Survival in low light: photosynthesis and growth of a red alga in relation to measured in situ irradiance

Abstract

Reduced light availability for benthic primary producers as a result of anthropogenic activities may be an important driver of change in coastal seas. However, our knowledge of the minimum light requirements for benthic macroalgae limits our understanding of how these changes may affect primary productivity and the functioning of coastal ecosystems. This knowledge gap is particularly acute in deeper water, where the impacts of increased light attenuation will be most severe. We examined the minimum light requirements of Anotrichium crinitum, which dominates near the maximum depth limit for macroalgae throughout New Zealand and Southern Australia, and is a functional analog of rhodophyte macroalgae in temperate low-light (deep-water) habitats throughout the world. These data show that A. crinitum is a shade-adapted seaweed with modest light requirements for the initiation of net photosynthesis (1.49-2.25 μmol photons · m(-2) · s(-1) ) and growth (0.12-0.19 mol photons · m(-2) · d(-1) ). A. crinitum maintains high photosynthetic efficiency and pigment content and a low C:N ratio throughout the year and can maintain biomass under sub-compensation (critical) light levels for at least 5 d. Nevertheless, in situ photon flux is less than the minimum light requirement for A. crinitum on at least 103 d per annum and is rarely sufficient to saturate growth. These findings reinforce the importance of understanding the physiological response of macroalgae at the extremes of environmental gradients and highlight the need to establish minimum thresholds that modification of the subtidal light environment should not cross.

Photosynthetic light requirements and vertical distribution of macroalgae in newly ice-free areas in Potter Cove, South Shetland Islands, Antarctica

“…Consequently, changes in light availability due to climate change could lead to changes in the vertical distribution limits and in benthic primary productivity (Gómez et al 1997). Direct associations between increases in turbidity and decreases in macroalgal productivity have already been reported (Airoldi 2003;Anthony et al 2004;Spurkland and Iken 2011;Pritchard et al 2013).…”

Whole community estimates of macroalgal pigment concentration within two southern New Zealand kelp forests1

“…There was, however, a tendency for species to have slightly greater concentrations of all three pigments at 2 m compared to 10 m. This finding contradicts the well‐established relationship of increasing pigment concentration with depth (Ramus et al , ; Wheeler ; Lopez‐Figueroa ; Talarico and Maranzana ). This may suggest that light availability at 10 m could be low enough to limit the synthesis of pigments, indicating that these species may be living on the edge of their photosynthetic capabilities (see Pritchard et al ). Further work is required to assess whether this is the case.…”

“…Anotrichium crinitum and E. formosissima composed 58% and 31% of the community biomass respectively in the low‐light site, while their combined contribution to the Rhodophyceae community biomass in the high‐light site was <0.5%. At the low‐light site, these highly pigmented species may hold a competitive advantage which is not the case in the high‐light site where light is not limiting (Peckol and Ramus ; Pritchard et al ). These findings indicate potential costs and advantages that come from pigment investment and this, among other factors, may help to explain certain differences observed in community structure between sites.…”

Restricted use of nitrate and a strong preference for ammonium reflects the nitrogen ecophysiology of a light‐limited red alga

“…For an alga with a C:N ratio of around 6:1 (typical for field‐harvested Anotrichium crinitum , Pritchard et al. ) it takes 23% less energy to produce one mole of the amino acid glutamate with ammonium as the nitrogen source compared with nitrate (Thompson et al. ).…”

“…A. crinitum is a filamentous macroalga, typically found growing near the maximum depth limit of non‐calcareous macroalgae or in shallower water under light limiting conditions (Pritchard et al. ). A. crinitum is a dominant component of subtidal, temperate reefs in New Zealand (Hepburn et al.…”