nitrogen growth rates of Nitzschia could be up to the max more
rapidly and grow exponentially, which was obviously different from
Navicula.
Study results also demonstrated that the nitrogen and phosphorus
nutrients of high concentrations could boost cell divisions
and were considered to be limited nutrient elements maintaining
five diatoms growing normally. In experiments of nitrogen and phosphorus
nutrients, the benthic diatoms reached the late log growth
phase generally in the 9th culture day while silicon with higher concentration
than that in f/2 medium seemly could not prolong the
diatoms log growth period (6 days).
In this study, going through the four nutrients single factor tests,
we found that the culture days which different benthic diatoms grew
in at the highest growth rates occurred regularly. Generally, it was
mostly ranging from the 2nd day to the 6th day when the five strains
grew at the max. In particular, the max of 7# in iron single factor
test turned up in the 9th day while that of 9# in nitrogen single factor
test turned up in the 12th day. Furthermore, the highest growth rates
of Nitzschia (6# and 8#) reached the max mostly earlier than that
of Navicula (5#, 7# and 9#).
In the orthogonal tests, it was indicated that the optimal nitrogen
and silicon concentrations of Nitzschia (6# and 8#) were lower
than that of Navicula (5#, 7# and 9#) yet the optimal phosphorus and
iron concentrations of Nitzschia were conversely higher. In addition,
the influence orders of nitrogen, phosphorus, iron and silicon
on diatoms growth also made an important point that, affected by
silicon much more greatly, three strains of 5#, 6# and 7# could grow
much better while, in comparison, 8# and 9# were proved to be more
in need of iron nutrients. It turned out that, as the limited nutrients,
if the nitrogen and phosphorus nutrientswere sufficient indeed,
appropriately adding up the silicon and iron nutrients content could
apparently boost benthic diatoms rapid growth.
3.2. Five species of benthic diatoms optimal concentration ratios of
nitrogen, phosphorus, iron and silicon
Li [8] studied the optimal concentrations as well as their ratios
suiTab. for different species of benthic diatoms growth and the results
revealed that the individual differences were existed exhibiting as
follows: 5.0:0.25:0.5:2.0 for Cocconeis scutellum var. pava,
2.5:1.0:1.0:2.0 for Amphora coffeaeformis and 5.0:0.15:0.5:2.0 for Navicula
mollis. Chen [20] put forward a conclusion, described as
N:P:Fe:Si = 10:1:0.1:1. In the meanwhile,Wu [6] found the optimal
concentration ratios of nitrogen, phosphorus, iron and silicon were