Abstract
A striking characteristic of endemic Antarctic brown algae is their broad vertical distribution. This feature is largely determined by the shade adaptation in order to cope with the seasonal variation in light availability. However, during spring-summer months, when light penetrates deep in the water column these organisms have to withstand high levels of solar radiation, including UV. In the present study we examine the light use characteristics in parallel to a potential for UV tolerance (measured as content of phenolic compounds, antioxidant activity and maximum quantum yield of fluorescence) in conspecific populations of four Antarctic brown algae (Ascoseira mirabilis, Desmarestia menziesii, D. anceps and Himantothallus grandifolius) distributed over a depth gradient between 5 and 30 m. The main results indicated that a) photosynthetic efficiency was uniform along the depth gradient in all the studied species, and b) short-term (6 h) exposure to UV radiation revealed a high tolerance measured as chlorophyll fluorescence, phlorotannin content and antioxidant capacity. Multivariate analysis of similarity indicated that light requirements for photosynthesis, soluble phlorotannins and antioxidant capacity are the variables determining the responses along the depth gradient in all the studied species. The suite of physiological responses of algae with a shallower distribution (A. mirabilis and D. menziesii) differed from those with deeper vertical range (D. anceps and H. grandifolius). These patterns are consistent with the underwater light penetration that defines two zones: 0–15 m, with influence of UV radiation (1% of UV-B and UV-A at 9 m and 15 m respectively) and a zone below 15 m marked by PAR incidence (1% up to 30 m). These results support the prediction that algae show a UV stress tolerance capacity along a broad depth range according to their marked shade adaptation. The high contents of phlorotannins and antioxidant potential appear to be strongly responsible for the lack of clear depth patterns in light demand characteristics and UV tolerance.
Highlights
Seaweed communities from the Antarctic Peninsula and adjacent islands are characterized by an exuberant abundance whose standing crops support most of the biogeochemical processes at coastal ecosystems [1,2,3,4]
The present study examines the physiological performance of four endemic Antarctic species (Ascoseira mirabilis, Desmarestia menziesii, Desmarestia anceps and Himantothallus grandifolius) with broad vertical distribution in the context of solar UV radiation
P-I curve based parameters measured in the different species varied along the depth gradient (Fig 3)
Summary
Seaweed communities from the Antarctic Peninsula and adjacent islands are characterized by an exuberant abundance whose standing crops support most of the biogeochemical processes at coastal ecosystems [1,2,3,4]. Species of the order Desmarestiales such as Himantothallus grandifolius, Desmarestia anceps and D. menziesii, as well as some fucoid species such as Ascoseira mirabilis and Cystosphaera jacquinotii can extend between 2 and 40 m [6,7,8,9]. This remarkable ability underlies efficient photobiological adaptations, especially very low light requirements for photosynthesis and growth of both early stages and adult thalli [10,11,12]. The capacity of Antarctic algae to maintain a low respiration rate and a positive carbon balance at compensation irradiances tuned to the seasonal light conditions have been claimed as the factors explaining this shade adaptation [10,11,14,15]
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