment of rare
taxa including diazotrophs in the oiled beach sands (Rodriguez-r et al. 2015; Shin
et al. 2019). A diazotrophic hydrocarbon degrader was subsequently isolated,
sequenced and found to be globally distributed in other oil-affected marine sediments (Karthikeyan et al. 2019). Experiments also provided evidence for selection
of diazotrophs in simulated water column oil plumes from the northern and southern
GOM and the Beaufort Sea which had been amended with Macondo-like oil (Sun
and Kostka 2019). Hence, use of hydrocarbon degrading diazotrophs may provide a
pathway for biologically enhanced oil remediation going forward (Kostka
et al. 2020).
9.5 Overcoming Challenges for Future N 2 Fixation Research
Many challenges remain to accurately and sensitively identify the presence and
identity of marine diazotrophs and quantify their role in N 2 fixation in marine waters
and sediments. Continued development of technologies and approaches will continue to be important in overcoming limitations of obtaining data on relevant time
and space scales (see also Chap. 5).
The quantitative polymerase chain reaction (qPCR) (Church et al. 2008) made it
possible to quantify microbes by the abundance of the nitrogenase gene (specifically nifH) even if not able to observe by microscopy, but has limitations. New
methods have been developed to measure the H 2 evolved by nitrogenase (Eichner
et al. 2019; Wilson et al. 2010), by using isotopically-labelled acetylene in an acetylene reduction method (Zhang et al. 2016) or by using highly purified acetylene to
improve sensitivity (Kitajima et al. 2009). Nanoscale Secondary Ion Mass
Spectrometry (nanoSIMS) has been used in several studies to directly identify
active diazotrophs at the cell scale (Foster et al. 2011). Stable Isotope Probing (SIP)
(Buckley 2011) which allows direct identification of active diazotrophs, has been
used widely in soils and very recently in a marine setting (Kapili et al. 2020). This
approach could provide insightful into elucidating the role of heterotrophic diazotrophs in the sea.
9.5 Overcoming Challenges for Future N 2 Fixation Research
human population and the collapse of many wild marine fisheries (UN-FAO 2018),
marine aquaculture (or mariculture) is an emergent area globally which can supply
needed food resources. Marine N 2 fixation could potentially play a direct role in
promoting macroalgal, fish and multi-trophic mariculture, particularly in more oligotrophic marine areas, by subsidizing nutrient demand within the system. Marine
N 2 -fixers, such as Trichodesmium and the symbiont UCYN-A may hold important
clues for bioengineering N 2 -fixing plants.
Another area of biotechnology which can potentially exploit marine N 2 -fixers is
the area of bioremediation. Petroleum hydrocarbon releases are a perennial problem
in coastal waters (Howarth 1989). Oil discharges in nutrient poor waters often
require fertilization to promote biodegradation (Pritchard et al. 1992). Research in
the wake of the 2010 massive release of hydrocarbons during Deep Water Horizon/
Macondo Well event in the Gulf of Mexico (GOM) noted the enrichment of rare
taxa including diazotrophs in the oiled beach sands (Rodriguez-r et al. 2015; Shin
et al. 2019). A diazotrophic hydrocarbon degrader was subsequently isolated,
sequenced and found to be globally distributed in other oil-affected marine sediments (Karthikeyan et al. 2019). Experiments also provided evidence for selection
of diazotrophs in simulated water column oil plumes from the northern and southern
GOM and the Beaufort Sea which had been amended with Macondo-like oil (Sun
and Kostka 2019). Hence, use of hydrocarbon degrading diazotrophs may provide a
pathway for biologically enhanced oil remediation going forward (Kostka
et al. 2020).
9.5 Overcoming Challenges for Future N 2 Fixation Research
Many challenges remain to accurately and sensitively identify the presence and
identity of marine diazotrophs and quantify their role in N 2 fixation in marine waters
and sediments. Continued development of technologies and approaches will continue to be important in overcoming limitations of obtaining data on relevant time
and space scales (see also Chap. 5).
The quantitative polymerase chain reaction (qPCR) (Church et al. 2008) made it
possible to quantify microbes by the abundance of the nitrogenase gene (specifically nifH) even if not able to observe by microscopy, but has limitations. New
methods have been developed to measure the H 2 evolved by nitrogenase (Eichner
et al. 2019; Wilson et al. 2010), by using isotopically-labelled acetylene in an acetylene reduction method (Zhang et al. 2016) or by using highly purified acetylene to
improve sensitivity (Kitajima et al. 2009). Nanoscale Secondary Ion Mass
Spectrometry (nanoSIMS) has been used in several studies to directly identify
active diazotrophs at the cell scale (Foster et al. 2011). Stable Isotope Probing (SIP)
(Buckley 2011) which allows direct identification of active diazotrophs, has been
used widely in soils and very recently in a marine setting (Kapili et al. 2020). This
approach could provide insightful into elucidating the role of heterotrophic diazotrophs in the sea.
9.5 Overcoming Challenges for Future N 2 Fixation Research