hence mannitol and subsequently laminarin accumulate. In the meristem tissue-N declines during the summer as it is diluted by other cellular components eg. carbohydrates, and cell division and displacement of tissue distally
removes N from the meristem as external N limitation prevents
■ f
rapid uptake and accumulation. In the mature frond tissue -g
i
where growth (cell division and cell enlargement) has , 'q
ceased the N content also declines. The dry weight of the frond increases during the summer (as carbohydrates accum- ulate) and the decline in tissue-K relative to the dry weight may merely reflect this increase. Using data for L . digitate.
(mature tissue), the mean dry w t . of a disc of tissue (2.5 cm diameter) increases from 42,94 + 1,34 mg in April to 71.09 + 3.22 mg by September; the actual amount of N
(pg) in that area of tissue drops from 1339.39 to 514.95 p g . y| In April this N content represents 3.06% of the dry wt. If
"■i
the amount of N remains constant then it would comprise -I
only 1.92% of the dry wt. in September, The observed K j
content (0.84% dry wt.) is less than the predicted value and '%
.the decline in N therefore, represents an actual loss of H from that area of tissue.
The I! may be lost in three ways:
i Loss to the seawater (efflux) ii Losses by respiration
iii Basipetal translocation 162
f
1 63
1 5
Despite the availability of N no bidirectional flux
measurements of N0_ (or NK^) appear to have been made in algae (Raven, 1975), possibly due to the experimental difficulties resulting from the requirement for aseptic
conditions to eliminate surface microbial effects. Evidence
for other anions eg. P0~ and suggests that efflux is
low relative to unidirectional influx (which approximates to the measured net influx) (Vallee & Jeanjean, 1968; Raven,
1974), Loss of nitrogenous materials during adverse conditions is reported by Sieburth (1969) but this is unlikely to be
important in midsummer. Unicellular algae are reported to release proteins, peptides and amino acids into the seawater under normal conditions (Mhailov & Burlakova, 1969) but
there is no evidence for this in macroalgae. Loss of II by these means is, therefore, probably 1 o\/.
It has been suggested that amino acids rather than
mannitol are utilised as the principle respiratory substrate in L . digitata (Hellebust & Haug, 1972b); however, this
would only result in a drop in tissue-K content if their N is subsequently exported. Quantitative measurements of utilisation of amino acids and subsequent export (if it occurs) have not been attempted.
Translocation of amino acids from mature to meristematic tissue has been shown to occur in the Lam i na ri al es , but
despite extensive studies on the rates of translocation and composition of the translocate, there is little infonation on the actual amounts of L’ exported. All measurements are relative to the amount of labelling eg, amino acids comprised
164
14
32% of the total basipetally exported C-labelled trans
locate in L, s a c c h ar i na ; 71% of the total basipetally exported translocate accumulated in the lowermost 20 cm of the frond in 96 hours (tuning e_t 1973) and actual amounts (pg N ) exported over the course of the experiment are not estimated. It is, therefore, not possible to
establish whether the loss of N from the distal frond tissue (824 pg from April to September) ,may be accounted for by basipetal translocation;' but since efflux is presumed to be low, N must, by inference, be exported from the mature frond tissue either by basipetal translocation of amino acids
or N-products from respiratory breakdown of amino acids. .To summarise: evidence presented suggests that during the winter luxury consumption of K occurs by Laminaria spp; the stored K reserves are utilised to maintain maximum
growth rates after N in the seawater has declined. However, prolonged external limitation causes a decline in linear
growth rates, by restricting protein synthesis, once internal K reserves have been depleted, despite the probable trans location of N from the mature to the meristematic frond tissue.
