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Marine Ecology Progress Series

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MEPS 125:229-237 (1995)  -  doi:10.3354/meps125229

Photosynthesis in reproductive unicells of Ulva fasciata and Enteromorpha flexuosa: implications for ecological success

Beach KS, Smith CM, Michael T, Shin HW

Photosynthetic performance of adult vegetative tissues is modified following the formation of free-swimming reproductive unicells by Ulvafasciata and Enteromorphaflexuosa. Comparisons of photosynthetic performance of adult haploid or diploid thalli versus gametes or zoospores (respectively) yield the following generalizations: (1) Pmax (maximum net photosynthetic rate) for motile cells (e.g. gametes of U. fasciata and zoospores of E. flexuosa) can be equivalent to Pmax of appropriate adult tissues when expressed on a chlorophyll a (chl a) or chl a+b basis; (2) respiration rates for all motile cells were 1.5- to 6-fold higher than rates for adult thalli when normalized to chl; (3) as expected with increasing respiration rates, compensation irradiance (Ic) and saturation irradiance (Ik) also increased for all motile cell types; (4) chl a:b ratios were lower in all motile cell types than in adult cells; (5) P:R (photosynthesis:respiration) ratios were 3.3- to 7.7-fold lower in zoospores of U. fasciata and E. flexuosa respectively. Photosynthetic performance of settled zoospores of U. fasciata was more similar to the vegetative parent than to the free-swimming zoospores in terms of Ik and Pmax even with altered chl a:b ratios. An unexpected finding was the increased invivo absorbance in 490 to 550 nm range for reproductive tissues of U.fasciata only. This absorbance increase suggests a potential photoadaptation. This potential photoadaptation, coupled with alteration of the chl a:b ratio, could allow reproductive unicells of U. fasciata to harvest light more readily for photosynthesis as part of the plankton when compared to zoospores of E. flexuosa. Such a difference between motile cells may enhance the competitive ability and subsequent ecological success of one over the other. High rates of Pmax have the selective advantage of subsidizing high respiratory costs associated with motility in reproductive unicells while still allowing for rapid growth upon eventual attachment to the substrate. Comparably, species such as late successional kelps do not show this pattern and may represent a different photoadaptive strategy.

Photosynthesis. Zoospores . Gametes . Sporophytes. Gametophytes . Invivo absorption . Ulvafasciata . Enteromorphaflexuosa

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