Krause-Jensen D et al. (2016), Long photoperiods sustain high pH in Arctic kel...

ECB-ART-45084

Sci Adv 2016 Dec 01;212:e1501938. doi: 10.1126/sciadv.1501938.

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Long photoperiods sustain high pH in Arctic kelp forests.

Krause-Jensen D , Marbà N , Sanz-Martin M , Hendriks IE , Thyrring J , Carstensen J , Sejr MK , Duarte CM .

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Concern on the impacts of ocean acidification on calcifiers, such as bivalves, sea urchins, and foraminifers, has led to efforts to understand the controls on pH in their habitats, which include kelp forests and seagrass meadows. The metabolism of these habitats can lead to diel fluctuation in pH with increases during the day and declines at night, suggesting no net effect on pH at time scales longer than daily. We examined the capacity of subarctic and Arctic kelps to up-regulate pH in situ and experimentally tested the role of photoperiod in determining the capacity of Arctic macrophytes to up-regulate pH. Field observations at photoperiods of 15 and 24 hours in Greenland combined with experimental manipulations of photoperiod show that photoperiods longer than 21 hours, characteristic of Arctic summers, are conducive to sustained up-regulation of pH by kelp photosynthesis. We report a gradual increase in pH of 0.15 units and a parallel decline in pCO2 of 100 parts per million over a 10-day period in an Arctic kelp forest over midsummer, with ample scope for continued pH increase during the months of continuous daylight. Experimental increase in CO2 concentration further stimulated the capacity of macrophytes to deplete CO2 and increase pH. We conclude that long photoperiods in Arctic summers support sustained up-regulation of pH in kelp forests, with potential benefits for calcifiers, and propose that this mechanism may increase with the projected expansion of Arctic vegetation in response to warming and loss of sea ice.

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Genes referenced: LOC100887844 LOC115923516

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	Fig. 1. CO2, O2, pH, and light in subarctic and Arctic kelp forests.Changes in pCO2, O2, pH, and light during field deployments in kelp forests in subarctic Nuuk at a 15-hour daylight photoperiod (A) (27 August to 5 September 2013) and in the Arctic Disko Bay at a 24-hour daylight photoperiod during midsummer (B) (16 to 25 June 2014). The slopes of linear regressions of the steady decline in pCO2 and increase in pH in the Arctic are indicated.
	Fig. 2. pH and CO2 levels in aquaria.Average pH (A) and CO2 concentration (B) over time in aquaria (n = 3) exposed to different photoperiods (left to right: 12-, 15-, 18-, 21-, and 24-hour light; shading illustrates dark periods) and treatments with different CO2 supply (~200, 400, and 1000 ppm). Latitude/season equivalents of the tested photoperiods are shown at the top of the relevant photoperiod panel; that is, the 15:9 and the 24:0 photoperiods represent the field studies in Nuuk and Disko Bay, respectively.
	Fig. 3. Macrophyte effects on CO2 in relation to photoperiod.(A) CO2 end point as a function of photoperiod (mean ± SE across pCO2 treatments) with an exponential fit (y = 12.82e−0.076x, R2 = 0.96). (B) Mean CO2 removal rate (in micromolar per hour) as a function of photoperiod (mean ± SE across CO2 treatments). (C) Linear regression slopes of mean relative maximum electron transport rate (rETRmax) per unit of CO2 as a function of photoperiod for the three tested species.