Effects of Nitrogen Supply and Continuous Darkness on Growth and Photosynthesis of the Arctic Kelp Laminaria solidungula

Sporophythes of the arctic kelp Laminaria solidungula were collected from the Beaufort Sea and kept for 7 months at$0\pm1.5^\circ C$in a factorial experiment under two light (continuous darkness or 25 μ mol photons m-2 s-1 on a 12:12 L/D cycle) and two nutrient regimes (with or without added nitrate). We monitored growth rate and at the end of the experiment measured the following on both new and old blades: carbon and nitrogen content, chlorophyll and carotenoids, photosynthetic light-response (P vs. I) curves, and photosynthetic unit size (Chl/P700). New blade initiation was independent of light and esternal nitrate, but blades grew minimally in darkness. Both new (B1) and old (B2) blades of plants held in darkness (with or without added nitrate) had the same quantum yield (φm) and area-normalized photosynthetic capacity (Pm) but a slightly lower respiration rate (Rd) and compensation irradiance (Ic≤ 1.5 μ mol photons m-2 s-1) as B2 of plants grown in the light with added nitrate. Nitrogen starvation in the light, but not in darkness, caused reduced pigments, Pm, and φmand increased Ic. Tissue C and N were primarily affected by light, whereas pigments were more strongly affected by N and light× N interaction. Based on these results, previous results or field-collected plants, and continuous in situ light data, we conclude that total annual growth in this population is severely light-limited, with N having an important but secondary role.