Trichodesmium IMS101 Research Articles

Phosphorus dynamics in cultures and natural populations of Trichodesmium spp.

Trichodesmium spp. fix atmospheric N2 and so an element other than N limits produc- tion by these species in the oligotrophic ocean. Because dissolved inorganic phosphorus (DIP) is in short supply in many marine systems, it has been hypothesized that P is a growth-limiting nutrient for these species in nature. However, Trichodesmium is capable of hydrolyzing dissolved organic P (DOP) compounds and the inorganic products from hydrolysis may provide an additional source of P for growth. We investigated P dynamics and alkaline phosphatase activity in cultures and natural populations of Trichodesmium from the Atlantic Ocean and the north coast of Australia to determine whether hydrolysis of DOP could supply enough P to fuel growth. During the Atlantic cruise, con- centrations of DIP were lower and chlorophyll (chl a)-specific rates of alkaline phosphatase activity by Trichodesmium were higher than during the Australian transect. However, because Tricho- desmium were much more abundant during the Australian transect, where they represented the bulk of the surface chl a biomass, total water column rates of alkaline phosphatase activity were higher along the Australian transect than in the Atlantic. In both systems, DOP could potentially supply a significant portion of the cellular P necessary for growth. In cultures and natural populations, alkaline phosphatase activity was inhibited when DIP was present and increased in the presence of DOP. Cul- tures of Trichodesmium IMS101 grew equally well on media enriched with DOP or DIP at all but the highest enrichment levels. From these studies, we conclude that if the supply of appropriate DOP substrates is adequate, DOP can represent an important P source for Trichodesmium growth which should be included in estimates of P availability in surface waters.

Stoichiometry of nitrogen and carbon utilization in cultured populations of Trichodesmium IMS101: Implications for growth

Rates of particulate organic carbon (POC) and particulate organic nitrogen (PON) accumulation, N2 and CO2 fixation, and NH4+ uptake were measured in cultures of Trichodesmium IMS101 growing on an artificial culture medium without added N substrates. Cultures exhibited a doubling time of about 4 d based on the accumulation of PON, POC, chlorophyll a, and cell number. Cell‐specific rates of N2 and CO2 fixation were highest during midday and during the initial stages of the growth cycle. Up to midlog phase, integrated estimates of CO2 fixation (based on short‐term 13CO2 uptake) closely tracked increases in POC. However, after day 8, estimates of C accumulation based on CO2 fixation exceeded observed POC increases. Within the error of the measurements, N2 fixation estimates (based on C2H2 reduction) also closely tracked, and could largely account for, PON increases over most of the experiment. N demand based on CO2 fixation also followed PON increases during early to midlog phase but diverged during late exponential phase and thereafter by greater than 100%. Estimates of short‐term C fixation rates during and after late exponential phase overestimate net growth of Trichodesmium IMS101, whereas estimates of N2 fixation rates approximated net growth. These observations may partially explain the disparity of estimates of growth in field populations based on 14CO2 fixation compared to N2 fixation. Ammonium accumulated in the culture medium during the experiment, and rates of NH4+ uptake were high throughout the day and over the whole growth cycle. Rates of NH4+ uptake generally exceeded N2 fixation. The turnover of N was therefore much greater than the net accumulation of N in biomass and implies a concurrent high rate of NH4+ release.

Nitrogen fixation, uptake and metabolism in natural and cultured populations of Trichodesmium spp.

Uptake rates of several combined N sources, N 2 fixation, intracellular glutamate (glu) and glutamine (gln) pools, and glutamine synthetase (GS) activity were measured in natural populations and a culture of Trichodesmium IMS101 grown on seawater medium without added N. In cultured populations, the ratio of GS transferase/biosynthetic activity (an index of the proportion of the GS pool that is active) was lower, and intracellular pools of glu and gln and the ratios of gln/glu and gln/α-ketoglutarate (gln/αkg) ratios were higher when N 2 fixation was highest (mid-day). There was an excess capacity for NH 4 + assimilation via GS, indicating that this was not the rate-limiting step in N utilization. In natural populations of Trichodesmium spp., the gln/glu ratio closely approximated the gln/αkg ratio over the diel cycle. High gln/glu and gln/αkg ratios were noted in near-surface populations. These ratios decreased in samples collected from greater depths. Natural populations of Trichodesmium spp. showed a high capacity for the uptake of NH 4 + glu, and mixed amino acids (AA). Rates of NO 3 - and urea uptake were low. NH 4 + accumulated in the culture medium during growth and rates of NH 4 + uptake showed a positive relationship with the NH 4 + concentration in the medium. Although rates of N 2 fixation were highest and accounted for the majority of the total measured N utilization during mid-day, rates of NH 4 + uptake exceeded rates of N 2 fixation throughout much of the diel cycle. In exponentially growing cultures, only 23 % of the total daily N utilization was due to N 2 fixation while NH 4 + uptake accounted for more than 70%. Based on N 2 fixation alone, the N turnover time for this culture during exponential growth was on the order of 9 d. This is consistent with the observed chlorophyll-based growth rates for these cultures suggesting that N 2 fixation was responsible for net growth. Our results contrast with the view that natural populations of Trichodesmium spp. acquire their cell N exclusively through N 2 fixation. C productivity may overestimate N demand for net production if regenerated production is significant in these populations.