Algal lineages, life cycles and aquatic productivity

Algal roles span different host histories

Alga is a useful term for many photosynthetic organisms outside the usual land-plant category, but eukaryotic algae do not form one exclusive clade. The green lineage includes green algae and land plants. Red algae have another ancestry within Archaeplastida. Diatoms and brown algae are stramenopiles, while photosynthetic dinoflagellates are among alveolates. These examples are not a complete list of aquatic producers.

Aquatic photosynthesis also involves cyanobacteria, which are bacteria rather than eukaryotes. Ecological terms such as phytoplankton can gather organisms with different cell organisations. Plankton describes organisms carried in a water body whose movement does not overcome its currents at the relevant scale; it is not another ancestry group.

Lineages differ in coverings, pigments, storage compounds, development and ecological roles. Not every member of a broader lineage has the same nutritional function. Photosynthesis can be absent or combined with ingestion, and the history of a plastid may differ from nuclear ancestry. A comparison should state which organisms, stages and conditions it includes.

Follow fusion and meiosis rather than appearance

In the first assigned life cycle, most vegetative life is haploid: one chromosome set per nucleus. Two haploid gametes fuse to make a diploid zygote. DNA replication prepares that zygote for meiosis, and meiosis produces four haploid products. A haploid vegetative state is familiar from Chlamydomonas, but the diagram is a simplified teaching pattern, not its exact genome or all its reproductive routes.

In the second assigned pattern, both haploid and diploid stages grow into multicellular generations. The haploid gametophyte produces gametes without halving its ploidy. Their fusion starts a diploid phase that develops into a sporophyte. Meiosis in a spore-producing cell yields haploid spores, which develop into haploid generations.

Many Ulva life cycles offer a context for multicellular haploid and diploid phases. Real routes can also include unfused gametes developing further and changes of ploidy through additional mechanisms. The drawn sexual route deliberately leaves these out. Similar-looking macroscopic stages do not establish identical chromosome sets.

The model assigns two chromosomes per haploid set. A diploid nucleus before replication has four chromosomes and four chromatids. After replication it still has four chromosomes, now comprising eight chromatids. C denotes DNA amount relative to an unreplicated haploid nucleus: these states are 2C and 4C respectively. Replication changes DNA amount without creating a new homologous chromosome set.

Counts beside a multicellular generation describe one indicated nucleus, not every chromosome in the whole body. The four meiotic products each have two chromosomes and 1C. A gamete's role is fusion in this route, while a spore can begin development without fusing. Swimming alone does not distinguish these roles.

A flow is different from the standing stock

Primary production creates organic material from inorganic carbon using an energy source. For photosynthesis, gross primary production records the total assigned fixation over an interval. Subtract the producers' own respiration over that interval to obtain net primary production. In this lesson the account uses carbon units, not a direct measurement of energy, oxygen or cell number.

New organic carbon can enter producer biomass, be released into the water or move to consumers. Net primary production therefore does not equal the increase in biomass remaining at the end. Existing biomass, feeding losses, release, transport and other processes affect that stock.

The original fixed-volume account assigns gross production of 120 carbon units and producer respiration of 45. Net production is 75. Grazers ingest 30 and dissolved organic release contributes 15, leaving an increase of 30 in producer biomass. All amounts refer to the same assigned interval and volume; no global ocean percentage is inferred.

A high production rate can coexist with a small standing stock if consumers or other losses remove material quickly. A high stock can also reflect accumulation over earlier intervals. To compare sites, researchers must specify time, volume or area, measurements and the processes included in the balance.

Use the pattern at the right scale

Some algae are major components of microscopic food webs; others form large attached bodies. This chapter includes an Ulva example to show why algal diversity extends beyond a purely microscopic size category. It does not treat seaweeds as a single clade or repeat the detailed evolution of multicellularity from earlier chapters.

Life-cycle stages alter which cells are counted and which comparisons are meaningful. A concentration of motile reproductive cells, the area of a seaweed body and a rate of carbon fixation are different measurements. One may help explain another, but they cannot be substituted without a model and evidence.

Likewise, a number measured during daylight must be related carefully to losses across a whole interval. Night-time respiration and consumer activity can continue. A useful ecological explanation follows both the organism's stages and the quantities transferred, rather than equating visible greenness with total productivity.

Worked example

Check your understanding

1. Why does premeiotic replication change the assigned diploid nucleus from four to eight chromatids without making it tetraploid?

   Show reasoning

   Each chromosome is copied into two sister chromatids. The number of homologous sets remains two and the chromosome count before separation remains four; DNA amount rises from 2C to 4C.

2. Must the assigned haploid gametophyte use meiosis to make n gametes?

   Show reasoning

   No. It already has one chromosome set. The shown route produces gametes without ploidy reduction. Meiosis occurs in the diploid spore-producing stage and supplies haploid spores.

3. With gross production 120, producer respiration 45, grazing 50 and dissolved release 15, what are net production and producer-biomass increase?

   Show reasoning

   Net production is 120 − 45 = 75 carbon units per assigned interval. The increase in remaining producer biomass is 75 − 50 − 15 = 10. Neither value is a count of cells or the total biomass stock.

What to carry forward

Algal producer roles span several lineages. Follow the actual life-cycle transitions and keep production flows separate from remaining biomass.