Seawater K concentration and tissue M content are positively correlated and in areas where the seawater H is high (as a result of pollution) the internal total H content is also proportionally higher. However, growth rates are not directly determined by total available external H c on c entration since a small rise in external K results in a
■1
■g 165
(NC^+ HC g+ NH ^) does not explain the higher growth rates
and similarly, the higher phosphate concentration in addition to the K at the Sewer site predicts a lower growth rate
than is observed. Plants at St, Andrews Sewer site are
growing at extreme low water of spring tides (ELWS) and are S
therefore, immersed for considerably longer periods than plants higher up the shore (ie. at St. Andrews site and also at Kingsbarns and Fifeness), and during parts of the winter (from December until February or March) they are only emersed for short periods during each tidal cycle or sometimes not at all. As a result, there is an almost continuous movement of water past the frond surface (except when emersed) which effectively reduces the width of the boundary layer
(V.'heeler, 1980) and enhances nutrient uptake (b'hitford & Schumacher, 1961; Doty, 1971; Meushal, 1972). Nutrient uptake of plants higher up the shore during periods of
emersion is restricted to the diffusion of ions retained in the film of moisture on the frond surface. Following from
this it would be expected that Laminaria growing in pools 4
isolated at lov; water would also have higher growth rates than plants which are emersed. This was not found to occur at St, Andrews and may result from only limited water m o ve ment in the pools during low tide and, therefore, similar diffusive limitations on nutrient uptake. It is proposed that the high growth rates of plaints at the Sewer site result from increased periods of water movement past the frond surface in conjunction with the elevated N (NO^ + NO + NH^ ^ and phosphate concentrations found in this locality.
166
L . digitata has been shown to take up nitrate, nitrite and ammonium simultaneously from the medium; this confers a competitive advantage over other macroalgae and phyto plankton many of which show either a preference for one N form or suppression of uptake of one form of K in the presence of a no ther. This advantage is particularly important in
the summer when external N concentration is minimal.
Simultaneous uptake occurs in a number of algae, where uptake of one is not affected by the other forms; in Gelidium
nudifrons (Bird, 1976), Fucus spiralis (Topinka, 1978),
Macrocystis pyrifera (Haines & hheeler, 1977), L, longicruris i|
(Harlin & Craigie, 1978). In other species ammonium
causes suppression of nitrate uptake: in Hypnea muse i f orir.i s #
NO uptake over 8 0 minutes was reduced by half in the presence of eouimolar IJH^-H, but NO^ had no effect on NH uptake (Haines & V/heeler, 1978) and in Gracilaria follifera and Ne oagar dhi el la baileyi NO uptake was suppressed at 5 pM(NH^) but simultaneous uptake occurs at unsuppressed rates at lower concentrations (D’Elia & DeBoer, 1978),
In L. d i g i t a t a , NO uptake was suppressed in the
presence of NO^ only but NO^ was unaffected by high concen trations of NCg. In L. l on gicruris, Harlin & Craigie (1978) found a similar suppression of nitrate as the molar ratio of NO :NOg increased, and in Codium fragile (Hanisak, 1979b) uptake of NC^was unaffected by the presence of NO^, but
M0_ uptake was reduced by half when NO :NO^ was in the ratio 5 pM:5 p H . However, comparison between individual studies is difficult; uptake is frequently measured at concentrations
167
higher than would normally be found situ and the
physiological state of the experimental plants (whether N limited prior to the experiment and the levels of internal N reserves) are not always stated.
Uptake of nitrate by L. saccharina and L. digitata was very similar despite the use of whole plants of
L. saccharina and discs cut from the mersitem of L. d i g i t a t a , emphasising the validity of using tissue discs in nutrient uptake experiments.
The 2 phase uptake of nitrate has been previously observed in corn (Van Den Honert & Hooymans, 1955) and in Macrocystis, ammonium shows a similar biphasic uptake (Haines & V.'heeler, 1978). The mechanism to explain this biphashic uptake (at present the multiphasic model (Nissen, 1973) takes precedence over the parallel (Epstein, 1972) and the series (Laties, 1969) models) need not concern us here. Uptake of nitrate by Laminaria is saturated at nitrate
5