Quantitative Significance of Dissolved Organic Losses Following Fragmentation of Kelp (Ecklonia maxima and Laminaria pallida)

Abstract

Kelp E. maxima and L. pallida both undergo fragmentation from the tip and thus release structural components as particulate matter and a dissolved fraction from the cell contents into the water column. Analysis of the dissolved fraction released as mucilage from kelp during fragmentation yielded a 92-94% gravimetric recovery comprising 4.74-5.27% protein, 0.17-0.18% fat, 7.17-7.56% sugar and polyols, 4.77-5.01% alginate and 4.6-7.2% laminarin with 69.8% ash. Of the lipids, fatty acids of the linoleic (18:2) and, to a lesser extent, the linolenic (18:3) series are represented. These may represent a major source of essential fatty acids in inshore waters when the large annual volumes of mucilage released during fragmentation are taken into account. Analysis of protein hydrolysates show that aspartate, glutamate and alanine, all of which occur in the side chains of mucopolypeptides, amount to approximately 68% of the total spectrum in E. maxima and 50.58% L. pallida. This may primarily represent protein linkages between polysaccharide side chains in kelp exudates. Sugars amounted to 6.5-7.1% of crude dried mucilage from both species (ca 23% of ash-free dry weight). Of these, acyclic polyol mannitol dominated the ethanol extract accounting for approximately 77% of the free reducing sugars and polyols present in both kelp species. Alginate accounted for 4.77% of dried mucilage from E. maxima and 5.01% in that from L. pallida. Laminarins amounted to 7.23% by weight of the dried mucilage from E. maxima and 4.59% in L. pallida when losses in recovery of laminarins are taken into account. Calculation of the energetic contribution of dissolved organic components in kelp mucilage, compared with the measured energetic content, yields 91-95% recovery if the 6-8% loss in gravimetric recovery represents organic matter. Energy released as dissolved organic components during kelp fragmentation amounts to approximately 34% of the total annual energy production of L. pallida and 16-17% of that by E. maxima. Total particulate energy production from both species in a small kelp bed of 700 ha is estimated as 43.25 .times. 1010 kJ/yr, whereas that released as dissolved organic matter amounts to 10.74 .times. 1010 k/yr. This is equivalent to 20.24 .times. 107 l mucilage released each year during fragmentation into the shallow into the shallow water of the kelp bed, to which should be added undetermined losses of mucilage from the surface of the fronds. Studies of the concentration of dissolved organic matter in the water near a kelp bed suggest that the exudate released during fragmentation is rapidly utilized within 24 h by microheterotrophic organisms rather than by phytoplankton.