That sinking feeling…

Image: Hannes Grobe, Alfred Wegener Institute for Polar and Marine Research, Bremerhaven, Germany/Wikimedia Commons.

Image: Hannes Grobe, Alfred Wegener Institute for Polar and Marine Research, Bremerhaven, Germany/Wikimedia Commons.

Whilst forests – aided and abet­ted by cryp­to­gams (see my pre­vi­ous post) – have a major role as biotic car­bon sinks on land, in the oceans that role is largely down to the activ­ity of cryp­to­gam­ous phyto­plank­ton, which ‘draw-down’ vast amounts of CO2 dur­ing pho­to­syn­thesis. However – and unlike trees – much of that aquatic primary pro­ductiv­ity is con­sumed by herb­i­vores, which in turn are preyed upon by vari­ous levels of car­ni­vores. Ultimately, a lot of the CO2 that is fixed is released quite soon there­after in res­pir­a­tion. Which is why attempts to con­sign such fixed car­bon that is retained in the bod­ies of the algae (before it can be con­sumed and respired by hungry herbi-/carnivores) to the ocean depths – and thereby place it out of reach of the atmo­sphere where it could con­trib­ute to global warm­ing – are quite attract­ive. Hence the notion of iron fer­til­isa­tion, which aims to pro­mote phyto­plank­ton growth by addi­tion of that essen­tial plant nutri­ent, which is in short sup­ply in large parts of the oceans. Whilst attempts at this manip­u­la­tion to date have suc­ceeded in pro­mot­ing algal growth, none have unam­bigu­ously demon­strated the neces­sary mass deep-sinking events of the fixed car­bon that would lead to car­bon being appro­pri­ately sequestered at depths that pre­clude its rapid return to the atmo­sphere. However, Victor Smetacek et al., ana­lys­ing the European Iron Fertilization Experiment (EIFEX) – car­ried out early in 2004 in ‘the closed core of a ver­tic­ally coher­ent, meso­scale eddy of the Antarctic Circumpolar Current’ – con­clude that at least half the diatom bloom bio­mass sank far below a depth of 1000 m and that a sub­stan­tial por­tion is likely to have reached the sea floor as a ‘fluff layer’. And – encour­agingly – iron-fertilised diatom blooms ‘may sequester car­bon for times­cales of cen­tur­ies in ocean bot­tom water and for longer in the sed­i­ments’. But I’ve yet to find out why these prom­ising res­ults were seem­ingly sequestered from pub­lic gaze for 8 years… And, if this bio­lo­gical sequest­ra­tion isn’t up to the job, we’ll just have to hope that the oceans them­selves con­tinue to soak up the excess CO2.

Nigel Chaffey. ORCID 0000-0002-4231-9082

Nigel is a botanist and full-time academic at Bath Spa University (Bath, near Bristol, UK). As News Editor for the Annals of Botany he contributes the monthly Plant Cuttings column to that august international botanical organ. His main goal is to inform (hopefully, in an educational, and entertaining way...) about plants and plant-people interactions.

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